I want to help other JK owners to avoid the hassles that I have gone through when using the Hypertech programmers to correct their speedometers. I had to program mine 5 times before it registered the speed correctly :sad:

What I am thinking is a chart of sorts which lists the tires you have, wheel size and what you had to enter into the programmer to get the speed to register correctly.

Here is what I have:

Toyo Open Country M/T
P265/70R17
7.5 wide stock wheels
Calculated diameter = 32.1875
Programmed diameter = 30.75

Stock Rubicon BFG M/T
P255/7517
7.5 wide stock wheels
Calculated diameter = 32.00
Programmed diameter = 31.25

Send me a PM and I will update this post with your information

 

All you really need to do is measure your tires from the ground up to the shoulder edge on top - NOT the very top at the center of your tread. After doing this a few times, I have found that the tire shoulder edge will give you an accurate measurement that will work well with your programmer.

 

That would be way too easy

Why don't the instructions from Hypertech tell you to do that? By the way, I just checked mine and it is 30-7/8 so this would have saved me a LOT of time.

Thanks, Eddie

 

I just did the math as described in the Hypertech instructions. I came up with 33.5" for my 35" MTRs on 15"x8" wheels. My speedometer is on the money.

 

I seems that measuring from the ground to the center of the hub would give an accurate radius. Then multiply by two for the correct diameter.

Wouldn't measuring from the ground to the top of the tire should not take into consideration loss due to tire bulge? The measurement from the hub to the top shoulder would remain basically constant while the ground to hub would change with vehicle weight, and air pressure. I know I lose quite a bit of radius (ground clearance to the axle) the more I air down on the trail.

Is it possible Hypertech has built in a correction for this?

 

I don't want my speedo to be accurate, cuz if it is then my mileage would be much much higher! I GPS if I am trying to refresh my memory on my speedometer! at 60mph I am really running about 67.5ish!

 

Mesauring from the ground to the center of the hub would not be the most accurate method. There is always the tire bulge, as well as some tire slip that screws you up.

WOL's "shoulder edge" is probably the best practical method, because the effective tire diameter will be a little less than the actual, unloaded tire diameter (again, due to tire slip/rolling resistance). If it works well, that is great and quite easy.

According to BFG, the LT255/75R17 Mud Terrain tire (JK Rubicon) gives 649 revs/mile at 45 MPH, which converts to an effective diameter of 31.1 inches. The actual tire diameter is listed as 32.05 inches. See what I mean?

If you want to get carried away, the best way for speedo corrections would be to mark a tire, and have someone count revolutions while you slowly drive the vehicle for a known distance, say 500 feet. You have to drive it, not push the Jeep in neutral. Then convert the revs/mile to a diameter.

Hope that isn't too much information overload