Gear change speedometer

[Texasgrillchef]

Since there is a FORScan option for changing tire size. Say from 35 to 7’s or even 40’s, so that the speedometer is correct., what about gear changes? Won’t that effect the speedometer accuracy as well?

When I upgrade to 37’s, I want To change the gears to 5.13, or even the 5.35’s. Won’t I need to make some changes besides the tires to keep the speedometer accurate?

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[Brian_B]

I tested this out once - gear ratio doesn't appear to affect the speedometer calibration, but tire size does. I suspect, but don't actually know, the speedometer is being driven off the axle speed sensor. Axle speed for a given road speed wouldn't change with just a gearing change, since the gearing change occurs before the speed sensor (but it would change with different sized tires, which is after the sensor).

That said, there is a setting for gear ratio (in both Forscan and ProCal), and it does appear to have some effect on traction control / ABS. Just not the speedometer - that is driven off tire size (entered in either revs/mile or circumference in mm, depending on what you are using to enter the number).

Old style speedometers came off the transfer case or transmission output - which was before the gearing, so you could "cancel out" the effects of gearing change and tire change the way you are describing. But the 6G Bronco doesn't appear to go that way.