Sorry, I think we're talking about two different things here. From a loaded radius, you calculate a "loaded circumferance" (actually the distance a loaded tire rolls in one revolution, at low speed). We agree there will be some error in this.

The calculator programs or the table in the Edge manual give you an "unloaded circumference" based on tire specs. There's no particular error in this other than manufacturing tolerances. I just determined that using a circumference of around 2480mm instead of the 2559mm given in the Edge table makes my speedometer and odometer accurate. The difference in the two numbers is approximately 3%, so that's where I'd start with any other tire. I worked backwards, and my technique accounts for centrifugal force at highway speeds.

- Jack

 

That's cool too Jeremy. You came up with a number that works through testing - just like I did. Bottom line, whatever works is what you want to use.

Why you DON'T apply "squish" and I do, is just one of those mysteries of life, I imagine.

- Jack

 

I calculated per the Edge instructions and was off. I did a 3% adjustment and was still off compared to GPS. I made an additional adjustment and it was right on. I did the math and it required a 5% adjustment. Now my Edge is identical to the GPS, but of course the truck speedo is off the customary 2 mph.

 

It DOES look like tire size, manufacturer, construction, inflation obviously all have a bearing on this doesn't it Jim? And, what's right for one tire will not exactly match another.

So, the key is to experiment.

- Jack

 

I agree.