[QWKSNKE]

Originally Posted by redhorse

Yes to the first question. We have 275-35-18 all the way around. The car came with 235 - 45 - 17 and we switched and changed tires and wheels when the car was new. The speedo has never shown the correct mph.

Seems as though the car came with the wrong tire size initially. It should have came with a 235/55(according to all mustang stuff I have)


Let's do some figuring.

Based off the 18" wheels and the 235/50/18 tires that came on my car, the overall diameter to keep the speedo correct regardless whether you are running a 17", 18", or 20" wheel is a 27.25". 235/55/17's are the same height as well

Your current combination is 25.57" tall which is a good bit shorter overall height than it should be. Now unless there are limits in the FRPP tunes, this should have easily been corrected but I am not positive that the FRPP tunes can adjust the speedo for tire size (I know the Predators and SCT tuners can though)

Since you obviously wanted a wider tire (275's) you could have gotten a closer with a 275/40/18. Its still a little short (26.66") but is closer than a 275/45 (27.75")

I am currently looking at doing a 18x9/18x10 combo for my car and will probably run a 255/45 front and a 285/40 rear. This will keep the overall diameter right at 27" and hopefully the tuner I use will take care of the minute difference.

Does any of that jibberish make sense?