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Username Post: Speedometer help        (Topic#345511)
Forum Newbie
Posts: 27

Reg: 02-17-11
06-18-17 08:50 PM - Post#2696463    

Speedo jumps around while driving down the road. Started out bought the truck with a bad rear end. So bought different axles went from 305 to 373 gears. I upgraded my gauge cluster to a Dakota digital gauges. Tried to set the speedo up but can't get it accurate with it jumping sround so much. Do I need to change the speedo gears to compensate for gear and tire size change? Or is the needle bouncing from the speedo gears being bad? Or is it a problem with the cluster? I don't believe it bounced around be for the Dakota digital install. But original speedo was slow by 10 mph which I knew would happen with gear and tire change. Thought I should be able to set the electronic one with out messing with the speedo gears. Any one else have any problems with the speedo jumping around or know much about the Dakota digital gauges? Any help or input would be appreciated. Or what speedo gears are needed for the gear and tire change. It's an 86 k10 with sm465 and np 208 t case. I have 3.73 gears with 33" tires.

Honored Member
Posts: 27372
Loc: San Francisco, CA
Reg: 11-26-02
06-19-17 04:01 PM - Post#2696609    
    In response to Keithk

  • Keithk Said:
....Thought I should be able to set the electronic one with out messing with the speedo gears.....

There is no need to replace the speedo gears in the T-case. All aftermarket electronic speedos I have seen had onboard provisions for calibrating the speedo to work accurately over a wide range of pulse per mile inputs.

However, you appear to be leaving out a rather important piece of information here. How are you interfacing the mechanical cable drive output from the T-case to the electronic input required by the speedo?


Those who choose an automatic transmission want transportation. Those who choose a manual transmission want to drive.

Senior Chevytalk Moderator
Posts: 24297
Loc: Austin, TX 78748
Reg: 05-09-00
06-20-17 09:12 AM - Post#2696739    
    In response to raycow

Use this speedometer gear calculator. alcula...

Find the drive and driven gear ratios. Find a combo with each gear as close to a whole number as possible.

Possible gears:
Drive gear tooth count / color:
15 - gray
17 - red
18 - blue
19 - yellow
20 - brown

Driven Gear tooth count / color
Truck Gears:
18 - brown
19 - dk aqua
20 - silver
21 - red
22 - gold
23 - maroon
24 - dk blue

'95 Jimmy SLT (Bought @131,814)
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS Ret. @236651 miles

"15th Year" Gold Supporting Member
Posts: 18770
Loc: Colorado Springs, CO
Reg: 08-08-04
06-20-17 01:49 PM - Post#2696786    
    In response to gchemist

Where is the electronic signal generator installed? At the transmission or at the end of the speedometer cable inside the truck?

If you have the signal generator on the end of the speedo cable, it is probable that the speedo cable is bad. When they start to wear really badly, they can make the speedometer jump around quite a bit.

If there isn't enough adjustability within the digital dash to iron out the speed with dip switches or whatever else they are using, you can switch the drive and driven gears inside the tailshaft of the transmission (or transfer case if it is 4wd). Find out what colors the drive and driven gears are, then you can adjust the output by using different driven gears (usually) or possibly by changing the drive gear (more involved, you have to take the (2wd) transmission tailshaft off to get at the drive gear. Floor shift 4 speeds can be a little more challenging than just taking off the tail shaft housing, you may need to drain gear oil to do it.

If you have a 2wd 3 speed trans, car style 4 speed, or an auto trans, it is relatively easy to get at these gears. Find a decent slope or hill to back the truck up on so that the rear is pointed uphill (so no gear oil or auto trans fluid gets out), Block the wheels so the truck can't roll downhill while you are under it. Then remove the driven gear by removing the hold down clamp, then twist the driven gear out with a pair of channel locks. Look at both gears, and record their color. If you have already done this and have the right gears, you can replace the driven gear, or you can take the tailshaft housing off and replace the drive gear if needed in addition to doing the driven gear.

You can then adjust the speedometer output ratio by replacing the drive and/or driven gear with the correct ratio to give the right speedometer output, by looking up the correct color of gears to adjust the ratio.

There are a fixed number of drive/driven gears made by GM:

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