Speedometer off after diff (pumpkin) swap

[Cobra Jet]

Not sure if you had come across this thread in your searches, but this user was having an issue with speedo calibration too with a different gear set. He ended up posting some good tech on how he solved the problem,; even though it’s a different gear set than yours, the tech may be helpful to you (and others):

https://www.mustang6g.com/forums/threads/2019-up-4-09-rear-gear-swap.175422/page-6#post-3722406

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[MrMike]

Not sure if you had come across this thread in your searches, but this user was having an issue with speedo calibration too with a different gear set. He ended up posting some good tech on how he solved the problem,; even though it’s a different gear set than yours, the tech may be helpful to you (and others):

https://www.mustang6g.com/forums/threads/2019-up-4-09-rear-gear-swap.175422/page-6#post-3722406

Thanks, this thread (again) confirms I need to modify the PCM.

 

[tdstuart]

At the risk of being boring, this makes me lean even further to the Procal lol. I've read almost all of your custom tuning thread, and that is not a rabbit hole I want to go down.

If you are just trying to fix the speedo and not tune the car might not be too hard.

Here are the options I see for 18+ car adjustment

 

[ice445]

Agree for anyone else who may find this thread, but mine is a 16.

Oh sorry, my bad lol. For 15-17 the procal is just a laptop program, but they'll probably make you buy the entire power pack to get access.

 

[MrMike]

Oh sorry, my bad lol. For 15-17 the procal is just a laptop program, but they'll probably make you buy the entire power pack to get access.

I wanted the Power Pack 3 eventually but I am kinda a mix of disappointed and frustrated to be forced into buying it sooner. I've decided to just set the parts from it aside for now and just use the tool to set my rear axle ratio to 3.73 and tire rpm to 776.

 

[Garfy]

I had the same gear ratio so I didnt have to change anything. There is a special function for speedometer calibration that hptuners has, same type of function like crank relearn. PCM tec doesn’t have functions like that as far as I know but you can change speedometer settings in the tune file.

Ive tried playing with the speedometer calibration but I would need to watch the digital readout on a logger in real time to tell if it’s accurate or not.

The car has some learning features for the speedometer and tire sizes. I’m not sure how it identifies that it needs to make a change and how it determines what the correct values should be.

Later tonight I can pull up the options you would have with pcm tec. You could probably disable the learning but I don’t know if this is the technically correct route or if there may be another value you have to change.

I'm wondering if the car will automatically change the speedometer calibration by using the onboard GPS? Since FordPass always "knows" where the car is it must be using GPS for that so perhaps when you're driving, it'll compare the speed detected by the GPS and calibrate accordingly?

 

[MrMike]

I'm wondering if the car will automatically change the speedometer calibration by using the onboard GPS? Since FordPass always "knows" where the car is it must be using GPS for that so perhaps when you're driving, it'll compare the speed detected by the GPS and calibrate accordingly?

There is no onboard GPS or FordPass.

 

[MrMike]

I had no issues using the FP Procal to change the tire size and rear axle ratio. Car runs great now. Speedo is dead on and does not pull power.

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