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      11-16-2021, 07:46 AM   #1
///d
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Wheels, Tires, and Fitment Information Thread

There are always questions regarding wheels, tires, and fitment, so I decided to create a thread with information . Understanding how offsets and fitment works can be a little confusing, so hopefully this will help folks get a better understanding.

WHEELS:

First things first, lets get some common terminology out of the way:

Wheels – Found on vehicles
Rims – Used for basketball nets
ET – Einpress tiefe (German), or insertion depth. The distance of the mounting surface from the center of the width of the wheel.
Offset – another word for ET
Backspacing – also another word of Offset or ET
Wheel Width - The width of the wheel, measured from bead seat to bead seat, not edge to edge.
Wheel Diameter - The overall diameter of the wheel's bead seat, not the diameter of the wheel edge.
Lip – Surface area between the outer edge of a wheel and the beginning of the spokes
Center Bore - The hole in the center of the wheel machined to match the hub of specified vehicles
Hub Centric - The center bore hole of a wheel matches the hub diameter of the vehicle. This centers the wheel via the center hole rather than the lug nuts.
Lug Centric - When the wheel is centered by the bolt holes/ lug nuts of the wheel, rather than by the center bore.
Bolt Pattern – Numerical representation of the number of lugs/bolts by the diameter, in millimeter, of the imaginary circle formed by the center of the lugs. (Example: 5x120)
Wheel Spacer – A machined piece of aluminum that is placed between the wheel mounting surface and the hub to add negative offset to the fitment of the wheel.
Square - All four wheels and tires are the same size.
Staggered - Front wheels and tires are narrower that the rear wheels and tires, and have different offsets.
Stance – How a vehicle looks with wheels/tires/suspension set up.
Flush – When the edge of your wheels/tires are in-line with the edge of your fenders.

Types of wheels:

Cast Wheels – Alloy is heated and melted, poured into a mold and cooled. Once cooled, it is trimmed and machined to perfection. Most OEM wheels are cast wheels.
Advantages: Less expensive
Disadvantages: Easier to crack. Heavier.

Flow Formed Wheel - Similar to cast but with improvements. Once casting takes place, the wheel is spun and pressure is applied to the inner barrel to create its final shape. This increases strength while keeping it lightweight compared to traditional cast wheels.
Advantages: Stronger and lighter than traditional cast while staying less expensive than forged wheels.
Disadvantages: Still prone to cracking. Not as strong or light as forged.

Forged Wheels – These wheels are machined out of a single piece of block metal. The block of metal is heat and pressure formed making the material stronger.
Advantages: Stronger. Lighter.
Disadvantages: Expensive.

One Piece Wheels – Exactly how they sound, made out of one piece.

Two/three Piece Wheels – The outer barrel, inner spokes, and sometimes mounting hub are separate pieces that will either be bolted or welded together. Usually the lightest and strongest, but also the most expensive.



OEM F15/F16 wheel packages and offsets

18 inch wheels
Square (all four)
18x8.5 ET46
OEM Tire Size: 255/55

19 inch wheels
Square
19x9 ET37
OEM Tire Size: 255/50

Staggered
Fronts: 19x9 ET37
OEM Tire Size 255/50
Rears: 19x10 ET21
OEM Tire Size 285/45

20 inch wheels
Fronts: 20x10 ET40
OEM Tire Size: 275/40
Rears: 20x11 ET37
OEM Tire Size: 315/35

21 inch wheels
Fronts: 21x10 ET40
OEM Tire Size: 285/35
Rears: 21x11.5 ET38
OEM Tire Size: 325/30

All OEM BMW wheels are cast aluminum one-piece wheels. The F chassis X5 uses a 5x120 bolt pattern.

Wheel Width:
Wheels come in multiple widths in half inch increments. Generally speaking the wider the wheel, the fewer tire options and the more expensive the tire.

Wheel Diameter:
Diameters usually come in 1 inch increments, but there are some half inch increments sizes as well. Generally speaking the larger the diameter, the fewer tire options and the more expensive the tire.

Wheel Offset:
Ah yes, this is what gets most people riled up. Understanding offsets can be confusing and frustrating, but once you understand what an offset is actually doing to your wheels position within your wheel well you’ll find it much easier to figure out.

Offset is the position of the wheels mounting surface in relation to the centerline of the wheel width, and is measured in millimeters.

A Zero-Offset means the mounting surface is exactly centered in the width of the wheel.

A Positive-Offset (sometimes signified by a + symbol) means the mounting surface is moved closer to the outside edge of the wheel from center. This will effectively move your wheel farther inside your wheel well, or closer to your vehicle.

A Negative-Offset (signified by a – symbol) means the mounting surface is moved closer to the inside edge of the wheel from center. This will effectively move your wheel farther outside your wheel well, or away from your vehicle.

See these diagrams below for visual representations.



It’s important to note that OEM wheels will already be a positive offset. When buying custom wheels it’s important to know your OEM offset so you can properly determine what your offset needs to be for your desired fitment. We will discuss offset more in the Wheel Fitment section.

Wheel Load Rating:
One thing that a lot of people overlook or are not aware of is load ratings for wheels. Yes, they do have them! To determine what your wheels need to be rated for, take your vehicles heaviest Gross Axle Weight and divide it by two, the result is what each individual wheel needs to be rated for at minimum. For example, if your vehicles max Gross Axle Weight is 4,000lbs, your wheels need to be rated for at least 2,000lbs each. Aftermarket wheel websites do not always list the load rating for a wheel and you may need to check with the wheel manufacture to find out what they are rated for.


Tires:

Common terminology:

Aspect Ratio - The dimensional relationship between tire section height and section width, a.k.a. Series. Example: 275/35 (Section height divided by section width)
Bead - The area of the mounted tire which seats against the wheel
Contact Patch – The part of the tire in contact with the road surface.
Curb Weight - The total weight of a vehicle with no passengers and a full tank of gas.
Extra Load, Light Load. Standard Load – Determines a tires load capabilities at a specific PSI.
Gross Vehicle Weight (GVW) - The total weight of the vehicle, including passengers, fuel, cargo, and attachments.
Load Index - A numerical code associated with the maximum load a tire can carry at the speed indicated by its Speed Symbol under specified service conditions up to 130 mph.
Load Range - Replaces the former ply rating term and identifies load and inflation limits.
Overall Diameter - The maximum height of a tire when mounted on a wheel and inflated to rated pressure.
PSI - Pounds per Square Inch. Measurement of air pressure inside of the tire.
RFT – Run Flat Tire. A tire designed to temporarily maintain its shape when all air pressure is lost to allow you to safely reach a repair center.
Section Height - The distance from the bottom of the bead to the top of the tread.
Section Width - The distance from sidewall to sidewall, exclusive of any raised lettering. Example 275/35
Series - This is the part of the size designation in tires, which gives the ratio of the height of a tire (from the rim to the top of the tread) to the width of the tire (from sidewall to sidewall). It is also referred to as the aspect ratio of a tire. (Example: 35 series, 275/35 )
Sidewall - The side of a tire between the tread shoulder and the rim bead.
Speed Rating (Speed Symbol) – An alphabetical system describing a tire’s capability to travel at established and predetermined speeds. An indication of the tire’s performance.
TPMS – Tire Pressure Monitoring System – System that monitors the pressure inside your tires, and alerts you if that pressure is outside of a pre-determined limit.
UTQG - Uniform Tire Quality Grade labeling. A performance measurement of a tire, based upon its test results in three categories: treadwear, traction, and temperature resistance.

Ok, tires can get a little technical, so I will try to keep this as simple as I can. If you’re buying tires from a place like Discount Tire or from the dealership, they will ensure that any tires you purchase are rated and sized properly for your ride, but if you’re looking to order your own tires you will need to know and understand what you are ordering to ensure you don’t buy something that can not be used with your vehicle.

How to read a tire:

Reading a tire is not as difficult as it may seem.

295/35 R20

295 is your Section Width. This is how wide your tire is from sidewall to sidewall. This is measured in millimeters.

35 is your Aspect Ratio. Simply put, this is how tall the sidewall is on your tire. This number is the section height divided by section width. The smaller the number, the skinnier your sidewall.

R20 is your wheel size. In this case, it is a 20 inch wheel.

Now there is a lot more to a tire than just what is stamped on the sidewall. Things like Load Range and UTQG are other important areas to pay attention to. These are areas that any tire shop should already consider for you, but again, if you are purchasing tires online you want to make sure you check these areas.

Load Index/Range is important because you need to make sure the tires you are selecting meet the load requirements for your vehicle. If you plan to tow with your vehicle this is even more important, as you need to ensure that the tires can handle the additional tongue weight when a trailer is attached. Keep in mind this is per tire, so if the Gross Vehicle Weight Rear (GVW) for your vehicle is 5,000lbs, the combined Load Index/Rating of your tires needs to meet or exceed 5,000lbs. This can also be broken down by front (GVWF) and rear (GVWR), and the same principle applies to your combined front tires rating or your combined rear tires rating.

UTQG should also be considered. Simply put, UTQG is the expected lifespan of your tires. This can get a little complicated and varies by tire manufacture, but I’ll give the technical explanation first and then give my simple explanation afterwards.

Technical explanation, the first portion of the rating is a number that represents how long the tire's tread is expected to last against the manufactures test tire. This test is completed on a 400 mile test loop with a total number of 7,200 test miles. If a tire is rated 100, it is expected to last as long as the test tire lasted, however a test tire may last the entire 7,200 miles, or it may only last 5,000 miles, and this number is usually not known. When talking UTQG, a number of 200 means the tire is expected to last twice as long as the test tire, a number of 300 is three times as long as the test tire, and so forth.

Simple explanation, the lower the number, the lower the life expectancy of the tire. Because each manufacture uses their own test tire, there is no industry standard that this number is based on.
The below is a chart from Tire Rack in 2006 showing UTQG ratings and their general life expectancy. This chart is only for reference and actual mileage will vary.



After the UTQG number are letters that represent traction and temperature. Simply put, A is the highest or best, and C is the lowest or “just passing”.

Tire Categories:

Touring Tires:
Touring tires offer a smooth, quiet ride with reasonable wet and dry traction, as well as reasonable winter traction. These tires are similar to all-season tires, but they are typically designed for extra tread life.

All-Season:
While widely regarded as 4-season capable tires, all-season tires are technically designed for 3-seasons, spring, summer, and fall. These tires will provide reasonable wet and dry traction, and fair winter traction. They can be used in winter climates, but will not provide the traction of a winter dedicated tire. All-season tires are the most common tires found on passenger vehicles.

Performance Tires:
These tires are designed for responsive handling, high dry traction with adequate wet traction, and fair to little winter traction. These tires have a stiffer sidewall resulting a little rougher ride compared to touring tires, and a softer compound to aid in traction, and therefore have shorter tread life than touring tires.

Performance All-Season:These are all-season tires with more focus on performance over comfort. They will usually have stiffer sidewalls and softer compounds to aid in better performance while still being reasonable for everyday street use. While they may still provide some traction, these tires will suffer in winter conditions.

Summer Performance Tires:
Summer tires are meant for summer season use only. They have excellent dry grip, fair wet grip, and should not be used on winter roads. They will have softer compounds and shorter tread life.

Track and Competition Tires:
These tires are all about maximum performance and dry grip. These tires have very soft compounds and short tread life, and may not be suitable for highway or street use. Some tires may not be DOT approved for street use.

All-Terrain Tires:
These tires are meant for combinations of street and off-road. They provide excellent off-road traction, fair to adequate wet and winter traction, but they will ride firmer and louder on the street. These tires will have reasonable tread life, but the more street use they see the shorter the tread life.

Winter Tires:
These tires are meant for cold, winter climates. They offer excellent slush and snow traction, fair ice traction, and will increase cold surface traction. These tires should only be used in cold climates as the compound is not designed to be used above 40 deg F. Some winter tires are studded or studdable, meaning they have small metal points sticking out of the tread to help with ice traction. Be careful when looking at studded tires because they may not be allowed on all roads.

Severe Weather Rated Tires:
Some tires may be Severe Weather Rated and stamped with the 3 peak mountain/snowflake symbol. To qualify for this designation a tire must provide 10% more snow traction that it’s standard or all-season tire. Some areas may require Severe Weather Rated tires during winter months.

*Shameless Winter Tire Plug* There is a common argument about winter tires vs all-season tires, and that argument is that all-seasons provide plenty of traction or that winter tires don’t last long and are too expensive, or that you need two sets of wheels. I have lived in Minnesota for my entire life and we get some of the worst winter roads and coldest temperatures in the country. I used to run all-season tires and thought they were fine, but after making the switch to winter tires there is simply no comparison. Winter tires are far superior to all-seasons on winter roads. It’s not only about the traction winter tires provide to get moving, but also the ability to control your vehicle, and stopping distance is drastically reduced with winter tires. If you’re concerned about the lifespan of winter tires, keep in mind you’re only using them for a portion of the year, which also means you’re only using your all-seasons or summer tires for a portion of the year. The benefits and added safety of winter tires far outweighs the additional cost of the tires or extra wheels.

Tires and Ambient Temperature:

Tire performance is affected by ambient air temperatures in multiple ways, but the two important ways are traction and air pressure.

Temperatures and Traction:
Tire compound (rubber) is designed to provide the best performance in a certain range of ambient temperature. Some compound will become hard and lose stickiness when it becomes cold, and some compounds are meant to be soft and sticky in cold temperatures but will be too soft for warm temperatures. This is especially important when running winter tires or performance tires.

Most Touring, All-Season, and All-Terrain tires are designed to work in a wide range of ambient air temperature, and can be used in both summer and winter temperatures. They will still retain a reasonable amount of traction performance in cold temperatures, but caution should still be used as you may experience traction loss sooner than you would in warmer temperatures.

Performance and summer tires are generally not meant to be used in temperatures below 40 deg F. Anything below those temperatures and you risk a decrease in traction, which can result in quick and unexpected traction loss under braking or cornering. Some tires may even come with a warning from the manufacture stating not to use them below “x” outside air temperature.

Winter tires are designed to be used in temperatures 40 deg F and colder. The tire compound is soft and sticky to help maintain traction on slippery surfaces, and remains flexible to help “grab” icy surfaces. Because of the soft nature of the compound you do not want to use winter tires above 40 deg F for long periods of time as the tread life will be significantly reduced.

Tire Pressure:
As the outside air temperature changes it will affect the air pressure inside your tire. As a general rule of thumb, for every +/- 10 degrees F in ambient air temperature change, your tire air pressure will change +/- 2 psi respectively. For example, if your tire pressure measures 35psi with an outside air temperature of 75 deg F, it will read around 31psi with an outside air temperature of 55 deg F. This is why a lot of TPMS systems start to show low tire pressure in the fall once outside temperatures start to get colder. Because of this, it’s important to check and adjust your pressures with the changing seasons to ensure that you’re maintaining the correct pressures in your tires.

OEM tires should always have pressures set to OEM specifications, and this can be found on the label inside your drivers door jamb. Running non-OEM size tires, well this is surprisingly a controversial subject. Even with aftermarket tires you should run the vehicles recommended pressures. You may find that you have to adjust pressures for comfort or if you have improper tread wear, but keep in mind you may have to reset the TPMS, and as you change pressures, you change the overall diameter and need to keep the front and rear within 1% of each other. Some people think they need to set their pressures to what the tire states on the sidewall, but this is a MAX pressure for the tire, not a nominal operating pressure.

When you drive your vehicle your tires heat up, which causes the air inside your tires to heat up, and when air heats up it expands. Because of this some manufactures have a hot and cold PSI listed. Be sure to use the correct PSI listed for either a cold tire (hasn’t been driven on before testing), or a hot tire (has been driven on before testing). If they don’t specify hot or cold, assume it is cold and set those pressures before driving the vehicle.

Tires, ride quality, and performance:
The type of tire and size of your tire will have a direct effect on how your vehicle rides and handles. When choosing a tire you need to keep in mind what you plan to use the vehicle for. Wide tires may look cool, but the wider the tire the more surface area you have making contact with the ground and the more friction you will have causing a decrease in fuel economy. Wider tires tend to grab the grooves on roads more and may make the vehicle feel wobbly on some highways. Wider tires also hydroplane easier and will have less traction on slippery surfaces because the weight is spread out over a larger area. The smaller the aspect ratio, or sidewall, the rougher the tire will ride because there is less sidewall to cushion bumps in the road, however, this usually means the tires will perform better when cornering and have a faster response to steering input because the sidewall is stiffer and does not have much flex. Cost is also a factor when it comes to tire sizes. Smaller diameter, narrower tires are usually less expensive and have more options vs larger diameter or wider tires. Smaller tires will sometimes have a longer lifespan vs wide or large diameter tires.

Common Wheel/Tire Combinations:

It’s difficult to give specific tire sizes for different wheel sizes because every tire fits differently. What I can do, however, is provide a guide as a starting point for tire section widths for different wheel widths, and the tire series that should go with them.
Keep in mind that you want to keep your overall diameter as close to the OEM diameter as you can in order to keep your speedometer accurate, and must keep the front and rear overall diameters within 1% of each other, so always check before you buy.




Wheel/Tire Fitment:

Now for the part everyone wants, fitment! How do you determine the wheel and tire specifications for the look or fitment you want? This can be overwhelming, especially with the cost of aftermarket wheels and tires, and ordering something that you find doesn’t fit the way you wanted is never fun.

First and foremost, it’s important to know where you’re currently at in order to know where you need to be. Knowing what wheel specs and tire specs you currently have can help immensely when trying to determine what specs you need to purchase to get your desired result.

Determining Offset:
Everyone has their own stance they are looking for. There are many ways you can determine the offset you need for your desired result, but I will show you one easy way using a tape measure and a bubble level to get a flush stance (even with fenders).

Park on a flat, level surface. Using a bubble level, place the level vertical against the outside edge of your fender. Keep the vertical bubble centered to ensure the level is straight up and down. Using a tape measure, place the tape measure against the sidewall of your tire, and take note of the measurement from the sidewall to the level. Do this for both front and rear, as they will be different measurements. The photo below demonstrates how to take the measurement. In this case, we have a measurement of about 1.25 inches.

*NOTE* The reason I do this against the sidewall will be explained later on.



If you are from the land of Bald Eagles and Fast Food (U.S.), you will want to convert your imperial measurement to metric. To do so, take the decimal equivalent of your measurement, in our case 1.250, and multiply it by 25.4 to get the measurement in millimeters.

Once you have your measurement, we will use the Wheel-Size website to calculate our offsets.
https://www.wheel-size.com/calc/

Here you will see two wheel/tire setups, the one of the left is your current configuration, the one on the right is your new configuration. Enter your current wheel/tire specs into the left side. On the far left you will see setting for fender clearance, wheel well clearance, etc. Enter the measurement you took into the Fender Clearance. If you know your Wheel Well clearance, suspension clearance, etc. you can enter those as well.



Next, enter the specs for the wheels/tires that you want to upgrade to on the right side. Once you get the wheel diameter, width, and tire size entered, you can try different offset amounts to see how it changes the fitment of the wheels compared to your current setup. In the case of our example, we wanted to get flush with my fenders, and we know we need to move our wheels outwards which is negative offset from zero, so we went with a smaller offset number than OEM.

Why do I measure off the tire and not the wheel? Well because this website measures the fender distance to the edge of the tire sidewall, and not the edge of the wheel. This helps to prevent tire bulge from poking past the edge of the fender.

*Note that the website measures to the inside of the fender lip, not the outside like we did. When adjusting your offsets on your new wheels, don’t adjust them to the outside edge of the diagram on the website, only adjust to the point where it is measured from.



If you scroll down farther you will see information about the changes you made. The two areas I pay the most attention to are overall diameter and fender clearance.



Overall diameter will tell me the difference between my current setup and new setup. I don’t want too much of a difference as it will affect my speedometer. In this case I have only 2mm of difference.

Fender Clearance tells me my tire will stick out 32mm farther than my current setup, which is pretty much what my measurement was (31.75).

There is a lot more information you can look at as well, like speedometer changes, scrub radius, etc.

Another thing you can use this tool for is determining the overall diameter different from front to rear. BMW needs your front and rear overall diameters to be within 1% of each other, so you can enter your front tire/wheel sizes into the OEM setup, and your rear tire/wheel specs into the new setup and see what the diameter difference will be.

Wheel Spacers:
Wheel spacers basically add negative offset to your wheel specs. If you want to bring your wheels out without buying a new set, you can install wheel spacers to effectively push your wheels outwards. If you have wheels with an ET40, you can add 10mm spacers to basically make those wheels fit as if they were an ET30.

Always opt for hub-centric spacers. These spacers are machined to fit perfectly center on your hub. You will need longer wheel bolts when you run spacers, and most spacers will come with appropriate length bolts. Never run your OEM bolts with spacers.

Do not run multiple spacers on one wheel.

Avoid running too thick of spacers. My general rule of thumb was never go thicker than 20mm. I usually try to stay 12mm and less.

Some folks like to order wheels with a slightly higher offset than what they need, and then run a spacer to bring them down to where they want them. This leaves a little bit of room for error, and if you find that what you thought was flush was a little too much poke (sticking out too far), you can remove the spacers and gain some room back.


If there is anything else people would like to see in this post please let me know and I will add it. I always feel like I manage to add way more information that I intended when I create these threads.

Last edited by ///d; 11-17-2021 at 03:14 AM..
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      11-16-2021, 07:55 AM   #2
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As soon as I posted this I thought about adding information about winter tires and such, so thats coming soon.
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      11-16-2021, 09:19 AM   #3
Xolinlevh
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Please sticky this! Also, thank you fellow 35d owner in MN
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      11-16-2021, 10:01 AM   #4
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Great post.

Just an FYI...there's also a staggered 19" wheel (style 467) that has the specs below...you can use all 4 (front) wheels for a square set-up...or use the wider (rear) wheel for a staggered set-up:
  • 9JX19 ET:37 (front) - 255/50R19 (oe tire size)
  • 10JX19 ET:21 (rear) - 285/45R19 (oe tire size)
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      11-16-2021, 10:40 AM   #5
///d
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Quote:
Originally Posted by Qsilver7 View Post
Great post.

Just an FYI...there's also a staggered 19" wheel (style 467) that has the specs below...you can use all 4 (front) wheels for a square set-up...or use the wider (rear) wheel for a staggered set-up:
  • 9JX19 ET:37 (front) - 255/50R19 (oe tire size)
  • 10JX19 ET:21 (rear) - 285/45R19 (oe tire size)
Sweet thanks. I'll add this in tomorrow when I update with winter info.
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