It's time for me to get new tires, both front and rear. Now I was wondering if I could put wider tires on my wheels. 245/40/18 in the front and 275/35/18 in the rear. I believe the front wheels are 18x8 or 18x8.5. The rear is 18x9. I am pretty sure I can fit these tire sizes on these wheels. What do you guys think? Anyone have this setup? Thanks.
-Andrew

I think I'd go with 245/40-18 and either 265/35-18 or 255/35-18. Your goal should be reducing the understeer the car creates. Either of the above setups would improve handling of the car. If you just want to throw the widest tires on the car for looks, go with the setup you listed - it is the widest you can safely fit on the rims, but the car will understeer as bad (if not worse) than it does now.

The sizes you mention will fit the OE wheels and on the car fine. They are also the correct height for the car. Jblack is correc though in that you will not reduce understeer with the 245/40R18 and 275/35R18 sizes.
If I can help let me know.

Well with the setup I listed, would understeer definately be worse, or would it not really affect it at all? I wanna weigh my options because I do want to have the widest tires I can put on there, but I don't want the understeer to drastically increase. Thanks guys.
-Andrew

It all depends on your style and skill, as the M3 has a tendency to Understeer all on its own. It is extremely predictable, but unnerving to someone who does not like the car to slide a bit (-'
So if you want a more neutral driving experience 245-255 or 265 will give you that. If it is all about looks, or the joy of getting a bit sideways, go 275 or some even go 285.