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TR6 Need help with '74 TR6 speedometer.

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2wrench

2wrench

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Hello, to everybody. I've been off the Forum for a while, but now I'm back and looking for some assistance.

My speedometer is not calibrated properly after installing my overdrive. I took it to a mechanic who looked at it and
said: "You need a new speedometer head."

Okay. So can anybody point me in the right direction to get a speedometer head? I would think that if a good speedo shop
could rebuild what I have, that that is what would have been recommended.

So what is the best course? I have not seen anything on line nor at the usual parts suppliers.

Any help is appreciated.
 
Or... visit gaugeguys.com (APT) and have them build you a ratio conversion box so you don't have to touch your speedometer at all. The ratio box converts turns-in to the needed turns-out so your existing speedometer and odometer will read correctly. General prices ($95 to $125) are on their web site.
https://www.gaugeguys.com/speedcorrect.htm
 
Installation of Overdrive should have no effect on the speedometer as it connects behind any gearing. Why do you think it's changed? Did you change the rear end ratio? That would have an effect.
 
JerryVV said:
Installation of Overdrive should have no effect on the speedometer as it connects behind any gearing.
Click to expand...
That is true enough, but not all TR6 gearboxes and overdrives have the same speedometer drive ratio. Early TR6 (and previous TRs) all had a 5:2 ratio in the speedo takeoff, but later TR6 had whatever ratio made the speedo calibration come out to 1000 tpm. I'm guessing that 2wrench's OD came from an earlier TR (or some other car entirely, Volvos are popular donors).
 
A huge thank you for each of your replies and input. I am a bit embarrassed to admit that what I finally did was to pay the Palo Alto, CA speedometer shop to
repair/rebuild/calibrate my speedometer. It works well and I am happy with it. It was a pricey repair, though. $500! I just wanted it done so badly that I paid the price.
After downloading the PDF and thumbing through it, I knew it was time to pay up and just get this thing done. Now I have returned the car to the Pros once again to
address heater core issues. Wishing you all a very Happy and Prosperous New Year!
 
I liked the sound of this, Doug, but I have had such a fit getting the speedo angle drive installed without leaks that I would have been reluctant to
"mess with success."
 
TR3driver said:
That is true enough, but not all TR6 gearboxes and overdrives have the same speedometer drive ratio. Early TR6 (and previous TRs) all had a 5:2 ratio in the speedo takeoff, but later TR6 had whatever ratio made the speedo calibration come out to 1000 tpm. I'm guessing that 2wrench's OD came from an earlier TR (or some other car entirely, Volvos are popular donors).
Click to expand...

Right, Randall. I bought that overdrive off the net from a gentleman selling rebuilts at very good prices. Problem was, I really didn't know its history. Made it a little
more interesting.
 
A few days after I finally got the restoration done on my TR3, I figured that it was time to see how far my speedometer was off, as I had made a couple of changes in the car that would affect the reading. I had installed the HVDA Toyota conversion, and also changed the tire size to 185/70-15. I stuck my GPS on the windscreen and set out to see about how far off the speedo was. I brought the car up to 30mph according to the GPS and glanced at the Jaeger speedometer...oh my, it read 30mph. Well, that just at slow speed, right?? Let's go faster...40 mph, then 50, then 60, all the way to 85 mph, and that Jaeger instrument read exactly the same as the GPS.

I got lucky with two changes that surely would have resulted in speedometer error just happily canceling each other out. Go figure.
 
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