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Speedometer Recalibration

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So_Calif_Guy

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So I finally got t <font color="blue"> </font> he overdrive from QuantuMechanics.com installed properly. After some leaks and a bad switch, it is running like a dream.... (John was extremely helpful and thorough - highly recommended) However, as I installed an OD into a non-OD car, my speedometer is reading ~10 mph higher than I am really traveling. My mechanic said that there are two different types of Speedometers... can anyone elaborate? Do I need a new speedometer or can I get mine recalibrated... I am located in the San Diego area. Thanks for the help guys!
 
Sean, it's Deja Vu all over again. I had the same exact experience as you with Quantum, speedo, leaks, etc. All is resolved now. I ended up sending my speedometer off to Nisonger in NY (yes, I know, west/east coast) and went ahead and had them completely overhaul my instrument (it needed it badly). There is a trick that will require you measuring the rotations of your speedo cable over a very short distance that you can roll your car. I have forgotten but if you want the process, just ask and someone with pop up with it from the forum.
Good luck.


Bill
 
Just went through the same thing, mine read 10mph slow though.
Pulled the speedo out, (remove the drivers seat so you can lay on your back with legs up). Because my odometer was reading o.k. all i had to do was pull the back off of the speedo rotate the needle drum to 50 and while holding it move the needle to 60.
All done within 1/2 hour.
See this link for all speedo answers, worked for me.
www.mywebpages.comcast.net/rhodes/speedo.pdf
 
Airfix,

That link comes back with an "unknown host". Am I doing something wrong??
 
Maybe I'm dense... but how does changing to an OD gearbox affect the speedometer calibration?

A change in the rear-end would affect it and many cars originally equipped with OD would also have a different rear-end ratio fitted and thus a different speedo, but wouldn't just swaping in an OD have no effect?
 
I dunno. When I had Quantum build me an overdrive, my speedo was way off. I asked John Esposito the very same question and he had no answer. I ended up doing the flag-on-the-cable-push-it-down-the-street trick, sent it to Nisonger, and, voila, everything is good now. I, too, thought the same thing. Go figure.


Bill
 
I guess that I got lucky with mine. John had my OD installed in CT and on the way home my wife and I were side by side on the Merritt Parkway. I asked to verify the speed at 10 mph intervals from 40-70 and I was off about 1-2 mph at 70 compared with her new Montego. That to me was negligible.

In the past, cars were geared in the transmission with a gear that was determined by the rear axle ratio. A 4:11 rear may have a green 42 teeth gear and a 3:73 would have a white 39 teeth gear. These drive gears for the speedo controlled the speed of the cable up to the speedo head, thus determining the reading to the driver. Now it's all done electronically via computers and sensors.

I haven't seen the breakdown of the TR6 transmission to see if there is s gear inside for this. But I think that I read where there were two different differential ratios, one being 4:11 and the other around 3:71 to 1. That would require some way of getting the same speedometers used in all cars to read correctly based on the axle ratio installed.
 
I'm not sure what kind of car SC guy has, but TR3's and 4's only had one gear coming off the tailshaft for the speedometer. All the magic was done inside the instrument via different gears for the cumulative and trip odometers and different spring rates for the speed reading. Which is why you can semi adjust by taking it partially apart and holding the needle and rotating the disc to get the desired speed reading but it will always be slightly off just above and below that speed. And your mileage will be always off unless someone puts in the correct gears. Biggest problems always arise from a big change in gearing, eg 3.7 to 4.5 or 4.1 to 3.5. Or changes in tire size will always throw things off significantly. I usually take someone with me with a stopwatch and time miles on the interstate at several different constant speeds and pick the most useful range.
Tom Lains
TS8651
TS58107
 
Just a comment about the Rhodes document and speedo calibration....

When you determine your speedo is reading low, pay equal (if not more) attention to your odometer error. If you've got an odometer error the Rhodes document won't help much. What you're looking at to fix that is either 1) a different speedo, 2) a rebuilt speedo with different/changed gearing, or 3) an inline (cable mounted) ratio box to match the speedo to the car's gearing. I don't see any point in recalibrating the needle of a speedometer if the odometer still reads incorrectly. Short of a gearing change (inside or out of the gauge) you can't address odometer errors. I believe an odometer needs to be close to accurate.
 
Indeed speedo error is often not linear (e.g. a weakened spring) and the best you can get with a DIY calibration is to get it pretty accurate over a speed range that is important to you. I set mine for accuracy in the 40-50 range as that is where the speed traps tend to be in our area.

Odo accuracy may be more of a personal preference. Back in the day when there was a TSD rally almost every warm weekend it was handy to have a good odo -- though even those required you to compute an adjustment to match the rally masters odo.
 
I tend to agree with Paul. Did they in fact make two different speedometers for the two different rear end ratio's? If so, how can you tell the difference in the speedo's?

Bill
 
Regarding the different speedometers.... On most Smiths/Jaeger gauges you'll find a three or four digit number adjacent to the odo or trip window. This will not be the model number, it is the turns-per-mile (TPM) number. I don't understand why there would be different speedometers unless Triumph used a different differential on cars with OD.

For me it's important that the odo read correctly if for no other reason than keeping track of maintenance. However, as stated by Geo Hahn, it's a matter of preference.
 
From page 4 of the Rhodes manual:

"Depending on the calibration required, the worm on the input shaft may have 20, 25 or 32 teeth (there could be others but I have not seen them). It appears that 32 teeth were very commonly used on the “old” and “intermediate” versions, with 20 and 25 are also seen. 20 and 32 teeth were used on the “new” styles. There was a wide variety of gears used on the odometer wheels to provide the final calibration. The calibration of the odometer is the number of teeth on the worm gear multiplied by the number of teeth on the
odometer wheel gear. This gives the number of input shaft turns for each odometer shaft turn."

Then again from page 6 of the same:

"New Style: Single main & trip odometer frame and drive (TR5/250? and TR6)
Type 1: 32 tooth worm gear (all 100, 120 mph) (MG only?)
Type 2 20 tooth worm gear (all 140 mph?) (TR only?)"

The Moss catalog shows a different design between the standard four speed and the O/D, but there is only one part number for each speedometer pinion gear for either transmission.

If that is the case, then the changes would have to be at the head, to allow for the different axle ratios and the possibility of different counts of teeth on the four speed than the O/D transmission pinion gears.

EDIT at this point...

And finally this from the Rimmer catalog on line:

"Speedometer, MPH (SN6411/04) - (1) - 218831R
Reconditioned exchange.
3.45:1 rear axle, miles per hour. P.I. cars.

Speedometer, MPH (SN6411/06) - (1) - 218827R
Reconditioned exchange.
3.7:1 rear axle, miles per hour. Carb cars to CF35000.
Speedometer, MPH (SN6411/11) - (1) - TKC2426R
Reconditioned exchange.
3.7:1 rear axle, miles per hour. Carb cars CF35001 on."

So there are at least three versions based on the axle ratio, but no apparent difference for 4 speed versus O/D from what I can see.
 
[ QUOTE ]
On most Smiths/Jaeger gauges you'll find a three or four digit number adjacent to the odo or trip window... it is the turns-per-mile (TPM) number.

[/ QUOTE ]

Specifically, this is the number of times the speedometer cable turns each mile.

[ QUOTE ]
I don't understand why there would be different speedometers unless Triumph used a different differential on cars with OD.

[/ QUOTE ]

I believe that is why.
 
From 1954 thru 1981 for the TR series there were available at various times the following rear axle ratios: 3.45, 3.71, 4.11, 4.34, and 4.55. coupled with a rolling tire diameter that is probably always 10 to 20 or more percent different from the original stock size should leave you with a lot of opportunity for experimentation. Unless of course your car is totally stock. On rebuilt cars it's anybody's guess.
Tom Lains
TS8651
TS58107
 
Tom, I don't even want to attempt to do the math on those combinations. But you are correct. It's just more combos than any of us thought.
 
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