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MG MGB Technical - speedo too fast, and out of ideas

I am getting around again to fixing the speedo on my 71 mgb, non overdrive. It has been reading about 25% higher than normal. Took a ride, with a GPS, and the 25% is consistent throughout the speed range, the odometer is also reading 25% high. Because the odometer is direct drive, it seems like everything would be accurate if the cable turned 25% slower. I have tried numerous 1280 T/mile speedos and they all read high by the same amount.

I also pulled the right angle drive and gear at the top filled transmission and it may be my imagination but it looks like the right angle drive is not a 1:1 ration, but more like a 1.25 ratio. (I turned the gear and viewed the output shaft rotation)


So question is, does anyone know what would make the speedo cable turn "faster" than it should?
J Johnson

Different year MGB transmissons have a different "turn-per-mile" output as do different year speedos..you have to have a matched set. Did your speedo ever read close to accurate?

Cliff
Cliff Maddox

Cliff, I have owned the car since 1979, and I do believe at one time it was working and reading accurately. I did replace the speedo several times, always with a 1280 T/mile but never could get it accurate again. Never touched the right angle drive or anything at the transmission end.
r schwartz

Tire size?


Dan Robinson

J. You need to take the car into a competent shop that does speedometer testing. They shoud be able to adjust, through the gearing, your speedometer.

Les
Les Bengtson

Well if you have never changed anything in the drive train that would matter to the speedo calibration, except the speedo itself, and maybe it was reading correct sometime in the past I would first swap in another speedo or two and see if this corrected things. Are you sure your trans is set up for a 1280 speedo? Even if the book says this is the correct ratio could your trans and earlier speedo have been swapped out at some point and now the 1280 speedo is wrong?

Hope you can figure it out.

Cliff
Cliff Maddox

J, I'm not clear if you tried the speedometer without the angle drive on the transmission. You mention it appears to be a 1:1.25 ratio. It should be 1:1. I know from experience that a side fill transmission with a 1280 tpm speedometer the speed readings will be low. I also had another car with a top fill transmission and a 1000 tpm speedometer, speed readings were too high. If you haven't tried it without the angle drive I would remove it and see what happens. You may have to use the longer speedometer cable for OD transmissions.

Clifton
Clifton Gordon

There is a realitively simple way to find out how many turns per mile your driveline is outputting (OUTPUTTING?) to the speedo head.

Remove the speedometer cable from the back of the speedo head. Inflate your tires to their normal inflation. Place a piece of tape on the end of the cable doubled over to act as a flag to allow you to count cable revolutions. Mark off 52.8 feet on a flat surface. Push the car along (in neutral) and count the revolutions of the tape at the end of the cable as you transit the 52.8 feet. Multiply the result by 100. With careful preparation and execution of this simple test you will know what your tires, rear axle, and out put gear on your gear box require for an accurate reading.

If you send your Speedometer with the turns per mile you determined above to these guys: (no affiliation, etc.) www.gaugeguys.com or APT Instruments, you will get back a working speedometer calibrated for your application.

warmly,
dave
Dave Braun

pull the speedo pinion out , if it has a white end its 1280tpm and if its red its 1000tpm
ste
Ste Brown

Thanks for the direction, I did the push test on 52.8 feet twice and got somewhere between 12 and 13 both times, which tells me that the cable is delivering 1280 T/mile. It has a white pinion which confirms the T/mile. The thing that I can't understand is if T/mile is right at the cable, why is the "odometer" reading high?
I thought the odometer was a direct drive, and therefore not subject to any recalibration.
J Johnson

Sounds like someone put a 1000 tpm speedo head in a 1280 tpm speedometer case. Beg, borrow or steal a working 1280 speedometer and check it out.

dave
Dave Braun

Well I think I resolved the issue, thanks to all your suggestions. Dave I think you are exactly right, the speedo I got from Ebay had the guts of a 1000tpm with a 1280 tpm faceplate. That explains the consistent speed and odometer overreading. Also I did the road test, and now know the cable is turning at 1280 tpm, which is correct. I pulled two other 1280 speedos out of my grab bag, one was reading high, and the other reading low, the good news is that the odometer was spot on in both. So...now I know it is either off to the calibration shop, or to my local british parts store, but this time with my GPS, to make sure I get one that works. Thanks so much for helping everyone. This board has never let me down.
J Johnson

my speedo reads anywhere between 60 mph and 120 mph at 3300 rpm. infact, while just drifting forward at a stop light it shows 20 mph, then gets stuck.

i've given up on fixing mine, just use the rpm's now...

Ken
70 B
Ken Harris

This thread was discussed between 20/06/2007 and 26/06/2007

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