Re-Calibrating Speedometer/ Different Tire Size

[svtfever]

So I have a non-HP car and want to add HP wheels and similar tire size but the sizes are significantly off from the 255-40-19 and 275-40-19s if I go HP sizing. How can I go about fixing this. If it were off 1-2 mph I would not worry about it but if its off 4-6 mph can this be adjusted? Thanks!

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[m3incorp]

Yes, you can adjust yourself using ForScan or a dealer can adjust it. Unfortunately, the dealer tried to adjust mine from reading 2 mph too slow and after the adjustment, it reads 5 mph too low. Good thing I wasn't charged for it. I have a ForScan device but just need to take the time to do it.

 

[svtfever]

excellent. thanks so much. I wish someone made a 315/35/19

 

[crzystang]

excellent. thanks so much. I wish someone made a 315/35/19

 

[Seanuf99]

I used the Pro Cal tool to change my tire RM when I switched from the stock 235/50/18 to 275/40/18. I used the RM spec provided by Continental. At varying highway speeds the speedometer is consistently 2mph slower than what Google Maps is showing on my phone. I am going to try it later with my Wahoo bike computer to see if it lines up. Anyone have any tips for validating the speedometer calibration is correct? I mean without setting cruise control for 20mph over and then asking the State Trooper what he clocked you at? I mean that would work, but would be an expensive option.

 

[m3incorp]

I use my radar detector with gps and my phone to validate. They are very close to the speedometer in my other vehicles and show the Mustang speedometer to be reading 5 mph slower than actual speed.

 

[Seanuf99]

I use my radar detector with gps and my phone to validate. They are very close to the speedometer in my other vehicles and show the Mustang speedometer to be reading 5 mph slower than actual speed.

Good idea. I should have compared the phone/bike computer (no radar detector) prior to swapping the wheels, but did not give it a thought. I will compare them in my wife's car today, see what the differences are, then try in the Mustang at same speeds.

 

[Cobra Jet]

This should help with tire size conversions:
https://www.mustang6g.com/forums/th...re-diameter-in-forscan-and-workaround.157039/

Or

https://www.mustang6g.com/forums/threads/tsb-19-7008-dtc-p1635-tire-or-axle-ratio-changes.117406/

 

[Epiphany]

I used the Pro Cal tool to change my tire RM when I switched from the stock 235/50/18 to 275/40/18. I used the RM spec provided by Continental. At varying highway speeds the speedometer is consistently 2mph slower than what Google Maps is showing on my phone. I am going to try it later with my Wahoo bike computer to see if it lines up. Anyone have any tips for validating the speedometer calibration is correct? I mean without setting cruise control for 20mph over and then asking the State Trooper what he clocked you at? I mean that would work, but would be an expensive option.

Can you use the Procal for anything else?

 

[Seanuf99]

Can you use the Procal for anything else?

Hey,

So I got the procal tool as part of the GT Calibration kit (fantastic kit, highly recommend). So I used it to back up factory tune and load the ford performance tune. Additionally, it can be used change the settings for gear ratios (up to 4.09) which I am looking to do as well.

Troubleshooting (DTC) , service and data logging. Scroll down to page four to see what it can do.

https://performanceparts.ford.com/download/instructionsheets/FORDINSTSHTM-12655-F.PDF

 

[Seanuf99]

I use my radar detector with gps and my phone to validate. They are very close to the speedometer in my other vehicles and show the Mustang speedometer to be reading 5 mph slower than actual speed.

Bike computer and phone GPS are exact match to each other as far as speed goes. Speedometer is reading .5-1.0 MPH slower depending on speed. I am aiming for perfection, so going to play with the Revs/Mi setting a little more, see if I can get it any closer (but I doubt it). Just thought I would let you know the progress.

 

[Epiphany]

I asked as I have an earlier ProCal and it performed similar functions. I had yet to see anyone report on the latest ProCal FP offers. Very nice.

 

[m3incorp]

Hey thanks for the update. It may be tough to get it read exactly the same as the gps.I commend you on aim for perfection.....can't say I would not do the same thing.

Bike computer and phone GPS are exact match to each other as far as speed goes. Speedometer is reading .5-1.0 MPH slower depending on speed. I am aiming for perfection, so going to play with the Revs/Mi setting a little more, see if I can get it any closer (but I doubt it). Just thought I would let you know the progress.

 

[RobZ71LM7]

Wonder if it's a more warranty friendly option when just changing tire sizes versus an aftermarket tuner.

 

[Charlemagne]

Perfection is useless, have it perfect with new tyres and it will show something else with somewhat worn tyres.

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