I have two sets of staggered 10-spoke OEM rims. I was thinking of using 245/40s on the 8.5s all around for summer tires and using the 7.5s with 225/45s snows for winter tires. This would allow me maximum flexibility for rotation and would give me hopefully better handling on summer tires. I also assume this would be better having the snows on 7.5s all around. Is there any problem running 245/40 up front on these rims with the stock suspension?
Using such big tires up front might cause you to oversteer. Especially in the summer when the tires get all hot and sticky. Stock on M3's are 225 up front and 245 in the rear.
Putting 245/40s up front does reduce understeer a little, but not nearly enough to make life interesting. The primary benefit to such a setup is the ability to rotate your tires to even out wear. I've autocrossed on both the stock staggered setup and with 245/40s all around extensively and have felt little difference in car balance.
If your suspension is stock, then the primary issue regarding fitment of 245/40s in front on stock wheels is the brand of tire. With BFG KDs, I needed 10mm spacers to clear the struts. Currently, I'm using S-O2 Pole Positions which barely fit (which is good enough.) I think they rub slightly under extreme cornering, but not enough to be of any concern. I've also heard that Pilot Sports fit like the S-O2s.
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