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Old 06-30-2020, 11:57 AM   #8501
Scooby Newbie
Member#: 418242
Join Date: Apr 2015
Location: PA, USA
2017 WRX


Originally Posted by Norm Peterson View Post
You have to look at more than just 'XL' and 'SL'. Basically, what you need to be doing is matching the rated load of your OE tires at their recommended inflation pressures. For the 2015-up WRX, I think the OE tires are 245/40-18, 97 in XL, and the inflation pressures are 33/32 psi.

After that it gets a little more complicated. The 255/40-18 is a 95 in SL, which actually corresponds to a 99 in XL (larger than OE). The 255/35 is a 94 in XL and is a slightly smaller tire than OE in terms of load capacity. In either case, you'd want to set inflation pressures slightly different from Subaru's 33/32. You could actually drop about 2 psi off of the door sticker number for the 255/40's, but you'd have to add about 4 psi for the 255/35's. Three load index numbers worth of correction is as far as I'm normally willing to consider.

Reinforcement is more for the XL tires being rated to 42 psi vs the SL tires to only 36 psi. I suspect any handling benefit would be mostly incidental.

Thanks Norm. Sounds like either option works fine. The 255/40 would fill the wheel well a little more and should be more forgiving of potholes and such, especially at 30/31 psi. compared to the 255/35 at 38/39 psi. . But maybe it's all semantics in day to day driving.

*edited. Tire Rack suddenly dropped the price of the 255/40R18 to $135/ea shipped so had to jump on that.
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Last edited by su_ba_roooo; 07-05-2020 at 11:13 PM.
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