Help with Speed Caliubration 06 Duramax

[wreedz71]

I am trying to reprogram my tire size with HP tuners. When I punch in new tire size and gear ratio it changes the Trans - Final Drive ratio from 4.10 to 3.73. My truck has 3.73 from the factory but the factory program says 4.10. Now the Ratio - Final Drive says 3.73.


Should I leave as 3.73 or should I change it back to 4.10 like the factory?

Also should I auto scale transmission tables?

[gofish7]

When I changed my tire size, just let the software redo everything and set the gear for what I had factory which was 3.73s. So far, everything seems to be working properly...I haven't had a chance to check my speedo to make sure it was set perfect for my tires. 29.5" stock to 30.5" now.

[roy67ss]

wreedz71 - I too need to change the speedo calibration on my '05 duramax. I do not see any place to change the tire size or gear ratio. Where did you find it? I do not get the same options with the Allison trannys that I do with my gas trucks. I have the most recent file version. With my gas trucks, I just changed the tire size and all else was done for me.

[Dashman]

Under speedo, speedometer, change the value in VSS Tire Revs per (Dist-Trans). Also, under Tools, unit conversion, you can enter your tire size and get a tire height or diameter. With the Diameter you can manually calculate a perimeter, with the perimeter you again calculate revs per mile. Good luck! I just guessed at it. I'll try it later, so if it has any merit.

The unit conversion tire calculator is performing a theoretical diameter, the diameter of your spare tire. The tires rotating at a certain speed have weight on them. Hence, the actual diameter is less than the theoretical. You need the radius from the ground to the center, and use that to calculate the perimeter.

[roy67ss]

Dashman - I have found that the tire diameter actually should be about 1/2" more than the calculator shows. I check my speedo by comparing to an actual known distance ( the longer the better ) to the odometer reading. I look for at least 30 miles and make my adjustments accordingly. I am also usually driving at about 60 mph which actually makes the tire 'grow' in size. When I have the odometer right, I know my speedometer is also dead on. Many highways have mile markers and that is what I use for my calibration.
My '99 1/2ton has 245/75/16 tires that the calculator shows as 30.5" diameter but I had to up that to 31.02" to get the odometer right on at highway speeds.

I also think that the number to be changing is the VSS pulses per - mile. I will be doing that this weekend and see if it works properly.

Does yours also show 3.73 gear ratio and 4.10 trans ?

[Dashman]

By chatting with Roy67ss, I kept thinking about how my calibration didn't make sense. It worked, it was roughly 1 mph off, but not correct. According to Roy, what is more important is the affect on the odometer reading over a known distance, like 5 miles. If you can adjust the speedometer to a smaller error, your error on the distance will be less. In other words, it is easier to determine 1/10th of a mile error in a 10 mile distance, than to see a .5 mph error (as an example) on your speedometer.

So, I built a spreadsheet to calculate the two values that should be adjusted. I believe we should be using an estimated height value that is less than the "unloaded" height, but greater than the "standing" radius multiplied by 2. I would call this estimated value, "rolling height".

Check it out, and let me know if it hits, or if it missed.

[roy67ss]

Hi Mario,

The spreadsheet looks good and well described. I have one that I found somewhere that keeps things in inches rather than feet like you have on columns F & G. It just eliminates all those decimals but does the same thing.

I have 2 trucks running the allison tranny and 3.73 gears and LT265/75/16 tires. My reads on them show different pulses per mile (98352 and 101008) but both have the same 680.63 tire revs per mile. I don't get that! Unless, as I suspect, the only number that affects the speedo is the pulses per mile. The other number has no bearing on anything and is for information only. I am thinking the 680.63/101008 numbers are from the factory original 245/75/16 tires. The 8.1 truck is out by 1.5% by my odometer check this afternoon and the D/A truck is about 4% out (yet to be verified). The 8.1 truck has a Westers tune in it and the speedo was corrected at that time.

Some food for thought.