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MG MGB Technical - Speedometer conversion from 1000tpm to 1280?

I recently converted my 77MGB to an overdrive transmission. The transmission is the earlier kind with a dipstick filler with overdrive in 3rd and 4th. My speedometer is 1000TPM and I think I need it recalibrated to 1280TPM. Does this number sound correct for this type of transmission? Also, are geared adaptors available so I don't have to recalibrate the speedo?

Thanks!
Craig Cootsona

Craig,

When I restored my '74 MGB a while back, I put in an o/drive gearbox. Rather than have the speedo recalibrated, I just put in one that is 1000 tpm (my original was the 1280). I was told this is the correct one for the o/drive. According to my service manual, with o/drive engaged in fourth, the speedo should read about 22 mph for every 1000 rpm (ie. 66 mph @ 3000 rpm, which my car does).

Sounds like you have the correct speedo already.
Simon Austin

Craig - The easiest way to correct the problem is to get the proper speedometer for the transmission. To my knowledge, the speedo gearing in the transmission cannot be swapped easiely if at all. A less espensive method is to have a ratio box made up. Using a navigator, with a stop watch, drive at a constant 60 mph (you may have to do this late at night to be able to maintain a constant speed) on the present speedometer between several mile markers on the freeway while the navigator uses the stop watch to time how long it takes for 3, 4, or 5 miles. Calculate what your actual speed was and what the ratio is between the actual speed and the 60 mph indicated. Take this ratio to a speedometer shop and have them make up a ratio box to go between the transmission and the speedometer. I don't know where you are in Washington state, but in the puget sound area, one shop that I know that will make up a ratio box is Tacoma Speedometer & Instrument at 10526 Steel St. S, Suite A in Tacoma.
Good luck - Dave
David DuBois

Thanks for the help guys! This forum is great! I'll check that shop in Tacoma and see what they can do.
Craig Cootsona

Craig, 1280 tpm is the correct speedometer for the dipstick four synchro transmission. I have travled that route twice. Clifton
Clifton Gordon

Craig: I should have a 1280 speedo in my parts box from my swap to a later year OD tranny. I'll check to be sure but if you want it its yours.
william fox

Were there any 1280 tpm speedos made that will fit in a 1977 dash? I thought that all later models used the same speedo, regardless of whether or not OD was fitted. This means that there may be no 1280 tpm speedos that will fit in the dash.

In that case, his only choice is to have his speedo recalibrated or buy an expensive variable ratio gear drive.
Paul Noble

or if you're concerned about originality we can swap OD trannies!
william fox

Remember that recalibrating the speedometer won't correct a wrong odometer reading.
George B.

Since the 1977 MGB uses a speedometer that is 100mm in size the older 1280 tpm speedometers will not fit. APT Instruments has a page on doing a ratio test. http://www.gaugeguys.com/ratiotest.htm Using a ratio adapter is your only option other than finding a OD unit from a 77 or newer car.

Jim
Jim Lema

OOOPs! I forgot about the dash change. What Nisonger Instrument? http://www.nisonger.com I believe they can recalibrate your current speedo. You have provide them some info about the turns per miles (turns over a given distance). I fellow club member had his midget speedo recalibrated this way.
william fox

Craig, I have the opposite problem. PO has put a later gearbox into my 74 Roadster so I have a 1280 speedo and 1000 gearbox. It is reading slow and I wondered if it had been fitted with a different drive gear to convert the ratio. I think I also have some friction in the speedo cable as it is not reading propotionally slow but just generally slow. It had been standing for years so I'll try that first. I found an excellent document on Smiths and Jaeger speedos (mostly Triumph based but also covers MG) try:
http://ourworld.compuserve.com/homepages/arhodes

or send me an email and i'll send it to you.
Richard
Richard Evans

I went to the speedo shop recommended by David and they set up a geared adaptor for me in under 30 minutes. It cost $75 but it was worth it. That way I can keep the original calibration on the speedo and my odometer will be correct both for its current milage reading and for function. I used the following equation to figure out the difference in speeds between instrument reading and actual MPH respectively: (1280/1000)= (x/60mph) solving for x gave me 76.8 The shop took that figure and designed the converter to those specs. It appears to be right on on the road. Thanks again for your help everyone! This site is a must-see for anyone with a british car! I will try to contribute to other postings where I can.
Craig Cootsona

Richard
If your 74 UK market car has the 100mm (4 inch) speedometer you should look for a Mk1 speedometer. These came in 1040 and 1020 tpm. It will not be correct, but closer than your current speedometer.

Jim
Jim Lema

This thread was discussed between 12/12/2002 and 13/12/2002

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