Both Speedometer and Tachometer are wrong

Weston Graham · Sep 12, 2017

Hello all,
Recently got a 1996 2 door Tahoe in my possession, and has it been a great truck to work on (is it considered a truck really?). Anyways, I noticed that the speedometer reads 5mph too high in the city and 8-10mph off on the highway (i.e, Im actually going 25mph but the speedo shows 30mph) as well as my tachometer is about 200rpm too high as well (idling at about 500rpm and it shows 700rpm). The reason I know this is because I connected a handheld obdII scanner and watched the live data as I drove. This tells me that theres some type of connection error between the computer and the dashboard. Im not really sure what to do with this kind of thing, as mechanical parts are my strong point and I tend to shy away from wiring. Any help would be appreciated to figuring out what the issue is! Also, the needles on both of them are a greenish hue, while the rest are orange. Not sure if this is significant but thought I may ad it.

Thanks!

95sierra2500 · Sep 20, 2017

try the drac or vssb box under your computer if you have one, are your tires oversized from stock?

east302 · Sep 20, 2017

Are your tires the stock size per the label in the door jamb? Larger tires will impact speedometer accuracy.

Here are the diagnostics for a 1998, maybe it will help.

Tachometer not accurate:

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Speedometer not accurate:

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GMCTRUCKS · Oct 27, 2017

I had 3.42 gears on my truck now I have 3.73 well I bought a Drac from a Z71 put it on well I’m closer to being right but I’m still off and I’m the same way at a lower speed it’s off by less and at a higher speed it’s off more now I just live with it and my radar detector lets me know my speed when hit with radar.

Guys I still don’t get it my tires are about the same size as stock and I did put on the correct DRAC and I’m still off I’m confused?

I can’t get a ticket but I still want to know what speed I’m going because the only time I know is when I’m hit with radar which isn’t always.

I’m going to have my son ride besides me in his ride just to compare or know for sure.

Weston Graham · Nov 28, 2017

east302 said:
Are your tires the stock size per the label in the door jamb? Larger tires will impact speedometer accuracy.

Here are the diagnostics for a 1998, maybe it will help.

East, tires are stock size, and the rpm is about 300rpm faster on the dash than my scanner reads and the speedometer can be up to 15mph off. so it fails both those tests, is there any way i can fix the instrument panel cluster myself?

95sierra2500 said:
try the drac or vssb box under your computer if you have one, are your tires oversized from stock?

How would i check that? and no, stock tires and rims according to the label.

GMCTRUCKS said:
I had 3.42 gears on my truck now I have 3.73 well I bought a Drac from a Z71 put it on well I’m closer to being right but I’m still off and I’m the same way at a lower speed it’s off by less and at a higher speed it’s off more now I just live with it and my radar detector lets me know my speed when hit with radar.

Guys I still don’t get it my tires are about the same size as stock and I did put on the correct DRAC and I’m still off I’m confused?

I can’t get a ticket but I still want to know what speed I’m going because the only time I know is when I’m hit with radar which isn’t always.

I’m going to have my son ride besides me in his ride just to compare or know for sure.

to be more accurate, you can buy a $35 obdii scanner from walmart that can read live data, including rpm and VSS readings (to see if the computer is reading the correct speed)

Thanks guys!

east302 · Nov 28, 2017

I've seen rebuild services on eBay for 95-99 clusters that would, in theory, fix any issue with a defective cluster. They're around $300 or so.

I'd grab one from a salvage yard and test it out. If it works better, swap the odometers and you're good.

Car-part.com is a good search engine for local yards.

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Weston Graham · Nov 28, 2017

east302 said:
I've seen rebuild services on eBay for 95-99 clusters that would, in theory, fix any issue with a defective cluster. They're around $300 or so.

I'd grab one from a salvage yard and test it out. If it works better, swap the odometers and you're good.

Car-part.com is a good search engine for local yards.

Sent from my iPhone using Tapatalk

I may go with a rebuilt from CARID.com, they program in the mileage and vin(?) and then ship you a rebuilt one for about $220. I got an estimate for $235 to send mine in and get it repaired, and sent back. CARID has a lifetime warranty and the repair service only offered a 1 year, so Im thinking just a rebuilt may do it. If it doesnt work, I've always got the option of sending it back!

east302 · Nov 28, 2017

Good deal, if you go that route let us know how it works. I may be interested in that some day as well.

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GMCTRUCKS · Nov 29, 2017

to be more accurate, you can buy a $35 obdii scanner from walmart that can read live data, including rpm and VSS readings (to see if the computer is reading the correct speed)

Thanks guys![/QUOTE]

My trucks a obd1 and my tires are the same height as the factory ones like to day driving into Houston I was hit with a police radar my radar detector was saying I was off six miles off but if I’m going slower it will be off around three miles off it’s weird

95sierra2500 · Dec 5, 2017

How would i check that? and no, stock tires and rims according to the label.

You check by searching drac or vssb calculator, and in the little drac box by your computer there is a chip inside with I think 7 pins, you need a certain order of pins which you receive from the calculator depending on what you input. It gives you an answer in binary. So you'll have 14 terminals and 7 pins.
.......
.......
Like that.
Some will have pins, some will not, use the binary from the calculator to decide which pins need to be connected for instance 1001000 the first top and bottom dot would be soldered together and the 4th top and bottom. The other option is to install a 7 pin dip switch that you can find on amazon, that means less soldering and the option to fix your speedo any time you'd like if you change tire size etc.

Both Speedometer and Tachometer are wrong

Weston Graham

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95sierra2500

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east302

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GMCTRUCKS

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Weston Graham

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Weston Graham

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