Okay guys, I'm new to this site and I'm looking for a little help here. Recently put on 305 65 18 BF Goodrich All-Terrain T/A KO Tires on my 18 inch stock rims. When using my hypertech speedometer calibrator I enter 34 for new tire size and speedo is still off by 3-4 mph. How do I fix this or is the speedo always off? Do I change it back to stock? Change it to 33.5, 33.75 inch tire size? Need some info and anything you guys can help me with will be great, thanks!!!

 

It's not unusual for some F150s to have inaccurate speedometers. I know mine reads 3mph fast compared to what the vehicle speed sensor and GPS say. If it's consistently off and doesn't get worse with speed, this is most likely your problem. Your options would be to try to adjust the needle yourself or to find a shop that calibrates gauges. Or just live with it like I do.

 

Thanks for the reply and I've gathered that some F150's are a bit off. Do you think I should recalibrate my truck back to stock or leave it calibrated for the bigger tires?

 

If you have the hypertech did you do the rolling cercomferance measurement as described in the hypertech inst or just reley on the tire size which can be way different than the actual measurement? if you measure the distance the truck moves with exactly 1 revolution of the wheel in inches and devide that number by 3.14 it will give the actual rolling diameter of the tire then use that number to program the hypertech and you should come out real close to actual speed and distance