2610 is the unloaded (i.e. perfectly round) circumference.

Tires are not perfectly round. When loaded, they squish (anywhere from 2-5%). This reduces the effective diameter.

Thus you need to reduce the unloaded diameter by multiplying the unloaded diameter by .98, .97, or .96, etc. as a starting point. Then you need to confirm/verify with a known speed (i.e., GPS) or distance (Google maps) and adjust based on the percentage of your error.

 

i did the .97 squish and my gauges show about 2 mph higher than my programmer. should i go more squish or less squish. lol

 

The truck speedometer will always be about 2 mph faster than the digital readout. Ford builds them that way. Get you digital readout calibrated to a GPS to make sure you have it correct.

 

ok. but if i leave it the way it is now should i go by my gauges or the programmer. its an edge evolution.

 

Yes, but how do you know it's dead on? The digital could still be off.

 

lol i'll just have the cop in my town use his radar to check it. i can get by with warnings by saying the tires have my gauges off, the cop just says have it flashed and heres your free pass be glad im in a good mood. lol works about 95% of the time. and we took my gf's g6 on a trip and took the garmin and the garmin always read 3-4 mph lower than her gauges and its bone stock with the same tires it came with from the factory. so maybe i shouldnt rely on my gps

 

I would bet the gps is correct and the car speedo is off. I would set the Edge to match the Garmin, then ask the cop to check it for you.

 

hi shotgunz,
thanks for providing the tire size calculator. i will really help many users at this forum.
you did really a good job.
Thanks


9xm live tv

 

Here is imo the best tire size calculator I have found. Let's u put in stock size and compare up to 3 prospective sizes. Check it out.
www.1010tires.com

 

http://www.1010tires.com/TireSizeCalculator.asp
More direct link.

 

Can someone tell me what nimber they are using for 295/70/18 Trail Grapps. I had my truck broken into and my GPS unit stolen. My cell phone has a GPS unit, but it doesnt have and odometer.

Thanks

 

Should be between 2597 (5% squish) and 2652 (3% squish).

You can also use Google Maps for accurate distances...

 

I am running 2651 at 45psi.

 

ok i have a question. i have stock 20" rims on my f150. current tires i have right now are 275/55/20. i want to go bigger. would i be able to put a 35x12.5x20 tire on these rims?

 

Your question would best be asked in a new thread in the Wheel and Tire forum.. It is a little off subject in this thread, plus may not be seen by many.