Speedometer Calibration Wrong After ForScan

[Brian_B]

For those that have changed tire sizes, whether via forscan or via the ProCal, noticed any MPG differences? I’ve lost like 2.5 mpg after going to a slightly larger tire.

Just adjusting the tire setting without a different tire? I wouldn’t expect some due to the mileage being inaccurate from an incorrect setting, but 2.5MPG is a big jump - interested to hear if others are seeing anything. The mileage being off would affect both the computer and hand calcs - unless you were getting your mileage from a GPS and not the odometer.

New tire itself, regardless of Forscan - definitely.

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[Brian_B]

Tin foil hat theory:

Ford adjusts speedometers a bit fast from the factory to cheat you out of warranty miles

 

[hilmar2k]

Tin foil hat theory:

Ford adjusts speedometers a bit fast from the factory to cheat you out of warranty miles

That was one of the reasons I adjusted mine. I realize 2.5% isn't a ton, but extra miles on the odometer are extra miles...

 

[2021FordBronco2]

For those that have changed tire sizes, whether via forscan or via the ProCal, noticed any MPG differences? I’ve lost like 2.5 mpg after going to a slightly larger tire.

With my speedometer being 2 mph faster mine gained 2.5 MPG

 

[2021FordBronco2]

Tin foil hat theory:

Ford adjusts speedometers a bit fast from the factory to cheat you out of warranty miles

That is exactly it, you found it out. Now you better watch out, Ford is probably going to put a hit on you.

 

[BigHoof]

The tire diameter they list for the Grabber A/TX does not match with the Revs per mile spec. Using General's Revs per mile, it should be 8.7272 ft which works out to 2660 mm. That's close to the factory SAS tire.

 

[CarlSpackler]

You didn't lose MPG, it's just using corrected data for calculation. However much you adjust your speedometer is how much of an MPG change you'll see.

Just adjusting the tire setting without a different tire? I wouldn’t expect some due to the mileage being inaccurate from an incorrect setting, but 2.5MPG is a big jump - interested to hear if others are seeing anything. The mileage being off would affect both the computer and hand calcs - unless you were getting your mileage from a GPS and not the odometer.

New tire itself, regardless of Forscan - definitely.

With my speedometer being 2 mph faster mine gained 2.5 MPG

No, 285/70/17 OEM, replaced with 285/75/17. 3.X percent increase in revs/mile, speedo showiong much less off than that (76.2 actual vs 75 displayed) Gas mileage went from 19.1 to to (just went and checked) 16.6. Something else is going on here. 11 lbs heavier per corner.

 

[2021FordBronco2]

The tire diameter they list for the Grabber A/TX does not match with the Revs per mile spec. Using General's Revs per mile, it should be 8.7272 ft which works out to 2660 mm. That's close to the factory SAS tire.

So, I did the SAS tire size, and it is over 1 MPH on all speeds over 30. If I do 2660 will that mean that the speedometer would be even more over the exact MPH?

 

[Cormeister]

My speedo read 1.75 mph fast from the factory (I have factory SAS package with OEM tires). I adjusted it using ProCal, went from factory setting of 606 revolutions (per mile is the measurement if memory serves) to 617. It still reads about .5 mph low at 70mph, which if fine by me, so 619 or 620 is probably spot on...

Thank you! Using the ProCal I changed it from 606 to 620, appears to be spot on now. Was driving me nuts every time I drive by a “your speed” sign and it showing slower than what the Bronco displayed.

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