Modifing computer for tire size problem.

I jacked up my left rear tire and counted the turns untill my odometer went 0.1 mi. It made 123 turns. The distance on the ground for one round measures 100.25 inches. That computes to 1,027.56 feet. My confusion is, 0.1 mi. is 528 feet. Am I overlooking something or what?
I put larger tires on my 2005 Silverado and bought a Hypertech programer in order to update my computer for the larger tires and am just trying to verify the accuracy.

Any help appreciated,
John B. in S.C.

I have a tire size calculator program on my computor..tell me your old size and new size and I can tell you what is going on.

I have a tire size calculator program on my computor..tell me your old size and new size and I can tell you what is going on.

The truck came with 245-75R16's and I put on 285-75R16's.

you are getting a doubling effect from the spider gears in the rear carrier because only one tire is turning so driveshaft is turning twice as fast.

So, could I just divide the revolutions by 2 and expect that to be accurate?

as accurate as can be expected

The truck came with 245-75R16's and I put on 285-75R16's.

Your speedo is reading is 7.8% too slow
When your speedo reads 60 Mph you are really going 64.7 Mph

Thanks for all the good info. Now for a new situation. I tried to get extended warranty for my 2005 Silverado and was told that it would not be insurable because of the larger tires that I have installed even though I adjusted the computer chip with a Hypertech Programer to recognize the larger tires.
Any suggestions taken to heart,
John

You can use this to check size differences: Tire Size Calculator (http://www.miata.net/garage/tirecalc.html)
As far as warranty goes I wouldn't mention the hypertech to them...flash it back to stock when you take it in and then see what they say....you can always reprogram it after......also the difference between 245 and 285's isn't that much to notice visually.....at a nearby dealer a lot of last years Z71's were being equipped with 285/75/16(at the dealership)....

Yea, re-flashing is an option. When I was corresponding to an extended warranty rep., he was aware that I had reprogramed the computer to reflect the larger tires but that didn't seem to matter. He just had a problem with the larger tires, period. I wonder what the big deal is with larger tires? How can that hurt anything?
John