I am programming my Superchips Tuner, but I don't know what the diameter of my 275/60 20 tires are. The guy at Discount tire said that it was 33", but I am trying to verify. Thanks for the help!

Well on my ride to work this morning, I noticed that since my tuning yesterday, my speedo is now about 3mph too fast when compared to my GPS. Before tuning, my speedo was dead accurate with my GPS. When using the link below to find diameter, it looks as if I should have left the stock setting of 31.75" instead of changing it to 33" for my 20" tires. Interesting, anyway just thought that I would let ya'll know what I found. Oh, and I have Cooper Zeon LTZ tires.

 

it's 33". For future reference if you change tire sizes, use this http://www.miata.net/garage/tirecalc.html

 

Thanks for the info.

 

It is 33". You can always look it up at www.tirerack.com

 

Bump

 

Dude, why did you bump your thread? Did you not get the information you were looking for?

 

Guess not.

 

Dude, because it says updated. I added information to it for others benefit. Please read and not assume!

 

Over in the Charger section of this site they were recently discussing that the tuner needs the loaded diameter of the tire, not its normal unloaded size. The sag in the sidewall caused by the vehicle's weight actually changes the registered speed. I need to find someone with a GPS so I can fine tune my tires with the tuner.

 

Superchips calibrated the tire sizes by measuring from the ground up to the top as the truck is parked. If you get one of the measurements off of a tire size calculator it will be off a little. I don't agree with the way they did it, that's just the way it is.