kostonpower

Just put new wheels and tires on. 245-45-20 and 255-45-20. Drivin down the freeway Goin 65 I think but was passing a lot of people. Are these big enough to mess up the calibration on my speedometer

onederful100

you need to compare the revs/mile of the stock vs new tires.
if they are different then your speedo will be off and you'll need to adjust the speedo with a tuner.

kostonpower

I don't have stocks. Bought the car with 235-50-18. Its only a v6

alkemist

Google what stock tires came with the car.. compare it with what you have now and see if it needs to be corrected.

Kotobuki

Your 255/45-20s are about 1 3/4" taller than than the stock tires. At those speeds you're doing about 4mph over what's indicated on your speedo that's calibrated for the stock sizes of Mustang tires.

grabber blue gt

ahhh **** I just put mine on and realized I never checked the speedo. I went from 19" wheels to 18" wheels (cant remember the exact tire sizes but they are 255 & 285) so I am sure my speedometer is off a bit. I can fix this with my sct right?

Kotobuki

You can, but you may not need to. All the mustangs are calibrated for a 27"ish tall tire. If your tires are the same overall height, you should be okay. For instance, if you're running a 285/40-18 you're close enough to stock size that you shouldn't need to do anything.

Kixsters

I did the calculator for you and you should be at around 703 revs/mile... which you can edit them if you have a sct tunner

Kixsters

Yup you can edit it ... heres a website that should help you calculate with your new sizes...

http://www.performanceprobe.com/inde...calc&page=tire

Nuke

Even with the calculators and the manufacturers specs, you still encounter errors. I prefer to use my GPS and then adjust using the SCT, regardless of what the calculators say.