How do I adjust my e99 speedometer?

[alabamafrog]

My e99 has the mechanical style odometer and my speedo is about 8% off so at 70 I'm really doing like 78 because of the 315 tire size. This would not be a problem except for I use this truck in my business and count off mileage, at 20,000 miles times 8% times 50 cents a mile I lost about $800 on taxes last year because of this. Not to mention the increased possibility of getting speeding tickets because of the subconscious psychological effect of looking at the slow speedo.
How do I fix it?

[7.3PwrHse]

I'd like to know this as well..

[alabamafrog]

There has to be a way, my 2000 has 37" tires and the speedo is almost perfectly in time with my GPS.

[7.3PwrHse]

Originally Posted by alabamafrog

There has to be a way, my 2000 has 37" tires and the speedo is almost perfectly in time with my GPS.

If I get some time I'll research. Are you sure it's purely mechanical? Mine has been taking spells of not working over the weekend. If i shut the truck off, and restart it starts working again

Sent from my C771

[alabamafrog]

What I was talking about is it has the mechanical rolling numbers instead of the digital readout for an odometer. I think the digital ones are real quick and easy to reprogram but I have never heard how to fix the old style rolling number ones. I believe the speedo is actually controlled electronically by a sensor in the rear-end chunk that reads off the ring gear and sends a signal to the PCM. If yours is acting up its most likely that $20 sensor in the chunk.

Come to think of it I wonder if that’s why my white truck with the 37” tires reads right, wonder if it originally had a higher ration rear-end and whoever lifted it may have swapped to a lower gear rear end but with much bigger tires which balanced out to make the speedo close to the same again?
Hmmm, also wonder if that might have something to do with why it shifts so hard?

[Cllake]

You can do it with a programmer. My PHP tunes take into account my new wheels/tires and my speedo reads correct now.

[alabamafrog]

I have had 4 different programmers on old purple over the years, none of them would change the speedo, as best I remember they would say something to the effect of this option is not valid for this vehicle. This was with a hypertec, 2 different model superchips, and 2 different Predators. Now I run a custom 6 position flip chip.

[jmaroon]

drop to a 305 and it'll be dead on..

[alabamafrog]

Nope, last set of tires were 305 and it was not at all dead on.
I guess no one knows how to do this, must be impossible.

[dieselcrawler]

I've heard that the dealer can re-program the computer for the tire size you're running... not sure how a chip will effect this, or if it would need to be removed to have the dealer do the programming.

Frog, you are correct, the speedo gets it's info from the sensor on the rear end, as you mentioned. The sensor going bad will cause erratic speedo readings, and shift issues, as the trans uses the speed info to shift correctly.

Good luck to both of you... I am in the same boat, my red truck wears 35's, and has no correction, I'm sure it's off, I've been lucky to not have gotten nailed for speeding. Also wonder how much it's thrown off my MPG calculations, I'd bet I'm getting better than what it calculates out to.