This is an interesting statement. Why do Hypertech and AEV allow calibration for gear ratio changes?

 

So for those of us that do have the money to fix it, what is the best way to get the speedometer and odometer back to an accurate setting with 35's and 5:13's?

 

I honestly don't know. It must have to do with something other than speedo calibration. Think about it for a moment... when the speedo was controlled by the "speedo driven gear" at the output of the trans then yeah, the speedo was tied to the final gear ratio. But these rigs have speed sensors at each wheel so the computer "knows" how many revolutions each wheel is making. Until ya re-program the computer to "know" that the tire size is now "XXX" it's assuming that the tire size is whatever it was pre-programmed with. Final gear ratio is more related to engine RPM's vs. speed than the actual road speed.

 

That is strange. I guess I am used to old TJs and stuff where it had a speedo gear in the t-case. That was effected by tire size and final drive. I guess I assumed that it would be different here as well. I swear that my speed is now higher than what it says it is, and used to be much higher than it said it was. I guess it was all in my mind since final drive ratio has "NOTHING" to do with speedo. Human brain can trick ya. lol. weird.

 

This is the same thought I had, and is why I went back and edited my original post. However, this does not jive with what has been reported. The poster said his speedometer is reading faster than he is actually going. This would not be true if the speedometer uses data from the wheel sensors, since he just upgraded from 32's to 35's. The speedometer should be slow.

 

actually, I went from 33s to 35s while I had 4.10s. then I got 5.13s. I had the dealer set my tire size to 32 since it is the largest. It seemed to me that with the 4.10s and going from 33s to 35s the jeep felt like it was showing slower than it had before at the same speed. But going from 4.10 to 5.13, it seemed that my speedo said I was going faster than I really was. But I guess not, since the sensors are at the wheels not in the t-case or tranny. Must all be in my mind!

 

A bit of clarity! Larger diameter tires roll further in 1 rotation than smaller one's.

 

My wife sent me this link: tacomaworld.com/forum/tirecalc.php when I was trying to calculate how far off my speedometer was, I know it's a tacoma forum (she drives a tacoma ) but it's pretty nice, has gear ratios and everything.

 

i said that wrong. I meant I swear my speed is slower than the speedo says. I am about to pull out the gps and find out i think.

 

Use the GPS to go exactly 100 miles on the GPS. Zeroing out your trip meter on the Jeep will show your computer mileage. The difference will give you a good estimation of how much your off on your mileage. This is best done in one continuous trip.