How big a deal is it if the overall tire diameter is an inch different from front to rear. Taller in the rear by an inch or less. I am wanting to run p245/35r18 in front and p275/35r18 in the rear. Any issues like ABS codes or crazy wheel speed sensor issues I need to know about prior to changing sizes?

 

Your actual speed will be 12.6% higher than idicated by your speedometer. For example, at an indicated 70mph, you will actually be doing 79. This can be problematical...

Your ABS may also be compromised.

The overall tire diameter should always match the OEM diameter within 2-3%, unless you want to invest in a yellowbox.

 

Your speedometer wont be off that much.
According to Tire Rack, for the tire I looked at a 275/35R18 makes 823 revolutions per mile, and a the stock 245/40R17 makes 840 revolutions per mile which I believe is a 2% difference.

If you have an '06 or later it might impact your VSA. I doubt it would impact ABS.

I'm sure you will have to make some modifications to get those to fit.

 

I would say that an inch will impact the ABS. I am not sure why you would want to do it anyway. You can end up with rubbing in the wheel well, speedometer and odometer error and gear ratio will be off.

 

I know a lot of autocross guys are running 245s up front and 275s out back with no issues. But those are also on a 17" wheel. The numbers may be slightly different.

 

245/40R18s are the same diameter as the 275/35R18s.

 

I'm running 245/40 and 275/40 R17 hoosier a6s. my GPS confirms my speedometer is low, and my ABS doesn't work when they're on the car (although that may be due to something other than diameters).