Well my stock 265/75/15's were 31.65" in diameter. My new 295/70/18's are 34.26". I reset my ABS/Speedo settings with my AE tool to 34.5" and my speedometer is 2mph fast as measured with a GPS app.

Unfortunately the AE - or maybe my Ford firmware only allows 1/2" increments.

Going to reprogram to 34" and see if the speedo is more accurate. I'm wondering if the 1/4" difference will affect my ABS performance and or timing etc. I have noticed that my RPMs are seemingly lower for a given speed, but that might just be because my speedometer was about 7mph slow previously.

Moving in the right direction in any case. I also noticed that my mysterious ABS light that went on an off has stopped it's erratic behavior.

 

I upgrades tire size to the 285s and now my speedo is right on, GPS verified. No other changes.

 

I need to do this and I have the AE tool with the enhanced setup. Can I ask where you went in the software to do this?

 

Diameter of the tire isn't the same measured off the truck as when measured with it's load on it. The rolling radius (the distance from the center of the axle to the ground) is less than the radius from the center of the axle to the top of the tire. If you used double the rolling radius you'd have a more accurate number.

 

I use a tire diameter calculation tool. Given that the option is to do it by 1/2" an inch it will be closer but not exactly calibrated in my case.

Here's the procedure using AE: https://www.ford-trucks.com/forums/8...iguration.html

 

Thanks so much! Makes sense that the tire size adjustment is in the ABS section.