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Jul 21st, 2025 | | | | Joined: Jul 2005 Posts: 66 Shop Shark | | Shop Shark Joined: Jul 2005 Posts: 66 | My speedo in my '55 Chevy truck is off by about 20 MPH. Where can I get a different gear to try and how do I know how many teeth I need? I had the original transmission in it and it was off. Now I have an overdrive in it and it's off the same amount. Could it be my speedo or is it likely the gear in the trans? | | | | | Joined: Jul 2001 Posts: 3,887 Cruising in the Passing Lane | | Cruising in the Passing Lane Joined: Jul 2001 Posts: 3,887 | if it was off the same amount with the original trans, it's unlikely to be the wrong gear .... more likely that the speedo mechanism is gummed up after 55 years, and if the speedo has ever been messed with it could just be the needle is mis-positioned - notice whether it starts to move as soon as you let out the clutch or 'jumps up' once you're moving faster [or drops to 0 before you're stopped]
Bill | | | | | Joined: Jul 2005 Posts: 66 Shop Shark | | Shop Shark Joined: Jul 2005 Posts: 66 | That's a darn good point about the needle. Does it just pop off so I can turn it. It is off about 20 mph across the board. It barely starts coming up when I'm in town. | | | | | Joined: Feb 2000 Posts: 4,886 'Bolter | | 'Bolter Joined: Feb 2000 Posts: 4,886 | How does the odometer track with mileage markers on the highway?
The odometer is gear driven and is a direct reading from the transmission, the speedometer is magnetic driven and relies on magnetic pull to spin the needle. A odometer error is due to transmission gearing, both odometer and speedometer error (if off same amount)is the gears, and just a speedometer error is in the speedometer head.
Drive the truck ten miles on the interstate, watch the odometer closely, then figure the difference between the two after ten miles. Say you lost 1 mile out of ten, thats a 10 percent error. Now count the teeth on the driven gear in the trransmission and reduce the number by 10 percent.
Smaller gears turn faster which add miles, larger gears turn slower and count fewer miles.
If the odometer is close and speedometer is still off, your speedometer head need cleaned and/or repaired. Most will loose a percent the faster you go, but around 35 they should track right on. As the magnet gets old it weakens and just can pull the needle like it used to and if the needle assembly is dirty, it just adds to the problem.
Joe | | | | | Joined: Jun 2009 Posts: 2,201 Shop Shark | | Shop Shark Joined: Jun 2009 Posts: 2,201 | That is a great post Joe, but aren't our older speedometer needles regulated by a spring instead of a magnet? That is all good information to tuck away in the recesses of my feeble mind as I have at least three speedometers to get working again. How does the odometer track with mileage markers on the highway?
The odometer is gear driven and is a direct reading from the transmission, the speedometer is magnetic driven and relies on magnetic pull to spin the needle. A odometer error is due to transmission gearing, both odometer and speedometer error (if off same amount)is the gears, and just a speedometer error is in the speedometer head.
Drive the truck ten miles on the interstate, watch the odometer closely, then figure the difference between the two after ten miles. Say you lost 1 mile out of ten, thats a 10 percent error. Now count the teeth on the driven gear in the trransmission and reduce the number by 10 percent.
Smaller gears turn faster which add miles, larger gears turn slower and count fewer miles.
If the odometer is close and speedometer is still off, your speedometer head need cleaned and/or repaired. Most will loose a percent the faster you go, but around 35 they should track right on. As the magnet gets old it weakens and just can pull the needle like it used to and if the needle assembly is dirty, it just adds to the problem.
Joe | | | | | Joined: Jul 2001 Posts: 3,887 Cruising in the Passing Lane | | Cruising in the Passing Lane Joined: Jul 2001 Posts: 3,887 | 'mater - the speedos do have a spring in them, but the magnet is what drives them .... and note magnets don't "weaken with age", that's why they're called permanent magnets, dirt buildup will be the problem Bill | | | | | Joined: Jan 2008 Posts: 4,903 'Bolter | | 'Bolter Joined: Jan 2008 Posts: 4,903 | Too fast usually indicates a weak spring. Too slow is dirt. Too slow is much more common than too fast. In fact too fast is very uncommon. An erratic needle usually means the cable needs grease | | | | | Joined: Feb 2000 Posts: 4,886 'Bolter | | 'Bolter Joined: Feb 2000 Posts: 4,886 | OK, I stand corrected..... but mine looses speed the faster I go and the odometer is dead on. I just assumed the magnet was causing this, maybe its time for another cleaning!
Joe | | | | | Joined: Dec 2009 Posts: 12 New Guy | | New Guy Joined: Dec 2009 Posts: 12 | My problem, speedo is slow to come up especially when it is cold.How do you clean them? 59 Apache 235 | | | | | Joined: Jul 2001 Posts: 3,887 Cruising in the Passing Lane | | Cruising in the Passing Lane Joined: Jul 2001 Posts: 3,887 | My problem, speedo is slow to come up especially when it is cold.How do you clean them? 59 Apache 235 check the tech tips link, top of the page Bill | | | | | Joined: Mar 2010 Posts: 78 Shop Shark | | Shop Shark Joined: Mar 2010 Posts: 78 | taking it apart is not a good idea unless you have instrumentation experience.I would sent it to Scott's in detroit family business they will get you fixed up.Lou.
1951 chevy 3100 short bed frame off resto.
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