[deathray]

I just got quoted about $5500 for a set by my dealer (I am in Canada); it splits out to about half for wheels and half for tires (mounted and balanced).

Ive had some trouble trying to track down the spec'd Pirelli Scorpions 285/40R20....most online retailers either dont list them or have them listed as unavailable.

I have found the Scorpion Winter 275/40R20 at a few places and it is generally a fair bit cheaper too, however, the load rating on those is 106 vs the spec'd 108 (and the 108 on the stock summers). I know the short-hand response is generally don't go to a tire with a lower load rating than OE, but, I did the math and it seems to be permissible.

Any opinions on whether this is a bad idea or not? Ill list all of my numbers below for reference.

Size comparison

Stock Size -> Proposed Size = Difference%
285/35R21 -> 275/40R20 = 0.7%
325/30R21 -> 275/40R20 = 0.1%

The sizing seems to be well matched.

Load Rating

The load rating for the front tires wont matter since the scorpions have a higher load rating than the OE tires. The comparison for the rears is as follows.

Max load on rear axle = 1631kg
Max gross weight = 2971kg or 743kg per wheel
OE recommended rear inflation pressure = 36 psi
OE Tires = Michelin PSS with XL designation and 108 load rating

Load rating 108@36psi = 885kg * 0.91 = 805kg per rear tire (OE spec)

It seems to me that the OE recommended inflation is based on the gross weight and speed rather than strictly on max load per axle. I should also note that for speeds >160kph the recommended inflation pressure is 44 psi. Considering this is a comparison for winter tires, I believe using the lower numbers should be acceptable as I dont think it is likely that I will be faster than 120-130 in winter.

Load rating 106@36psi = 841kg * 0.91 = 765kg
Load rating 106@39psi = 902kg * 0.91 = 821kg

Thus, I would think that so long as I keep the rear tires inflated to 39 psi, I should have the same load bearing capacity as the OE spec despite having a tire that has a derated load index.

That seems to make sense to me....am I missing something?

Notes:
1. I have read in a few places that when passenger vehicle tires are used on SUV's (i.e. any tire that does not explicitly start with LT before the sizing), the load rating should be reduced by 10% to compensate for the higher center of gravity and the increased likelyhood of carrying cargo. That is why all of the load figures are multiplied by 0.91.

2. In the owners manual under the recommended tire pressures, the sized winter set (285/40R20 108) has a recommended rear pressure of 31 psi. I assume they have done this to allow the winter tire to maintain more contact with the road for better traction, however, this would seem to throw the very relevance of the load rating into question (unless, I suppose, it is highly dependant on expected speed and ambient temperature). Anyone have any opinions on this?

And now to find wheels.......