Speedometer not accurate

Tiny
AL52
  • MEMBER
  • 1963 CHEVROLET TRUCK
  • 5.4L
  • V8
  • 2WD
  • MANUAL
How can I correct speedometer? It only reads 35 mph when I am actually going 50 mph. Off by 30%. The transmission is a three speed manual. Speedometer gear has twenty teeth. I am sure either transmission or rear end has been changed. Probably rear axle because panhard bar seems to be too long.
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Thursday, January 5th, 2017 AT 1:25 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
You will need to find a service manual for that year and transmission. In it will be a chart that shows the rear axle gear ratio and the tire size. Match the column and row and it will list the number of teeth that should be on the speedometer gear.

Twenty teeth is already a real low number of teeth. Typical gears have in the order of twenty eight to thirty five teeth. Yours for a 1963 model might be a lot smaller in diameter than newer gears; then the low number would make sense.
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Thursday, January 5th, 2017 AT 1:56 PM
Tiny
STEVE W.
  • EXPERT
Do not be surprised if you have to pull the tail shaft housing to change the drive gear as well, the early ones had a different gear pitch and the later gears do not mesh correctly and chews them up. BTDT
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Thursday, January 5th, 2017 AT 9:13 PM
Tiny
CARADIODOC
  • EXPERT
Thanks Steve. It also occurred to me that some vehicles used a gear reduction assembly in the middle of the speedometer cable. No idea which vehicles or years, but I assume those were selected according to the specific needs of the vehicle.
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Friday, January 6th, 2017 AT 2:10 PM
Tiny
AL52
  • MEMBER
The gear that is in it looks to be a original, it is a metal gear longer than the plastic gears you find today. However a new shorter gear housing will fit allowing a shorter gear shaft but I can't find one with few enough teeth. Where could I find a gear reduction that would work in reverse to increase 30% to drive.
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Saturday, January 7th, 2017 AT 10:57 AM
Tiny
CARADIODOC
  • EXPERT
Start by researching some of the restoration companies. One of the most popular is "Year One, Inc.".

Do a search for "Pull-A-Part" and see if they have a salvage yard near you. I've been to 16 of them. A few have rows of very old cars and trucks.

Look at the classified ads in "Hemings Motor News". You can usually find them at Walmart and in your local library. There are many companies listed in there that restore speedometers. Someone there would be able to offer some help. The book is about an inch thick, and the covers are a yellowish-brown.
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Sunday, January 8th, 2017 AT 4:00 PM

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