My speedometer is a little off...

[mpyusko]

Seriously though...
How can I fix this? I was in Ohio doing 70 on the interstate when I took this photo. The VSS is reading correctly.

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[Supercharged111]

2700 RPM at 70?

 

[HotWheelsBurban]

2700 RPM at 70?

If it's in overdrive....

 

[Supercharged111]

If it's in overdrive....

With a 4.10 he'd be north of 70 at 2700. The plot thickens.

 

[yevgenievich]

Air core motors fail. Easiest is to try a different cluster. Took a couple of junk yard clusters to get a good working one for last vehicle I had with that issue. But the motors on the back are replaceable. As an option, it is possible to tune speed output from ecm depending on year of the vehicle.

 

[mpyusko]

With a 4.10 he'd be north of 70 at 2700. The plot thickens.

The tach is a little off too, but I also have 33's.
Call it "70ish" but still... Less than 80 and far less than 100.

But what can I do to fix this?

 

[Supercharged111]

The tach is a little off too, but I also have 33's.
Call it "70ish" but still... Less than 80 and far less than 100.

But what can I do to fix this?

Has it always been this way or did it recently decide to act up? If it's the latter I'm leaning toward what yev said above.

 

[mpyusko]

Air core motors fail. Easiest is to try a different cluster. Took a couple of junk yard clusters to get a good working one for last vehicle I had with that issue. But the motors on the back are replaceable. As an option, it is possible to tune speed output from ecm depending on year of the vehicle.

The ECM/VSS is reading correctly. It's off by 10% inline with my ~33in tires. When I bought the truck 8 years ago, 30 was 35 and 60 was 70. Now 60 is ~80 and 70 is ~100.

So really, the only way is a new cluster?

 

[stutaeng]

Mine reads 78 when I'm actually going 70. Same deal on mine: the ECU reports correctly, within 1-2 mph of GPS. I have 265/75/16 over the stock 245/75/16, so there is a bit error on that.

Anyway, I'd be interested in changing the air core motor if someone can provide a part number. I haven't seen many GMT 400 trucks at the wrecking yards the times I've been there. Then you got to worry about mileage being different on the cluster...

BTW, mine runs right under 2500 RPM (2450?) at 70 with 0.75 OD, 4.10s and those tires. I've got other trucks with the same tires, transmission, and gear ratio, and they also run at the same RPM at 70 mph.

 

[yevgenievich]

The ECM/VSS is reading correctly. It's off by 10% inline with my ~33in tires. When I bought the truck 8 years ago, 30 was 35 and 60 was 70. Now 60 is ~80 and 70 is ~100.

So really, the only way is a new cluster?

Easiest is a cluster change. Odometer section swaps out. Air core motors are for sale on amazon/ebay. With programming, on some it is possible to change the output to cluster signal while keeping the vss speed without change.