PSYCHO Speedometer
Thread Starter
Racer
Joined: Oct 2006
Posts: 396
Likes: 3
From: Buffalo New York
PSYCHO Speedometer
My speedo continues to baffle me. I have replaced the cable and the driven gear. I have 3:36 rear gears, the original ST-10 tranny, the brown 18 tooth driven gear, and 235-70-15 tires in my '76. The speedo, odometer, and trip meter all read 50% of actual speed and distance. I would suspect the speedometer calibration, but I believe that the odometer reading is a direct connection wirth the cable. With the original tranny, I can't believe that the drive gear is wrong, and I can't believe that it is worn because it's metal. Could it be the cable connection point at the speedo is worn?. I want to get it fixed this winter. I'm almost ready to go GPS! HELP
4th Gear
Joined: Aug 2013
Posts: 4
Likes: 0
From: Phoenix Arizona
I have ’73 vette that I have had for about a year. The speedo and odometer were off when I bought the car. It seemed to be off by a specific percentage. Consistent percentage at all speeds. I called around and found a speedo repair place in Phoenix Az (Dick’s speedo) who told me it was probably a calibration problem. He had seen this before and had a fix. It was calibration unit installed at the transmission. The calibration problem made both the speed and miles (odometer) to be off. It ran $200 but fixed the problem. It has been totally accurate for about 6 months (2k miles). The shop offered to put it on the dyno at about the 6 mo point at no charge to see if the accuracy was holding. I don't have the part number at my finger tips but the guys at Dick's Speedo could give it to you and if this is a good solution for you may be you can find a speedo shop in your area that can get it and install it (it's a small electronic box 2" x 2") . Good luck
Thread Starter
Racer
Joined: Oct 2006
Posts: 396
Likes: 3
From: Buffalo New York
Casey, it's a MPH speedo that spent most of it's life in Ohio. Actually, with a metric setup, it would read higher.
Thread Starter
Racer
Joined: Oct 2006
Posts: 396
Likes: 3
From: Buffalo New York
I have ’73 vette that I have had for about a year. The speedo and odometer were off when I bought the car. It seemed to be off by a specific percentage. Consistent percentage at all speeds. I called around and found a speedo repair place in Phoenix Az (Dick’s speedo) who told me it was probably a calibration problem. He had seen this before and had a fix. It was calibration unit installed at the transmission. The calibration problem made both the speed and miles (odometer) to be off. It ran $200 but fixed the problem. It has been totally accurate for about 6 months (2k miles). The shop offered to put it on the dyno at about the 6 mo point at no charge to see if the accuracy was holding. I don't have the part number at my finger tips but the guys at Dick's Speedo could give it to you and if this is a good solution for you may be you can find a speedo shop in your area that can get it and install it (it's a small electronic box 2" x 2") . Good luck
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
My speedo continues to baffle me. I have replaced the cable and the driven gear. I have 3:36 rear gears, the original ST-10 tranny, the brown 18 tooth driven gear, and 235-70-15 tires in my '76. The speedo, odometer, and trip meter all read 50% of actual speed and distance. I would suspect the speedometer calibration, but I believe that the odometer reading is a direct connection wirth the cable. With the original tranny, I can't believe that the drive gear is wrong, and I can't believe that it is worn because it's metal. Could it be the cable connection point at the speedo is worn?. I want to get it fixed this winter. I'm almost ready to go GPS! HELP
It helps if you mention what year car you've got...................
Thread Starter
Racer
Joined: Oct 2006
Posts: 396
Likes: 3
From: Buffalo New York
Mike, it's a stock '76 L-48, with original engine and tranny. I confirmed the diff ratio by rotating the rear tire and counting the driveshaft revolutions. 3 turns of the tire were exactly 10 turns on the driveshaft. The 18 tooth brown driven gear is correct for that ratio. The speedo needle moves smoothly, and does not jump around.
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Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
Some years ago, I got into it was a member known as Turtlevette, about my supposed speeds, and various affects on the car.....come to find out that the tire sites with a diameter/speed/ratio/rpm calculator are RONG!!!!!
I bought a GPS....hooked it up, and proved the point, the driven gear was off by some 15% in tooth count, and the tach was proven optimistic also....by 600 rpm at say 5000 rpm.....
NOW that I have the correct gear count, my speedo is dead on at any speed, up to 70, at 80 the correct speed is only 78.......
at 100 indicated, i'm really doing 94 I ran outta room and cancelled the test......IMO, it's designed that way......
I believe Garmin, not the old mechanical crap, and tires slip at higher speeds......which is my main suspicion not so much in the design....maybe.....I just dunno, except I believe my results.....
I bought a GPS....hooked it up, and proved the point, the driven gear was off by some 15% in tooth count, and the tach was proven optimistic also....by 600 rpm at say 5000 rpm.....
NOW that I have the correct gear count, my speedo is dead on at any speed, up to 70, at 80 the correct speed is only 78.......
at 100 indicated, i'm really doing 94 I ran outta room and cancelled the test......IMO, it's designed that way......
I believe Garmin, not the old mechanical crap, and tires slip at higher speeds......which is my main suspicion not so much in the design....maybe.....I just dunno, except I believe my results.....
4th Gear
Joined: Aug 2013
Posts: 4
Likes: 0
From: Phoenix Arizona
Mine (1973 vette) was off by 20% across the board. MPH and ODO. I don't know how long it had been that way but the car has 33 yrs of documentation and some 20 years ago (about 20k miles ago - probably needs to adjusted for the 20% error) the engine was rebuilt, clutch replaced and transmission rebuilt. It seems shortly after that you start to see receipts for the speedo being checked out. but apparently nobody was able to fix it. One shop even pulled it out. Maybe the owner was taking this car to the wrong places (??). Then when I got the car (about 1 yr ago) I looked into it and the speedo shop said it could be a calibration problem or a mechanical problem in the speedo or trans. Since it was off by 20% consistently at all speeds they thought it was calibration. That is when they installed the small calibration unit at the trans. If it is a mechanical problem with the speedo or trans gear, it will eventually just break or begin to be off again. But after 2k miles it is still accurate. The shop put it on a dyno and tested it up to 80mph. They don't test on their dyno above that for safety reasons. It was an extremely easy fix but as I said it did cost $200.
Thread Starter
Racer
Joined: Oct 2006
Posts: 396
Likes: 3
From: Buffalo New York
John, I think that you are talking about a gear reduction unit.One end has a male plug that fits into the driven gear and attaches to the tranny, and the other end has a female socket that the speedo cable threads onto. I know they are made in various ratios. The units I've looked at do not cost $200, so yours might be different. May be a simple solution if I can't solve the problem.
4th Gear
Joined: Aug 2013
Posts: 4
Likes: 0
From: Phoenix Arizona
When I crawled under the car after it was fixed, it pretty much looked like what you describe. I probably paid $25 for the part and $175 for someone to figure out which ratio/box it needed and then installing it! I suppose he needed to make a living too...right? oh well. At least it is fixed.
Instructor
Joined: May 2012
Posts: 119
Likes: 2
From: Kennebunk, ME
What about the Sleeve Assembly
Are you sure you have the correct sleeve assembly or that the previous owner didn't change it? Here is the chart from Willcox's website. When I change the rear on my 73 from 3.08 to 3.70
with a TH400 I had to change both the sleeve and the gear. The chart says there's only one choice for the sleeve for 76 with a 4-speed.
with a TH400 I had to change both the sleeve and the gear. The chart says there's only one choice for the sleeve for 76 with a 4-speed.
Thread Starter
Racer
Joined: Oct 2006
Posts: 396
Likes: 3
From: Buffalo New York
Are you sure you have the correct sleeve assembly or that the previous owner didn't change it? Here is the chart from Willcox's website. When I change the rear on my 73 from 3.08 to 3.70
with a TH400 I had to change both the sleeve and the gear. The chart says there's only one choice for the sleeve for 76 with a 4-speed.
with a TH400 I had to change both the sleeve and the gear. The chart says there's only one choice for the sleeve for 76 with a 4-speed.
Instructor
Joined: May 2012
Posts: 119
Likes: 2
From: Kennebunk, ME
The sleeve is the metal gear assembly that the plastic gear slides into (engages with). I figured it was probably the original but I thought it was worth asking. Adapter "E" is another name for the sleeve. Not sure why they call it a sleeve but that's what the other documents I have refer to it as.
Advanced
Joined: Jun 2013
Posts: 64
Likes: 1
From: Lambertville Michigan
This may sound like a dumb question but in the interest of spitballin... Do you have cruise control on your car?
The reason I ask is that perhaps "Bubba" visited and modified your transducer. If the internal gears were switched it could result in the output cable running from the transducer to the speedo to read a reduced amount (say 50%). When you are not the original owner you sometimes need to look deep. Just a thought. best of luck.
Is it possible that "Bubba" changed the gear ratio in the speedometer / odometer it self? Perhaps in an attempt to keep the original miles off the odometer.
The reason I ask is that perhaps "Bubba" visited and modified your transducer. If the internal gears were switched it could result in the output cable running from the transducer to the speedo to read a reduced amount (say 50%). When you are not the original owner you sometimes need to look deep. Just a thought. best of luck.
Is it possible that "Bubba" changed the gear ratio in the speedometer / odometer it self? Perhaps in an attempt to keep the original miles off the odometer.
Thread Starter
Racer
Joined: Oct 2006
Posts: 396
Likes: 3
From: Buffalo New York
Sorry Ely, but no cruise control. Willcox has offered to check the speedo calibration, but the odometer mirrors the speedo reading. I believe that the odometer is a direct gear drive from the cable, and doesn't require calibration. Strangely, the speedo didn't work at all when I bought the car. I installed a new cable, and it worked, but bounced around a little and was reading about 20% low. Now it's mysteriously reading 50% low.