On our mustangs, is the speed calculated at the front or rear wheels ?

I'm asking because why installed my new tires they have a difference in Rev/miles from front to back because I have larger wheels in the back.

The difference is minimal. The original OEM tire/wheels are 766 Rev/mile and if I compare my new wheels/tire in the fronts are (770 Rev/mile) which means 0.52% too fast and the rear are 752 Rev/mile) which is 1.78% too slow.

I'm wondering which value should I enter in my SCT to make sure my speed rating is accurate on the Speedo.

I know it's only 1.78% in the worst case and that means that I will actually be a bit faster than the speedo if the rear is being use.

My guess is that I should use the rear tires/wheel valus as I think the car uses the transmission rotation to determine speed but with all the fancy electronics and ABS sensors I was wondering where the calculation come from ?

Thanks,

 

The rear wheels ...

 

 

Thanks

 

Even after I used what was supposed to be the revs mine was off compared to my GPS. So I kept adjusting to match.

 

Use the tire manufacturers spec. There was another thread on this. We concluded the car's computer does actually match the GPS speed, it's the instrument cluster that's off. I confirmed it by using the datalog on my SCT tuner with my laptop.