Speedometer correction - 98 Vortec truck

[Truckracer]

With a tire change the speedometer in my truck was 6 to 7 mph off at higher highway speeds. I researched until I was tired of the many ways to bring the accuracy back.

I needed a low cost method. I wasn't willing to pay for a tune I don't need or a tuner I have no other use for. I know many have pried the needle off and reinstalled, that just didn't appeal to me.

I decided on modifying the signal from the VSS to the speedo.
Sounds easy on the surface, Use a Dakota Digital SGI-5E Universal Signal Interface, tie it in the sensor wires going into the ECM and adjust away. And that's what I did.
Worked like a charm, in five minutes of adjusting I had the speedometer right on.
Then I took off on a 500 mile trip. After the second gas stop the SES light was on. Turns out the computer reads the adjusted signal against the rpm/speed table and determines that something is slipping.

Back to square one.

And then I found the ECM out to the speedometer (black, Dark green on pin 4).
I tied the SGI-5e between the ECM and the speedometer.
I set the input/output to L /L
And in just a few minutes had the speed readings within 1mph over the full scale.

http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/category_id=287/mode=prod/prd126.htm

 

[df2x4]

Credit where credit is due, nice job making this work. In my personal opinion though that's quite a bit of effort for very little gain. Tunes for Vortec trucks are around $280, with free retunes for life. Even if you don't "need" it right now, it's the easiest way to solve your problem and you'll definitely pick up a few extra MPG even if extra performance isn't your goal.

Like I said though, nice job making it work.

 

[Jrgunn5150]

That's very nice work, since mine is off with stock tires and gears lol. 85 bucks, I might have to order me one.