Whats a good size tire to put on these rims for my 98 740il

235/50-18 front 255/45-18 rear. That's what OEM is and when I had M-Pars those sizes worked great for me.

I have 245/40/18 in the front, and 295/35/18 in the rear and that works great, a lot of traction.

I have 245/40/18's in the front and my back tires just went out so I was thinking if a 275/40/18 would work in the back or if the profile is too tall.

+1 for 235/50-18 front 255/45-18 rear.

What ever you do...make sure that there isn't a 3% difference between the front & rear's rolling diameter. If there is, you may expereince problems with your ASC+T/DSC (traction control) and ABS braking.

These systems work off of measuring the rolling distance between wheels on the same and/or front to rear axles (depending on which TC option your car has).

A tire calculator site like: http://www.1010tires.com/TireSizeCalculator.asp can help you determine and stay within proper specs.

Good luck. :)