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Speedometer Reads Way Too High

Dave01

Dave01

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Location
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The speedo on my '98 K2500 reads very high, about 23-25% higher than actual. 45 indicated is actually about 36, 85 indicated is actually about 69. Any ideas on why this is happening, and a fix?

Also, is the odometer linked to the speedo on these, or are they getting their signal separately?
 
They both use the same signal. Is this something that just started, or has it been this way since you've had it? The first thing you need to do is get a scanner on it that can read the vehicle speed from the data port, and compare it to the speedometer reading. This way you can determine if it is the speed signal coming from the VSSB or if it is just the speedo.
 
I've only had the truck for a few months, and it's been this way since I bought it. It shifts and otherwise runs great.
 
Get a scanner on it nd see if the speedo reads the same as what the speed reads on the scanner. If it does then it sounds like you have the wrong VSSB in it. If they are different then the speedo motor that makes it move could have failed.
 
Couple things could be going on since the truck is relatively "new to you"

Different tire size from OEM.

Gearing in the axles have been changed

The speedometer motor has been replaced at some time (they fail sometimes, but more rare in this era) and the needle was reinstalled slightly off

Someone has taken the cluster apart (popular for led installations and face color changes) and replaced the needle in the wrong spot.

Ferm makes an excellent suggestion on hooking up a scanner and seeing what the data feed from the vssb is.

If you have an android powered device, you can download an app called torque for free and buy a cheaply elm 327 blutooth module off eBay for around 10 bucks shipped. This will show you the speedometer signal the vssb is sending out to the speedo.

It will also read and clear trouble codes, but that's for another thread for another time....:)
 
Thanks for the comments! The difference is too large to just be from a tire size change, and it has 245/75-16's on it. I assume it came standard with 4.10's, I'm sure that's what's in it now from the rpm/speed that I see. Also, the speedo difference is progressive with speed so can't just be a needle location problem.

Thanks for the suggestion on the app, I'll download it and get the elm Bluetooth adapter, didn't know about them until you mentioned it. Once I get the speedo reading I'll report back.
 
You may need to modify your VSSB to make it read right. Sounds like somebody put one in from a truck with 3.42 gears. So it thinks your driveshaft is turning what it should be for 85, but due to the lower gear ratio it is only running 69.
 
Thanks Marty, that's a great link. It says it's for 92-95 VSSB, but I assume it continued on through 2000, is that right?

Ferm, you may be right, just calibrated for a much different gear set.
 
Dave01 said:
Thanks Marty, that's a great link. It says it's for 92-95 VSSB, but I assume it continued on through 2000, is that right?

Ferm, you may be right, just calibrated for a much different gear set.
Click to expand...

Good question, I have no idea. I hope someone else pipes in on this.

I am stumped, I thought all of these trucks used the vssb/drac.
 
Dave01 said:
Thanks Marty, that's a great link. It says it's for 92-95 VSSB, but I assume it continued on through 2000, is that right?

Ferm, you may be right, just calibrated for a much different gear set.
Click to expand...

DieselCash said:
Good question, I have no idea. I hope someone else pipes in on this.

I am stumped, I thought all of these trucks used the vssb/drac.
Click to expand...

Starting in 96 GM eliminated the VSSB/DRAC module EXCEPT for the lowly 6.5 powered trucks. All of the manuals out there say that GM stopped using them in 95, but it is a little known fact that GM kept them for JUST the 6.5's, and ONLY the 6.5's. So yes it applies to the 96-00 6.5 powered trucks, and I think even the van's to 04 or so when they stopped the 6.5's, and the HUMMERS until who knows when. GM just sort of left the 6.5 in the stone age for it's run while the gasser's kept up with technology, and the DURAMAX time warped to the future when it came out.
 
I wish someone had told me I was buying a stone age truck before I paid 1/10th of the cost of a Duramax powered truck for it.
 
These are good trucks. Parts are cheap! What more do you need in life?

I would not trade mine for anything in the world.
 
David, the scanner I remember you having should read speed. I did get those switches from Hong Kong on a canoe. I can build that VSSB for you.

Did you do a computer yet? How has the truck been?

Sent from my ADR6350 using Tapatalk 2
 
I didn't think that Actron would read speed, could give it a try. I ordered a 10 dollar elm 327 bluetooth as suggested above, sounds like a cool app to run on your phone. No tune yet, will order one at some point. Truck runs perfect! I might go for the dip switches once I figure this out, I'd also put 265's on it except there's a lot of life left in the 245's that came with it, so would be good to be able to just dial it in with different tires in the future.
 
I have to be in Concord mid afternoon, will stop in if you are there. I'm planning to put in 2 or 3 gauges, need to get a pillar mount. Not sure if I want to put just boost and fuel pressure, or do 3 and put egt as well.

It's not that the speedo is all that big of a deal, but I'd like the odometer to read right and you know me, I just like things to work the way they are supposed to!
 
I bought a Vgate ELM327 Bluetooth scan adapter. This is a cool little gadget for only 11 bucks, I loaded the Torque app on my phone and can read OBDII data.

Turns out the speed signal and the GPS speed match almost perfectly:

'98 GMC 2500 Speed Screen Shot.jpg

So, does that mean I just need a set of dip switches to calibrate the VSSB?
 
Sounds like your VSSB is fine, sounds like a speedometer problem. The VSSB sends the same speed signal to the ECM as it does to the speedo.
 
I also haven't checked the odometer. I'm curious if it's reading OK, or same error as the speedo.
 
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