Speedo off big time

[RustyShackleford]

98 k1500 5.7 285/75/16
Speedo is 15mph fast. It reads 70 when I’m going 55. It was like that when I got it. The ABS light is on. Where do I start? Please don’t say it’s the tires. I have a 97 that has same size tires and it’s only 2mph off. Is there a sensor issue? Does the ABS have anything to do with the speedo reading? The dealership wanted $400 just to diagnose because it’s “such an old truck.” It’s definitely worth fixing because the truck is in really good shape

 

[0xDEADBEEF]

It's the tires.

Have to set the correct pulses per mile in the PCM.

 

[Erik the Awful]

The dealership wanted $400 just to diagnose because it’s “such an old truck.”

The dealership wanted $400 because the truck was built before any of their technicians were born, and none of them would know what they're doing.

Compared to the stock tire (assuming 245/75-16) your tires only account for a 7% difference in speed. The ABS sensors don't affect the speedometer. I'd pull the VSS out and make sure it doesn't have any physical damage or metal shavings on it. Put a flashlight on the hole and see if you can see the tone ring that triggers it.

Past that you're probably looking at editing the PCM, like 0xDEADBEEF said.

 

[HotWheelsBurban]

Isn't 245/75R16 the standard size on the 400 era 16" wheels?

 

[any4xx]

I used to use a device called a “Speedo Healer” on motorcycles when changing gearing. All it does is convert input pulse counts to the desired output pulse counts. I’m not sure if that brand even exists any longer but I’m sure something like that is still out there somewhere. Easy to tune and a lot easier than having to program the PCM.

 

[Schurkey]

In '88, the speedo calibration is done in the instrument cluster via the DRAC.

Later years have a remote-mounted DRAC near but not in the vehicle computer.

By '97, it's integrated into the Vehicle Control Module (VCM) (Engine computer) for gasoline engines, or Vehicle Speed Sensor Buffer for Diesels.

 

[Trio]

It's NOT the tires, at all. You have the opposite problem that oversize tires would cause. Since your tires are oversize, the speedo should read too SLOW compared to actual speed. Your speedo is reading FAST compared to actual speed (as you said, it reads 70 when you are going 55 actual).

You've got something else going on that is flaky, but the tires are not the cause of this particular problem.

 

[Caman96]

This video explains it for the early version. But yeah you might need to have pcm updated, previous owner might have had different tires and pcm set for those. Hard tellin not knowin!

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[RustyShackleford]

Yeah it’s not the tires. It has 265’s stock. Going to 285’s wouldn’t do this. The guy I bought it from said the 4wd didn’t work. But…it works…I wonder if he screwed something up with the speed sensor in the transfer case when working on it.

I still need to know if the ABS light is related to the speedometer

 

[RustyShackleford]

Isn't 245/75R16 the standard size on the 400 era 16" wheels?

Not always. Both my 97 and my 98 came with 265’s according to the door stickers. Might be part of the Z71 package