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Speedo/overdrive curiosity

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yon_pie_eater

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Hi All,
I had my engine rebuilt during early summer, and I took the opportunity to have the sideshift gearbox and o/d looked at whilst it was out.
All is great with the engine, the 'box and the o/d, but with one curiosity. Some years ago I took my car down the autoroute and calibrated the speedo against the Kilometre posts, and found to my delight that the speedo was bang-on at 70mph.
Now, my gearbox came back from the repair shop without its speedo angle drive attached, so I had to buy and install a new one. Now, running the car up the autoroute, I'm finding that the speedo is reading 50mph when, looking at the kilometre posts, we're actually travelling at 61mph.
Any ideas? There's no gearing in the angle drive I suppose? Is there something afoot with the overdrive? Could it be that I have an overdrive from another?
Thanks
Andy
 
I could be off on this, but maybe the internal speedometer mechanism is becoming gummed up?

Mine is presently off as much as 7 or even 10 MPH, depending at which speed I'm traveling. I blame this on the Speedo and nothing else for I have a new angle drive, cable, lubrication and everything else left [supposedly] original.

From memory, my speedo was not working until I managed to clean up the gearing with the constant reusing of "Q" tips and PB Blaster (dial side facing up, of course, to avoid capillary action or the like contaminating the dial), via externally or by means of the "keyhole" route (that is, through the one and only applicable opening in the back after the cable is removed--could not succeed at that time to remove the glass bezel from the front).

This is not the correct way to thoroughly clean and re-lubricate the above, but I did get my speedo and odometer working but the needle acts nervous at certain speeds which tells me I still need of doing further cleaning, which may reduce my present miscalibration, maybe.

Paul
 
yon_pie_eater said:
Hi All,
I had my engine rebuilt during early summer, and I took the opportunity to have the sideshift gearbox and o/d looked at whilst it was out.
All is great with the engine, the 'box and the o/d, but with one curiosity. Some years ago I took my car down the autoroute and calibrated the speedo against the Kilometre posts, and found to my delight that the speedo was bang-on at 70mph.
Now, my gearbox came back from the repair shop without its speedo angle drive attached, so I had to buy and install a new one. Now, running the car up the autoroute, I'm finding that the speedo is reading 50mph when, looking at the kilometre posts, we're actually travelling at 61mph.
Any ideas? There's no gearing in the angle drive I suppose? Is there something afoot with the overdrive? Could it be that I have an overdrive from another?
Thanks
Andy
Click to expand...

Wouldn't matter if you did have a different O/D, even one with different ratios. The speedometer is only concerned with the final RPM out of the gearbox-O/D, only a rear-end ratio change would affect the speedometer (it's essentially a mechanical tachometer, calibrated to indicate speed instead of RPM). If you had an O/D with different ratios, your speed would still be indicated correctly but the tachometer readings would be different. Note that, if you work the accelerator just right, your speed doesn't change dropping into O/D, just the RPMs do.

I've had my speedometer apart more times than I care to think of (got tired of spending $200 every time something was wrong, plus I didn't have a speedo for a couple weeks). I finally bought a spare and started doing my own repairs. The speedo is (largely) calibrated by moving the indicator needle. If you look close, there is a small white dot at what would be -5 (minus five) MPH. This is the nominal starting calibration for the needle. You can check your starting calibration by pressing the needle stop in with a small blade--it's spring-loaded--then seeing where the needle rests with the speedo held in it's normal position (vertical). You can move the needle to a different location--it can be CAREFULLY pulled off its spindle--then moved to the correct side of the stop. If your speed is off a proportional amount--e.g. 1MPH at 10MPH, 2MPH at 20, etc.--just repositioning the needle should get you close.

The only other calibration I know of is a tiny brass screw on the brass bracket that serves as 'bearing' for the spindle. The bracket is two parallel brackets, and the small screw can move them together or apart, effectively shifting the plane of the speedo's driven disk relative to the moving magnets. My best guess is that this serves to increase or decrease the effect of the magnets on the disk, which affects the 'amplification' of speed, since otherwise indicated speed isn't linear.

The magnets are on a shaft that looks like a tiny camshaft (without lobes). There are no jewels or other bearings, and I think this little shaft needs lubrication else it presents too much resistance, which can cause the cable to wind up (hence a 'jumpy' needle).

If you have a needle that 'pulses'--i.e. jumps at a regular interval like once a second depending on car speed--you likely have a crack in one of the small fiber or plastic gears that ride on a large worm gear and drives either the tripmeter or the odometer.
 
I recently installed a 3.667:1 rear axle ratio on my 1954 100. I have calculated that the speedo now reads 11% less than my actual road speed.
Should I go to the trouble of having the speedometer re-calibrated? If I did, that would mean that if I were to revert to the standard 4.125:1 at some time in the future I would need to re-calibrate the instrument again. I wonder if there is a speedo angle drive that has a gearing that suits the lower numerical diff ratio?
 
I did some research when I put a Lempert 3.54 ring-and-pinion set in my BJ8 (that also reduces the speedometer indication by 11%). You (realistically) have two choices: a ratio adapter or new gears in the tripmeter and odometer (the speed indication can be recalibrated mostly by moving the position of the indicator needle). For reversibility, the ratio adapter is best because it can be removed easily (it's a small unit that goes inline with the cable). I chose the new gears because I intend to stay with the 3.54. It cost me over $200 and I was without a speedometer for a couple weeks. I think you can google 'ratio adapter' and get plenty of hits; they'll want you to count the number of turns the cable makes over a specific distance (100 feet, usually).

I suppose you could look for a speedometer that was initially designed for that rear-end in that car, but I suspect you won't have much luck.
 
Bob_Spidell said:
I did some research when I put a Lempert 3.54 ring-and-pinion set in my BJ8 (that also reduces the speedometer indication by 11%). You (realistically) have two choices: a ratio adapter or new gears in the tripmeter and odometer (the speed indication can be recalibrated mostly by moving the position of the indicator needle). For reversibility, the ratio adapter is best because it can be removed easily (it's a small unit that goes inline with the cable). I chose the new gears because I intend to stay with the 3.54. It cost me over $200 and I was without a speedometer for a couple weeks. I think you can google 'ratio adapter' and get plenty of hits; they'll want you to count the number of turns the cable makes over a specific distance (100 feet, usually).

I suppose you could look for a speedometer that was initially designed for that rear-end in that car, but I suspect you won't have much luck.
Click to expand...
Nisonger can recalibrate your speedo for the 3:54 rear end ratio.
 
You said the original problem started after the transmission and O/D were out of the car...

Perhaps the person who reassembled it left out the small copper washer that goes underneath the speedometer cable ? I had to replace my angle drive several years ago and my copper washer was missing.
Before it was replaced my speedometer needle jumped back and forth and then it quit working all together.
Here is a link to the washer I am talking about;
https://www.mossmotors.com/graphics/products/PDF/021-511.pdf

Ed
 
With the stock rear end gear set, my odometer was spot on but the speedometer was off by 10 %.

After I installed the Lempert gear set, the speedometer is spot on but the odometer now is slower at running up the miles by 10 %.

I like the speedometer being correct so I have do nothing to try to correct the odometer.

Ed
 
To correct the odometer and tripmeter you have to change the ratcheting gears on both. I think PA Speedo had to order the gears from England.
 
Hang on a minute. They make angle drives in various ratios for adjusting speedos to diffs.
I'll have to look....but bets are someone did that once, the shop lost it, now you've gone to stock, and it's wrong.

I'll find it and BRB
 
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