speedometer reads ~3 MPH slow

[cc333]

Hi,

My 94 K2500 Sierra's speedometer seems to be reading about 3 to 5 MPH slow. Is this fixable by replacing the instrument cluster, or is it something related to the VSS sensor, transmission or ECM? If I can fix the problem with a new cluster, this will be a good excuse to get one with a tachometer

Stock rims on stock-size tires, and as far as I know, all components in the drive train (front/rear differentials, drive shafts, etc.) are 100% stock or equivalent. Rear differential gearing should be matched to whatever the speedometer and related items are/were calibrated to, since everything is presumably factory original.

Thank you!

c

 

[alpinecrick]

Hi,

My 94 K2500 Sierra's speedometer seems to be reading about 3 to 5 MPH slow. Is this fixable by replacing the instrument cluster, or is it something related to the VSS sensor, transmission or ECM? If I can fix the problem with a new cluster, this will be a good excuse to get one with a tachometer

Stock rims on stock-size tires, and as far as I know, all components in the drive train (front/rear differentials, drive shafts, etc.) are 100% stock or equivalent. Rear differential gearing should be matched to whatever the speedometer and related items are/were calibrated to, since everything is presumably factory original.

Thank you!

c

3mph is most likely within specs of the speedo gauge.......

 

[cc333]

Hmm, OK.

I might still go get a new cluster, though, because I want the tachometer.

However, it's good to know that it's probably within tolerance, and there's nothing wrong aside perhaps for normal wear and tear.

c

 

[Supercharged111]

Some of the TBI trucks used DRAC modules with jumpers you could play with. 94 may be one of those years?

 

[PlayingWithTBI]

My 94 K2500 Sierra's speedometer seems to be reading about 3 to 5 MPH slow.

Is it constantly 3 - 5 MPH slow or is it more @60 MPH than 30 MPH? If it's constantly the same offset, you can pull the needle off and put it back on 3 - 5 MPH faster

 

[alpinecrick]

Is it constantly 3 - 5 MPH slow or is it more @60 MPH than 30 MPH? If it's constantly the same offset, you can pull the needle off and put it back on 3 - 5 MPH faster

 

[cc333]

It's hard to tell. I'll have to run some more tests, but at 55, the speedometer reads 51 or so. At 40, it seemed to read 47-50, so I somewhat doubt the offset is constant, but again, this is only a rough guess, and I have to run more tests to be certain.

There's probably an app for my phone which can measure speed. Probably no more accurate, but hopefully at least consistent, so I can set a baseline for comparison.

EDIT: Got an app downloaded, now I'll have to go take a drive and test.

c

 

[Supercharged111]

It's hard to tell. I'll have to run some more tests, but at 55, the speedometer reads 51 or so. At 40, it seemed to read 47-50, so I somewhat doubt the offset is constant, but again, this is only a rough guess, and I have to run more tests to be certain.

There's probably an app for my phone which can measure speed. Probably no more accurate, but hopefully at least consistent, so I can set a baseline for comparison.

EDIT: Got an app downloaded, now I'll have to go take a drive and test.

c

Google maps will show speed. I calibrate my speedometer at 70, figure that makes it good for 60-80. It is off down in the 30 range.

 

[LVJJJ]

cc, my 94 k1500 burb reads the same slowness. Interesting to hear that I can maybe pull the needle off move it. Just had the gage cluster apart though, guess I'll wait a while, was a lot of work.

 

[cc333]

OK, so I have run some tests, and it would seem that the following is true:

• At 0-10 MPH, truck's speedometer seems to be at most 1 MPH faster than what the phone app indicates
• Starting at 15 MPH, things get a little weird, with a discrepancy of ~2 or 3 MPH
• At 25 MPH, truck's speedometer is almost 5 MPH fast
• At 30, it's maybe 4 fast
• Then at 35 and over, it seems to average about 3 MPH fast up to at least 55 (where truck's speedometer reads ~58)

So, I sort of doubt simply moving the needle back 3 MPH will work completely, as the discrepancy seems to be somewhat nonlinear below 35.

I guess I can live with it for now, as I don't want to start messing around and making it worse, but I wouldn't mind fixing it eventually.

I have read cases where the speedometer was off by some highly variable amount at certain speeds, and replacing the instrument cluster solved it, so maybe that's what I'll have to do?

I don't yet have posting privileges in the classifieds, or else I'd make life easy and post a WTB for a gas cluster with a tachometer (I don't care about mileage, as I plan to swap the odometer over to the new console).

In the meantime, I guess I'll look around and see what I can find on eBay and the like.

c