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

 

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.

 

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

 

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

 

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.

 

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?

 

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.

 

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

 

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

 

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.