1998 Silverado Speedometer is reading too fast

[lindstromjd]

Not sure if this is a common problem or an easy fix, but the forum won't let me do any searches with "speedometer" in the key words. Anyway, I recently bought a 98 Silverado 4x4 with the 4l60e and a 5.0 that the speedo reads way too fast. I verified this with my GPS. When the speedo reads 50, I'm doing about 45. When it reads 61, I'm doing 55. When it reads 83, I'm doing 70. Etc etc. What's the fix for this?

The tires are not oversized, either. And as far as I know, the transmission has been rebuilt, but not too sure how accurate that statement is. Probably just something the guy I bought it from said to make it more "tempting" to a potential buyer. It does throw a P1870 code, but that's a different issue.

[UKNOWME]

Originally Posted by lindstromjd

Not sure if this is a common problem or an easy fix, but the forum won't let me do any searches with "speedometer" in the key words. Anyway, I recently bought a 98 Silverado 4x4 with the 4l60e and a 5.0 that the speedo reads way too fast. I verified this with my GPS. When the speedo reads 50, I'm doing about 45. When it reads 61, I'm doing 55. When it reads 83, I'm doing 70. Etc etc. What's the fix for this?

The tires are not oversized, either. And as far as I know, the transmission has been rebuilt, but not too sure how accurate that statement is. Probably just something the guy I bought it from said to make it more "tempting" to a potential buyer. It does throw a P1870 code, but that's a different issue.

That transmission doesn't have a gear so it's signal comes from a speed sensor. If that sensor was bad it should set a code. Maybe the transmission was taken from another application that has a different speed sensor. Another possibilty is if the gears have been changed. Does the speedometer register 0 when at a stop?

[lindstromjd]

Originally Posted by UKNOWME

That transmission doesn't have a gear so it's signal comes from a speed sensor. If that sensor was bad it should set a code. Maybe the transmission was taken from another application that has a different speed sensor. Another possibilty is if the gears have been changed. Does the speedometer register 0 when at a stop?

Yes, it works just as it should, only the reading is off. No bouncing or floating or anything like that. I don't think the gears have been changed, either. The RPM's are right about the same as my Suburban (with a 3.42) on the highway, so it doesn't sound like the engine is struggling or screaming.

Is there a way to re-calibrate the speed sensor?

[UKNOWME]

Originally Posted by lindstromjd

Is there a way to re-calibrate the speed sensor?

Not that I'm aware of
I think the speed sensor is on the tail shaft of your transfer case. Try swapping it out and see if that changes anything.

[ChevyTech]

On a 1998 there is no separate DRAC to calibrate.

To change the speedometer calibration, the PCM is reprogrammed.

[lindstromjd]

Originally Posted by ChevyTech

On a 1998 there is no separate DRAC to calibrate.

To change the speedometer calibration, the PCM is reprogrammed.

Sigh, crap. Looks like I'll just have to spend another $100 at the dealer on this truck then. Thanks for clarifying though.

[ChevyTech]

You're welcome!

[83GMCK2500]

Hi, I just wanted to clarify and ask a couple additional questions.

1. Speedo reads faster than ground speed = tires smaller than PCM programmed for.
2. Speedo reads slower than ground speed = tires larger than PCM programmed for.

You're experiencing #1, correct? (My '96 is a #2 truck, I've got tires 1-1.5" taller on it.) Please verify the original size of tire on the vehicle (sticker in door jamb or on door itself), and compare to the size of tire currently on it. If the sticker is unreadable it may be possible to determine it through your VIN, I might be able to run it if you PM the last 8 of your VIN (if need be).

[UKNOWME]

Originally Posted by ChevyTech

To change the speedometer calibration, the PCM is reprogrammed.

Are you sure you can program a 98 for speedometer calibration? The only reason you would want to reprogram anyhow would be for non-stock tires or gear ratios. GM only supports factory options anyhow.

[lindstromjd]

Originally Posted by 83GMCK2500

Hi, I just wanted to clarify and ask a couple additional questions.

1. Speedo reads faster than ground speed = tires smaller than PCM programmed for.
2. Speedo reads slower than ground speed = tires larger than PCM programmed for.

You're experiencing #1, correct? (My '96 is a #2 truck, I've got tires 1-1.5" taller on it.) Please verify the original size of tire on the vehicle (sticker in door jamb or on door itself), and compare to the size of tire currently on it. If the sticker is unreadable it may be possible to determine it through your VIN, I might be able to run it if you PM the last 8 of your VIN (if need be).

Correct, I am experiencing the speedometer reading faster than the actual ground speed. My door jam says the tires should be 265/75r16, and that's what I'm running.

[ChevyTech]

Originally Posted by UKNOWME

Are you sure you can program a 98 for speedometer calibration? The only reason you would want to reprogram anyhow would be for non-stock tires or gear ratios. GM only supports factory options anyhow.

No!!!
We would have to reprogram all the time when someone changed their tires size. That will make the speedometer inaccurate. Some people would have the tire swapped before they ever took delivery of the vehicle and we would reprogram them.

When someone changes the axle ratio, a reprogram is needed.


On the 1988-1991 with the DRAC in the instrument panel, and the1992 – 1995 with the separate DRAC, the ratio could be calculated for tires larger then the factory ever put on the vehicle. The charts from the manufacturer only showed how to program it for the tire that were available. Just because the chart does not show the answer, does not mean it can't be done.

The formulas for doing that calculation have been posted by me on Chevy truck boards over the years.

Having said that, I have never corrected the program for larger then factory tires, on the later trucks like his 1998. With the variety of axle ratios, and tire sizes from the factory, a person should be able to get the correct reading unless someone has gone very extreme where it would not be street-able anyway.

In all my years of doing calculations for this, I have never come up with a ratio that was outside the ratio range that was cover by the factory charts.

[ChevyTech]

Originally Posted by lindstromjd

Correct, I am experiencing the speedometer reading faster than the actual ground speed. My door jam says the tires should be 265/75r16, and that's what I'm running.

If your truck has the factory tire size on it, the speedometer should be close unless someone has changed the axle ratio.

[speedygonzales]

Why don't you look at the RPO code and then actually check the ratio of the diff?

[lindstromjd]

Originally Posted by speedygonzales

Why don't you look at the RPO code and then actually check the ratio of the diff?

Because I can't find the code. It's not on the driver's door and there's nothing in the glove box.

[83GMCK2500]

Originally Posted by lindstromjd

Because I can't find the code. It's not on the driver's door and there's nothing in the glove box.

Couple options: Call a dealer or stop by and have them run your VIN, they can give you a printout of all your RPOs and their meaning. Or you can try: http://www.decodethis.com/Decoder Or you can register on http://www.compnine.com/ (used to be free and one of the best decoders). Or you can post your VIN here and someone who may have access to the right software can run it and share the info with you.