How do you correct a 2015 Mustang speedometer error after changing the circumference in a wheel or tire size modification? I assume it requires a reflash of the computer to tell it the "new" circumference. Service manager is no help! Mustangs come with both 18" & 19" tires & they don't have the same circumference! I want to change from a 255/40/19 to a 255/50/19 to improve the ride! Speedo will show 60 when it is 64 MPH. Help!

 

The difference in the factory sizes is so minimal, it falls within accepted margins to not worry about.
Unless it has changed since the 05-14 models, yes it is a simple update to the computer, that the dealer should be able to do, they used to sell simplified tuners with that function, try to google it.

 

AK,

Going from a 255/40/19 to a 255/50/19 to improve the ride results in a speedometer reading of 60 when you are going 64 MPH & 70 when you are going 75. That could get you a ticket. I thought it would be a computer adjustment, but local Ford Service didn't know the answer. Yes, really! I imagine going from an 19" wheel to a 20" wheel must also cause a substantial speedo error.

 

First it seems like they should know, see if they would let you talk to an actual tech, Vs the desk jockey manager.

Second do the math on the tire sizes Ford offers, I'm pretty sure overall diameter variance isn't more than .25 inch.

 

Wheel size has nothing to do with it, tire height (diameter) does. You can have an 18" rim and a 20" rim with an overall tire height equal to each other and you would see no difference in your speedo.

If you do get a tire with a different height then yes, a simple adjustment via a handheld tuner is what is needed.

Here's SCT's instructions for example: http://www.sctflash.com/PDF/64Options.pdf

Your dealer is probably strictly by the book so they won't even mention an aftermarket device like the SCT tuner to access the ECM for the adjustment.

 

Thanx for the info. Derf00!

I think what you meant is the circumference of the tire that is want affects the speedometer reading. Of course mathematically the diameter will tell you that with a little math. In my case of changing from a 255/40R/19 to a 255/50R/19 the circumference causes the speedo to be off! At 60 mph you will be going 64 mph. I have concluded I will have to buy a tool to change the Mustang's computer. I'd like to get one that "only" does that since I'm going to put a Roush Supercharger on the car, so any tunes i pay for would not be used. It would be nice to get a tool that would change the speedo & save the Roush Supercharger tune on the tool for safety in case I have a problem with the Mustang's computer in the future!

Anyone have any suggestions as to a tool like that to change speedometers in tire/wheel changes?

 

speedometer recalibration tool.
As a lawyer would say asked, and answered, please try to keep up. LOL

 

Maybe if you were nice to your roush installer, he could put a copy of the tune on an Xcal4 or other tuner for you, on my old Xcal3 speedo and other small changes were actually separate from the main tune, as I messed with my fuel settings till I liked it, and it never took near as long as when loading my different octane tunes.
To be honest I'm not 100% sure it was separate, but as I said tune install time was different.

 

I need to fine a tool that will allow me to reset the speedo without changing the tune. I don't want to spend the price of a tuner just to reset the speedo. If I didn't get a supercharger that would be OK, but no supercharger, only 435 HP! Unthinkable!

Anyone know of a good OBD Tool at a reasonable price just for resetting speedos & reading trouble codes, etc.?

 

Sorry but you won’t find (or “fine” sic) such a tool. The speedometer is built into the tuning parameters. So the only way to change the speedometer is to go into the tune and adjust what the computer is reading.