timhutch
New member
Hi all, I know there are a lot of posts about how to use Forscan to correct speedometer error after installing oversize tires. But I've just gone through the process with my 2023 BRaptor and thought I'd share some info that I couldn't find in other places. First off, I went to 39" BFG KM3 Mud Terrain tires, adding about 2" in tire height. Actual, measured tire height is about 38.5" verses the 36.5" for the OEM KO2's. This change caused about a 4mph speedo error at 70mph (indicating 70 going 74). Not super happy about this as I felt I would be more likely to get a speeding ticket, it messed with mpg and range remaining data showing less range and mpg than actual, odometer was no longer accurate (fewer miles indicated than actual) and finally, it seemed to be effecting the transmission shift points. For all these reasons I decided to attempt to correct the speedometer error with Forscan.
I used Forscan release 2.3.63 to make the changes and it was very easy. There are a lot of You tube videos showing how to do this. What is more difficult to find is the correct value for the new tire sizes. There is a formula that appears to work properly that goes like this. Get your tire circumference (mine was 122.5") convert to millimeters (3111.5mm) and apply a multiplier value to reduce/scale the size value. Internet said .967 my calculations put it at more like .933 based upon the stock/default tire circumference value programmed into the truck and the OEM 37" tires. So applying the multiplier we get a new value to be written of 2903 (3111.5 x .933 = 2903) verses the stock value of 2753 (2951 x .933 = 2753). But you will find that when you go to write that value (2903) into the BCM that the relearn required on the PCM module fails with a "tire size value out of bounds" error. Through trial and error I discovered that the max value allowed in the BCM module for tire circumference is 2885. It appears that this value would be correct for 38" tires but it's really very close for the 39" KM3 tires (which are actually more like 38.5").
My speedometer error at 65 is now about .8 miles per hour (indicating 65 going 65.8) and has returned my odometer and mpg/range values to be much more accurate. If I decide to increase tire sizes above what I have now it will probably require something like the Hypertech Speedometer Calibrator that remains installed on the vehicle. Hope you find this useful!
I used Forscan release 2.3.63 to make the changes and it was very easy. There are a lot of You tube videos showing how to do this. What is more difficult to find is the correct value for the new tire sizes. There is a formula that appears to work properly that goes like this. Get your tire circumference (mine was 122.5") convert to millimeters (3111.5mm) and apply a multiplier value to reduce/scale the size value. Internet said .967 my calculations put it at more like .933 based upon the stock/default tire circumference value programmed into the truck and the OEM 37" tires. So applying the multiplier we get a new value to be written of 2903 (3111.5 x .933 = 2903) verses the stock value of 2753 (2951 x .933 = 2753). But you will find that when you go to write that value (2903) into the BCM that the relearn required on the PCM module fails with a "tire size value out of bounds" error. Through trial and error I discovered that the max value allowed in the BCM module for tire circumference is 2885. It appears that this value would be correct for 38" tires but it's really very close for the 39" KM3 tires (which are actually more like 38.5").
My speedometer error at 65 is now about .8 miles per hour (indicating 65 going 65.8) and has returned my odometer and mpg/range values to be much more accurate. If I decide to increase tire sizes above what I have now it will probably require something like the Hypertech Speedometer Calibrator that remains installed on the vehicle. Hope you find this useful!
