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TR2/3/3A Speedo reads way too low

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doc50

doc50

Jedi Trainee
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Does anyone know of a ratio-adjusting piece that will change the speedo to read about 15% higher?

I put one in our 71 Revcon motorhome to correct that speedo. It was a simple piece that had male threads on one end and female threads on the other and it went on the lower terminus of the cable. I'm sure the threads are different in the TR, though.

Or can we actually change the gearing in the speedo head?

Thom
1959 TR3
TS34909L(O)
 
Any speedometer shop should be able to make up a "ratio adapter" for you, with whatever ratio you want. It's been a few years since I asked, but back then the cost was about $100 and as I recall included a short cable to run between the adapter and the speedo head.

Changing the ratio inside the head is not out of the question, but finding the gears to match may be a bit difficult. You'd probably either have to machine them directly, or maybe you could find a stock gear that could be fastened to the existing shaft and crank pin (or whatever that thing is called).

15% is an awful lot. Why do you need that much change? Are you running tractor tires?
 
Just a few years ago I sent a speedo in to a company in No. CA to have it rebuilt. I had installed 3.45 gears in the rear axle. I requested they make appropriate changes to adjust For the difference. It came back rebuilt BUT without the adjustment. It was about 10% off. I contacted Nisongers and sent them the speedo. They corrected it for me.

Thom,
could you be running a speedo from a different car or axle ratio?
Charley
 
That's why I was asking the reason. John's way is probably easier, but I used simple home-made electromagnets to both remagnetize and weaken the magnet in the speedo. Here's the DC magnet I used to make it read higher

and the AC magnet to make it read lower
 
Ooops, I think I know why the speedo may be off. I bought this car as ''8 boxes o'stuff'', including an OD trans (the clue to this mystery)...So when I put it all together, it all worked well but the speed read low. My tag says there was no OD trans in the original configuration.
I am NOT sure of the amount, but I threw out 15% as an approximation. When speedo reads '90mph', it is ACTUALLY travelling at around 70mph (by GPS), so my goal is to have it correct at the speed I drive freeways.
I will look for a ratio adjuster (as I put on our 1971 Revcon that had a similar problem when I changed final drive ratio).

Thom

TS 34909L (O)
 
The real quick and dirty way to do it (works better for minor adjustments than major adjustments) Drive by one of the radar things they put by the side of the road to keep us honest and check your speed, aim for about 60 mph, see what the speedo reads alternatively time your speed with a GPS or by mile markers if you are taking 60 seconds to go a mile you are going 60 mph. Once again note what the speedo reads. Not take it apart (not going to go into it here, but not that hard, you need to take the mechanism out of the case, but leave the face and dial on. Now turn the needle/mechanism to the indicated speed at 60 mph, then hold the moving part that moves with the needle in the back gently but firmly in place and twist the needle to 60 mph. You should be in good shape for highway cruising readings, but as I am sure others will note it will still be off at the high end and the low proportionately more as speed goes up and down.

I used small powerful magnets to get it close, and the above method to get it more precise.
 
Thom,
putting an overdrive unit on the car would not change the speedometer if they were both Triumph trannies. But if you put a speedometer from a 4.1 ratio( which usually came with the overdrive outfitted cars) into a car with a 3.7 axle ratio, you will get a difference at the speedometer.
Charley
 
What Charley said. The speedo is the same for OD or not. The drive gear comes off the output shaft in both cases...
 
Have any information on the overdrive? Maybe one from a Volvo?

Scott
 
charleyf said:
But if you put a speedometer from a 4.1 ratio( which usually came with the overdrive outfitted cars) into a car with a 3.7 axle ratio, you will get a difference at the speedometer.
Click to expand...
Good thought, but the difference is only about 10%. Reading 90 at 70 is almost 30%! You can tell if it's the speedo for a 4.10 axle by checking the calibration number on the face. A regular 3.7 speedo will have a number around 1150 or 1182; while a 4.1 speedo will be 1350 or something like that.
 
"A regular 3.7 speedo will have a number around 1150 or 1182; while a 4.1 speedo will be 1350 or something like that."

Are those numbers supposed to be an approximation of revolutions per mile?

Thom
TS34909L(O)
 
Yes, that's the speedo calibration number, in turns (of the speedo cable) per mile (or per kilometer if you have a kph speedo). The numbers were not precisely correct even when new; because of the way the odometer and trip meter work, the calibration number has to be a multiple of either 20, 25 or 32. So not all ratios are possible, and the factory used the one closest to truth. For early cars there was only one each mph and kph calibration per final drive ratio, but later on they started fitting different calibration numbers depending on whether the car was equipped with bias ply or radial tires.

But the speedometer was originally calibrated to match the number on the face, even if it didn't exactly match the overall ratio quite right.
 
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