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MG MGB Technical - New Speedo Not Accurate (Calibration?)


I've got a problem with my speedo, it's a bit of a story to bear with me...

When I originally got my B (12 yrs ago) it had a Smiths 100mph speedo in it. It's a 1963 car, so I knew that wasn't original as it should have had a Jeager guage and I'm pretty sure all B's had 120mph speedos. I assumed the speedo came from a midget. The speedo had been fine all this time and as accurate/calibrated as you'd expect (pretty decent). One thing the speedo never had was a reset on the odometer - just a hole in the bottom so the reset plunger must have broke off at some point.

I'm in the process of preparing my B for historic road rallies, and a working odometer that you can reset is needed for this (as back-up for a tripmeter). So I got my speedo exchanged at Moss for a "proper" B 120mph speedo. The new speedo was appox '67 era, so it looks fine in my car.

However, now the new speedo is fitted, it was pretty apparent that it's calibration is way out. To check I got someone in a decent modern car to follow me. 40mph was approx right, 60mph was really just over 50 and 80mph was really 65.

Now I'm pretty sure my car has a standard axel and drivetrain, wheels/tyre sizes, etc.

So my question is, do you think it's the speedo that not been calibrated correctly or something odd/different on my car (given the non-standard speedo I'd previously had). If the latter, any ideas and how can I verify them?

Cheers,
Jamie
jamieo

What number is on the face of the speedo dial?
Tatty

I'm no expert here, but I'd guess when the 100 mph speedo was put in the car, the drive gear(s) for that speedo were installed in the transmission. When Tatty asks what number is on the face of the dial, this is where he's eventually going. If I'm not mistaken, normal MGB's have either 1000 or 1280 on the dial. This number is turns per mile, regarding the speedo cable.

I don't know how tough it is to inspect (or replace) those with the transmission in the car, but I'm sure there are those here who have done it.

Hopefully someone can help you identify it before you remove it (i.e. diameter & number of teeth). Which brings up a point. Did you call Moss and explain the problem?

Glad you're getting the car back on the road. Pat on the back to you.

Matt Kulka


I've exchanged the 100mph speedo now so no longer have it. However, I did note the numbers on the front, and kind of expecting something like this *may* happen I took photos of the fronts of the old and new speedo. I have those at home and can look tonight. I know they were different, and I do think the 100mph one was 1000 and the new one 1280.

So this explains my problem, now to the solution? If my transmission has been set up with 1000 I guess my options are:
- get a matching 120mph (1000 turn) speedo
- get the transmission out and have it converted to 1280

So can you get 1000 turn MGB speedo's? Is this what I should be asking for when I talk to Moss?

How much work is involved in converting the transmission to 1280? Is this more standard, is it something I should probably have done to the car at some point?

I have not yet spoke to Moss about this as I wasn't really sure what the problem was - was it the new speedo they supplied or a car/speedo mismatch (which it seems).

Cheers,
Jamie

Just an aside. I've hardly used my B for the last 7 years and have only recently got it back on the road. I drove about 50 miles at the weekend and it was lovely. Once again I am charmed by it :)

http://popdog.dyndns.org/interests/cars/pictures/mgb_lives/
jamieo


Ah, this page:
http://www.mgbexperience.com/reference/speedometers.html

seems to indicate what you guys have pointed me to is my problem. I need to check those photos tonight to find out what the numbers were on the old speedo.

Hopefully I can then find the correct matching 120mph speedo.

Cheers,
Jamie
jamieo

Jamie. According to the reference you listed above you should have a 1040 tpm speedometer if you have a 3 syncro transnission, if you have a 3 syncro OD it should be a 1020 tpm speedometer. You also said the speedometer is accurate at 40 mph and has big errors at 60 and 80 mph. If the speedometer drive gearing is not correct it should have the same percentage of error at all speeds and should not be correct at any speed but 0. Based on the information you have given it sounds like you have a bad speedometer.

Converting the transmission to different speedometer drive gears requires disassembly of the transmission

If you can get your hands on a handheld global positioning system (GPS) you can verify the the accuracy of your speedometer at any speed.

There is some additional speedometer information at
http://www.w-p-c.com/speedotach.htm

Good luck, Clifton
Clifton Gordon

You can also use the kilometer posts on a section of motorway, when used with a stop watch they can good practice for your navigator.
Tatty


Clifton thanks for that extra info.

Based on your information, my car, a 63, should have a 1060tpm Jeager speedo (120mph).

I do know my car doesn't have it's original (3 bearing) engine, but a later 18GB engine. What I do not know is if the transmission is the original or not.

It's a non-overdrive car and I know from original documentation that it was non-overdrive from new. So there's a chance that it's the original transmission, but I can't be sure.

So there's a chance that it could have a non-overdrive transmission from a 18GB era car (or in fact any other!). I haven't yet been able to check the photos, but I'm sure the old 100mph speedo I had in was a 10XXtpm speedo.

I guess I can only replace the speedo with a matching tpm one to the 100mph one I've just removed. I can then test it with a GPS to see how accurate it is. Hopefully then I can figure if the transmission is 1000/1040/1060tpm. Is this likely to be possible? I mean, there's at most a 6% difference in those tpm and that would translate to the speedo reading and could be within the normal tolerances you'd expect?

It's never easy is it!

Jamie
jamieo

I don't know if your looking for original/correctness, but there are speedo/tire shops that will cut the cable somewhere under the car and add a device to adjust it and then put it on a rolling road. It is then possible to get the car up to 60 or so and adjust the speedo to make it accurate as possible. I've not personally had it done, but have heard of people having them calibtrated.
J Arthurs


I'm not particularly looking for originality or correctness. I just want (relative) accuracy (for road rallying) and it to be reasonably period (again within historic road rally regs).

I've also got a tripmeter install, so using this over a known distance I can work out the %age inaccuracy in the odometer reading.

Jamie
jamieo

Jamie, Your error is around 16-17% at an indicated 60 and 80 mph. I agree the difference between 1000, 1040 or 1060 is within the normal reasonable variations. I guess the reference to a 1020 isn't correct. There was a thread on this subject a few weeks ago, it should be in the archives. I also agree the simple things never seem to be easy to solve. Hope you get it worked out, Clifton
Clifton Gordon

First thing I'd do is check the odometer accuracy. The odo is actually driven by the cable, whereas the speedometer section is a magnetic coupling that is subject to friction, incorrect return spring tesion and a host of other variables. You may find the odometer section is accurate and the speedometer section is not. If both are off by the same percentage, then it's time for gear changes or calibration.
Ken Lessig


Oh dear, I have problems :(

This was my old speedo:
http://homepage.ntlworld.com/jamie.oshaughnessy/misc/old_speedo.jpg

And this is the new one:
http://homepage.ntlworld.com/jamie.oshaughnessy/misc/new_speedo.jpg

So the old one was a 1472tpm (!!!) and the new one is a 1040tpm (I think it's a 1967 one).

Where the hell does a 1472tpm speedo come from and why is it in a B? (more to the point, why is it in *mine*)

So if my gearbox is geared for a 1472tpm speedo I'm pretty stuffed aren't I? Either find another 1472 speedo or get the transmission "fixed"?

Of course my car may have had a 1472 speedo and the tranmission for a 1280 speedo and it's been wrong all this time. I'd be a bit surprised though.

Ken - good point. I do think the odometer is out too. I did a run at the weekend and it read something like 70 miles. I'm pretty positive the distance I went should be in the 50-60 range.

Ideas?

Jamie
jamieo

Jamie,

What gearbox have you got in there? OD or non OD?

If the speedo is OK ish at 40 mph then I doubt it is a gearing problem - it is more likely to be the speedo - you didn't grease the top 6 inches of the drive cable did you?


Chris Betson

It's a non-OD box.

The accuracy tests I did weren't particularly scientific and, er, a woman was involved (sorry to generalize!). So the error may be a bit more linear that my figures above suggest. Then again, the speedo itself may also be bad.

I haven't greased the cable, but it was a bit greasy. Are you thinking it may be slipping and the faster it's going the more it's slipping?

Should the end be completely grease free?

Jamie
jamieo

Jamie,

Grease on the top 6 inches of cable can find its way into the speedo head and mess it up - you should grease the cable and wipe the top of it clean.

The new speedo you have is correct for the standard 3 synchro gearbox - if that is what you have fitted.

The later engine may have been fitted along with a later 4 synchro gearbox - do you have synchro on first gear?

It may also be a Marina gearbox!

The correct gearing also depends on the size of wheels and tyres fitted and the rear axle ratio.

The best way to determine what you have is to find a quiet stretch of road or car park and take the speedo off its cable, tape a piece of masking tape or sellotape to the cable end to act as a flag, mark the road where the rear wheel touches it and then drive forward in a straight line counting the number of times the cable rotates, stop when it has rotated ten times and mark the road again where the rear wheel touches. Measure the distance between the two marks.

Divide the distance between the marks in feet into 5280 and multiply the result by 10 - this will be the TPM you need for your speedo.
Chris Betson

Chris, thanks for the advice, I'll try that.

It is certainly a 3 sync box, I can tell when you shift down from 2nd to 1st (pretty much need to double declutch) and even just engaging 1st to pull away (it's much easier to select 2nd then 1st). I think it's a B gearbox, it's got the little dipstick in the right place under the dash anyway.

The wheels/tyres are standard size. I'm not sure about the axle, but I've got no reason to suspect it's anything other than ordinary (e.g. comparing revs vs speed in other Bs).

Thanks,
Jamie
jamieo

Jamie,

Its worth doing as someone suggested already, and checking the odometer calibration. I purchased a new speedo last year, and the odometer is accurate, but the speed always reads 10% less than im actually doing. I only found this out when my sister complained I was going to fast for her to keep up. Ive since had it checked on a rolling road.

Anyway, the fact the the odometer is accurate tells me that the gearing is correct but the speedo is out of calibration.

just a thought

Richard.

Richard


Another related question, how can I identify what gearbox is in my MGB? Is there any way?

It certainly is 3 synchro, but how can I tell if it's an MGB one and what era it is?

Jamie
jamieo

The front of a Moss catalogue or even a Haynes manual I think should show the marks.
Tatty

This thread was discussed between 17/02/2003 and 20/02/2003

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