Recalibration of Speedometer

I've been carrying the Superlift TrueSpeed speedometer recalibration unit around in the truck for the last month and finally got around to installing it. It was a fairly simple install with the exception of the c2 connector on my 2002 California truck is clear instead of red. Any way we got it installed and after 3 or 4 stops to make fine adjustments the sppedometer / odometer are whithin .3 mph at 75 mph according to my GPS.

I calibrated it according to the published tire sizes from Fireston/Bridgestone. Stock Steeltex 245/16 at 30.5 diameter and Bridgeston Dueler 285/15 at 32.9. These were obviously off either by stock being smaller or new tire bigger because I had to dial in more adjustment.

I had always figured my fuel mileage at about an 8% difference. But last tank I used after the install showed it to be about a 10% difference. last tank was about 125 city 300 highway came in at 17.9 mpg.

For all you guys out there that want to recalibrate the speedometer this thing works! It also has a switch so you can run a second size of tire wheel and have your speedometer be on with the flic of a switch.

allamerican1

Where did you get the recalibration unit from. Its the first one I've heard of for the Allison...

I ordered it from 4wheelparts in Southern CA. I guess it's been out for a while and they just got around to testing it on the Duramax/Allison. It goes inline with the vehicle speed sensor wire from the main PCM. It has two trim pots on it for 2 different tires sizes. You just turn the trim pots the correct number of times to adjust to your tire/gear size.

allamerican1

AllAmerican1,

How much does this Speedo Recalibration unit cost?

With freight and CA sales tax (7.25%) It was right at $195.00. It seems to shift quicker now and shouldn't have any problems with ABS brakes either.

allamerican1

ALLAMERICAN1 - I assume you miss-typed and meant 285/16. I have been trying to inform people that even though the static calculation for these tires is 8%, the tires "grow" with speed. I've calculated doing measured miles and gps that they cause almost an exact 10% error. As an extreme example, watch a dragster on takeoff. The tires look like they "grow" more than 50%.

A question immediately came to mind when I read this post...What would prevent a person who purchased this item to program, at the very least, 225/75R16's into the recalibration unit, but wear 255/85R16's in order to add an additional 13K of warranty on the engine?

Allison Jettester,
Why wouldn't the 8% (I calculate 7.2%) circumferential difference of 245's vs. 285's be the same statically and dynamically? Is there that much difference in the "grow ratio" of the 245's vs. the 285's? :confused:

Allamerican1,

Thanks for the info. I also found it on the superlift website but didn't see a price. I wanted to put a lift on my truck but didn't want to do it because of the speedo calibration issue with the Allison tranny. Guess I can do it now.
Thanks..... :D :D :D

How difficult was the install? I have one on order it should be here tommorrow. Are the instructions detailed enough? Which wire is the speed sensor wire (description)? I have nightmares when it comes to wiring!! I want to make sure I have the right one :(
Do you have any tips or secrets?

Thanks in advance
Kyle

Install was not that difficult with the exception of their directions refer to 2001. My truck is a 2002 and the red connector to the PCM they refer to in their instructions is clear on my truck. Luckily I purchased a full set of GM manuals and was able to figure it out. The wire color for the vehicle speed sensor was the same (Pin 50 and green and white I believe). Basically it was just cut that wire and solder in the Tru-speed, then hook up a hot wire and a ground and adjust it. After playing with the adjustment according to my GPS I'm within 3/10ths of a mile at 75.

I installed the module yesterday. Fairly easy and straight forward, just time consuming. I also wanted to make sure I did not make any mistakes :eek: :eek: With 315/75 tires my gps reads about 0.2 under to about 0.2 mph over what the speedo says. I don't think it gets much better than this. With my stock tires my speedo was fast about 1-2 mph compared with the gps. So far I am pretty happy :D

Moved to "2500HD/3500 Trucks & Drivetrain"

Wouldn't the best bet be to measure actual loaded runout of the tire, and use that instead of tire diameter?

Blaine

Isn't it possible to recalibrate the speedometer via PCM programming like on late model f-bodies and corvettes?

allamerican1,

Do you have the Allison Transmission? If so, how are things going with the calibrator?

Mike