VSS tire revs per TCM different than ECM

[ntilley52]

Hello, i had my tune and harness done by a professional when i did my swap. I have now purchased HP tuners, and am trying to get some smaller details lined out. The out of the box tune had my VSS reading 10MPH faster than my gps app. When i looked at the speed settings. the settings ratio and tire sizes were off. I used the diff wizzard, and tried to plug in my numbers. The first issue using this method is that it will only accept a tire size up to 40, i currently have 42s and plan on going 45s on my next tire purchase. I also have 5.38 gears and it accepted that number. So what to do about the size cap?

Also after using the wizard with the boxes checked for adjust speedo calibration and scale trans parameter. I looked into the VSS tire revs per, the distances are different for the ecm and tcm. Distance=313.13 Distance Trans= 504.23 Why are the different? Should they be?

thanks

[CP_CUSTOMS]

what is your complete tire size 42/xx.xx/xx

[ntilley52]

42*15.5*17

[CP_CUSTOMS]

You will have to manually calculate the tire turns per mile.

1. Do a roll out in inches.
2. 63360/roll out gives you tire turns per mile
3. Apply that value to the appropriate slots in the speedo file.
4. Take old tire turns/mile divided by new tire turns/mile & multiply your shift points & TCC lock values by that.

Remember as the tire diameter goes up the turns/mile go down and your shift speeds should go up with a larger tire.



Your calculation is based on a rear diff gear of 1:1. You have to multiply your sum by the ratio of the rear gear (in my example...4.88) to get the trans revs/mile...right? There isn't an area in the speedo breakout for revs/mile.

Example...315/75R16 tire = 34.6 or 108 1/2 inch roll out or 34.6 * 3.14 (pi) = 108.644.

63360/108.644 = 583.19 * 4.88 = 2845.963 trans revs/mile

Doesn't this really all depend on the number of teeth on the VSS sensor?