Fixing/Tuning Speedometer
Thread Starter
Cruising
Joined: Mar 2014
Posts: 11
Likes: 0
Fixing/Tuning Speedometer
On my '78 the Speedometer is off by about 10 mph or so anywhere under about 50mph. For example:
Going about 20-25 through a neighborhood, says I'm going 10-15.
Going down main roads at about 45- 50, says im going 38 - 45.
As I go faster it gets closer, but normally i just go as fast as everyone else is
Is this a quick fix or a common problem? If it's just a quick fix could you tell me how?
Thanks.
Going about 20-25 through a neighborhood, says I'm going 10-15.
Going down main roads at about 45- 50, says im going 38 - 45.
As I go faster it gets closer, but normally i just go as fast as everyone else is
Is this a quick fix or a common problem? If it's just a quick fix could you tell me how?
Thanks.
Le Mans Master
Joined: Aug 2010
Posts: 5,339
Likes: 546
From: Central Texas
There is a drive and driven gear in the tail shaft housing. The driven gear is attached to the end of speedometer cable where it goes into the tail shaft housing.
These gears can be altered to speed up or slow down the speedometer reading. You need to pull the driven gear out of the transmission and count the number of teeth on it first. Then you can change to a gear with more or less teeth to correct the problem. I believe the rule of thumb is a 3mph increase or reduction per tooth.
Search the forum for more information as this is a very common problem.
These gears can be altered to speed up or slow down the speedometer reading. You need to pull the driven gear out of the transmission and count the number of teeth on it first. Then you can change to a gear with more or less teeth to correct the problem. I believe the rule of thumb is a 3mph increase or reduction per tooth.
Search the forum for more information as this is a very common problem.
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
First step is to check if it's the speedo that is out of calibration, or if you have the wrong speedo gears (in the transmission).
Check your odometer against a known distance (best is to use a GPS). If the odo reading is off the same amount the speed is, you need to change the speedo gears.
If the odo reading is correct, the speedo itself is out of calibration. There is an adjustment inside the speedo, but it will only go so far. Over the years the speedo anchor demagnetizes, making the speedo read low.
On my '71 it read way low, yet the odo was dead on. The anchor can be re-magnetized but there's only a few places that can do it. Since my speedo had other issues (bad faceplate), I ended up just buying a complete new speedo. Wasn't all that expensive, a bit over $200.
Check your odometer against a known distance (best is to use a GPS). If the odo reading is off the same amount the speed is, you need to change the speedo gears.
If the odo reading is correct, the speedo itself is out of calibration. There is an adjustment inside the speedo, but it will only go so far. Over the years the speedo anchor demagnetizes, making the speedo read low.
On my '71 it read way low, yet the odo was dead on. The anchor can be re-magnetized but there's only a few places that can do it. Since my speedo had other issues (bad faceplate), I ended up just buying a complete new speedo. Wasn't all that expensive, a bit over $200.
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
thegazman
I had to change the drive gear inside my transmission tail housing on my 79 th350 transmission as it was worn causing the driven gear to skip a tooth once in awhile. Another option is to install an after market electronic speedometer. They can be calibrated right on the money.
Le Mans Master
Joined: Oct 2010
Posts: 6,937
Likes: 848
From: melbourne florida
2023 C3 of the Year Finalist - Modified
mine is the same, according to gps I'm 12 m.p.h slow around 30 and dead on at 70. thats not a gear issue. i need it recalibrated and possibly remagnitized I've been told that by three speedo shops. hard to get 3 car guys to agree on anything. now i just need to stop being lazy and pull the unit and send it off.
