[matai70]

I was thinking of buying new tires and rims for my car and I had a question about it. I went on a tire size calculator and when I entered the size of the tire it said, when changing tire sizes, we recommend staying 3% of the diameter of the original tire. Any more than this and you face thr risk of brake failure. Also the speed difference was 5.024% faster. Now the sizes I was thinking of were 255/40zr18 99y rd for the front and 285/40zr18 101y rd for the rear. Now when it says the 3% difference can there really be brake failure? and is there a way to fix the 5.024% faster speed difference or what does this mean?
Thanks in advance

[kj_cinci]

Your ABS system is very sensitive and has some very complicated programing, where it compairs speed of other axles etc.

The spedo can be fixed with your tuner.

Why not look at all brands and sizes? If you go 255/40/18 front and wanna go wider back I think you'll have to go 285/30/18 or 275/35/18. The profile number is lower but the way they read it because of the wider tire it will be the same hight... go figure...!?!

[wsd42]

A 285/40/18 tire has a 26.9" to 27" diameter and a 255/45/18 has a 27" diameter. On a Mustang GT the OEM tire is a 235/55/17 with a 27" diameter.

As KJ said you can use a DSP or SCT II tuner to adjust for your tire size, but it is best to try to stay within the OEM diameter range to reduce hassels in the ABS and traction control systems. Tire Rack (not trying to push them sales wise) is a good place to look at different tire sizes and see what the tire diameters are for a lot of different brands. I use them for my tire references data - diameter, rims sizes, tire tread width, etc...