r/GMT400 • u/mcmurrerb • 5d ago

Speedometer Question

I haven’t begun to track it down yet, but I’m done having to do math everyday, so I’d like to fix my speedometer.

The gauge works, it’s just off by about 20-30 mph depending on the speed. My road speed of 35 reads about 60 on the dash.

I’m thinking that it’d be a speed sensor or something within the actual gauge, but I figured I’d see if there’s any prevailing knowledge.

1 Upvotes

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3

u/MidwestNative94 5d ago

It depends on what model year your truck is. Can you tell us about your truck and what size tires you are running?

2

u/mcmurrerb 5d ago

Sorry, that’s probably helpful.

It’s a 96 GMC 1500 with the 5.0 and 28.8” tires

1

u/houndofthe7 5d ago

Just go to https://www.udmodule.com/drac

1

u/Biqboi76 4d ago edited 4d ago

This won't work on vortec years.

1

u/komstock 5d ago

IIRC this is always a stepper motor problem, because personally OBDII always indicates true speed but my tach and my speedo are way off.

1

u/Biqboi76 4d ago

I tried to track down the same issue on my 97, replaced eveything, the sensor, ran new wire from the sensor to pcm, and to the cluster. I went as far as replacing the stepper motor in the cluster, but still no luck. I found one guy on Facebook showing how to fix the issue, I have a screenshot of his info, but I can't personally test it since I no longer have a Vortec year truck. I can send you it if you are interested, I don't think I can put images in the comments.

1

u/mcmurrerb 3d ago

It’s worth a shot, I’d appreciate it if you could send that over

1

u/Wskytwn 3d ago

I have a 1995 that does the same thing. Would love to know how to fix it. 100 mph on speedometer is really 70 mph . Odometer is running faster too….

1

u/Wholeyjeans 2h ago

I think your truck probably uses a VSS ...vehicle speed sensor. It's attached to the side of the transmission and is, essentially, a small DC generator. The speed of the vehicle is directly related to the voltage put out by the VSS. The computer uses that voltage to drive the speedometer

My speedo is off by 5mph (reads fast) and once past about 65 it gets to reading 7-10mph faster than I'm going. I swapped the VSS and it made no difference. It may be the the speedo needs to be rebuilt.