I'm doing my lift and tires tomorrow morning.My tires are brand new toyo open country at 285/75/18.My speedo and edge have never been the same even with stock tires and entering in the tire size both ways.I'm not much on math so is there anyone you can tell me what I should enter in for the 285/75/18's? I do have a (borrowed)garmin to verify with.Also,how do I get the in dash speedo to match th edge? Not a huge deal but quite annoying!!! Dealership reprogram only? This is very important because I drive for a living and can not afford another ticket cause of this issue.Thanks for any help you can offer!!!!

 

YOu will never get the truck speedo to match your tuner or the gps. It is built by Ford to be 2-3 mph fast. Just get your tuner accurate.

 

Yea,just read that in another thread.How do I go about getting the tuner right? I guess I can try the driveway/chalk method but do I measure in inches,cm,mm? Hoe do I convert the 285/75/18 into what the edge needs? Thank you,by the way.That was fast!

 

Trial and error, start with about 2700, match GPS to Edge, go from there.

 

10 4 !atleast I know where to start now! Thanks!

 

Just so you know, I used this math:

Circumference = diameter x pi (3.14)

35 (inches) x 3.14 = 109.9

There are 25.4 millimeters in one inch.

109.9 x 25.4 = 2791

Need to subtract about 3% for squish.

2791 x 0.97 = 2707

 

thanks alot.going out to give it shot now!!

 

BUT - the CS is probably better programmed using the "inch" diameter of the tires rather than the circumference in mm. So, your tires have a diameter of 34.83" without factoring in the "squish" (flattening effect due to weight). Accounting for squish, try a diameter of 33.7" or maybe 33.8".

This is something you enter in the "Create a custom program" option when you program your PCM to a new Level. It is NOT a value you enter in the "Options" sub-menu of the Main Menu. In the "Options" menu you would enter the tire size as 27575R18, but it will not account for squish and it will NOT give you accurate readings.

Adjust up or down from the 33.7" value until the CS speed reading matches the GPS (stabilized, preferably at highway speed). Increase the diameter to increase the CS speed reading, decrease to reduce the CS speed. When you get them to match, the dash speedo will read about 2 mph faster at 70 mph, and the odometer will be correct.

- Jack