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MG MGA - Another speedometer inquiry:

I have another speedometer question to which I'm guessing someone out here knows the answer. After opening up my speedo and cleaning it and oiling it, I was successful in stopping the bouncing needle. But I still have another problem, which I was kind of hoping (but thought not) that the cleaning would resolve. The speedometer seems to be pretty accurate up to about 45 mph or so. But, as I go faster it becomes more and more inaccurate. So much so, that it reads 80 when I figure I'm doing somewhere around 60. I'm positive I have original wheels and the standard tire size. It's possible the tranny has been redone at some time. It works too good to be original. But I don't know if that would have anything to do with it.
So, does anyone have an idea for a speedometer that reads too fast?

P.S. I marked the little spinning plate (whatever it's called) before I took the speedo apart and put it back as it was so I don't think it is a calibration thing. But I could be wrong.
P.R. Fahey

If the speedo is accurate up to 45 then there is no problem with any other components on the car (wheels, axle or transmission). If you can check your speed with a sat. nav. you can get an accurate comparison of speedo vs actual at 10 mph intervals or so. It's possible that your speedo has a more linear error than you first thought, but is just much more obvious at higher speeds. (see last paragraph)

The gearbox should not affect the reading reading if it has the correct speedo drive. An MGB gearbox drive, a five speed box or a lower ratio rear axle (modifications that do affect speedo reading) would all make the speedo read low and not by the amount you mention.

One other thing, if you fit an early MGB Jaeger speedometer it will read high by 30-40% so that could well be the problem!
Neil McGurk

I may be out of my depth here but as I understand the workings of a speedo there is an aluminium disc attached to the opposite end of the needle axle. This disc is in close proximity to a bar magnet which is capable of revolving and connected externally to the speedo cable. When the car is moving the magnet rotates and induces a current in the aluminium disc causing it to start rotating itself. The amount of rotation is limited by a hairspring attached to the needle axle. This hairspring should be linear i.e. amount of axle rotation is proportional to the axial force caused by the revolving magnet which in turn is proportional to the car speed. If the hair spring is damaged (and it extremely delicate) then it may become non linear.
I think if I were you I would do a bench trial as follows:- Set up the speedo on the bench with speedo cable attached and attach the opposite end of the cable to an electric drill. For my bench work I used a battery operated drill but you may need a mains drill with higher speed to cover the full speed range. Now all you need is a watch with a second hand for the timing and the odometer reading for the distance. Distance divided by time is the car 'speed'. For this timing you will need to keep the speedo reading constant of course and you may need an extra pair of hands.
When I looked at my speedo I didn't calibrate it as above because I was only trying to find the cause of a non functioning odometer. Hope this helps Mike
PS presumably you have checked whether your speedo number, on the front face, is correct for your model.
m.j. moore

If the operation of the needle is smooth and continuous as you accellerate; it doesn't bounce; and there is no point where it jumps or sticks, you may have the needle misplaced on the shaft and/or you have misadjusted or seriously bent the hairspring. The difference bettween the 45 and 60 readings show a lot of gain in the hairspring requiring more turns being uncaptured or a very thick oil or grease on the needle shaft bearing. IF this is true, in order for the 45 MPH indication to match actual speed, then the needle is also mislocated.

C Schaefer

Something else that puzles me , my speedo is accurate by my sat. nav. but the odo. records approx. 6% low. I run a 3.9 diff. ? ?
S Sherry

S Sherry, thats an easy one! Lets assume your 6% is accurate together with your speed.

When the car was built the supplied speedo was calibrated to run about 6% fast (legal requirement for speedo reading was something like +2% to +8% at that time - someone correct those figures if you have exact information) and most manufacturers tried to be as close to the top end as possible (to make drivers feel like their cars were quicker, better high speed cruisers etc.)

Your 3.9 diff and tyre combination probably makes your car about 6% faster.

Your odometer (was more accurate when new) now runs 6% lower, because no-one told it about the diff.

Neil McGurk

PRF It's just dawned on me that a speedo should always be recalibrated after the speedo needle has been removed because it's possible to put it back in a position which would give a positive or negative offset, although a positive offset would be obvious at zero speed. However, this would not give rise to your apparent non linearity. mike
m.j. moore

PRF, I could believe that a possible cause for non linearity may be a non concentric aluminium disc. Because the rotational force on the disc will be greater if the disc is closer to the rotating magnet. If for example the disc is buckled it may become closer to the magnet as it rotates thus causing more deflection at higher speeds. I put this forward because when I removed my speedo insides I noticed that you had to be very careful how you gripped the mechanism with your fingers in order to avoid touching the aluminium disc.
Whatever it is though it would be a good idea to do a recalibration. mike

PS When you oiled it did you use a good watch or clock oil for the needle bearings? I think it needs some oil because it is a non-jewelled speedo. Any watch repairers out there?
m.j. moore

I had my speedometer repaired at Portland Speedometer Inc. in Milwaukie. They did a great job and calibrated it as well. They can test it without removing it from your car by putting your car on their rollers. They also made me a new cable to replace the bum one that I had purchased from a well known supplier. The price was fair as well.
Ed Bell

Manoschevitz! Lots of info here. Thanks! After reading the various suggestions of things to check, I'm thinking that incorrect calibration is probably the culprit. I did mark it before I took it apart so I would be able to put it back the way it was. I was also pretty careful in handling it when I had it apart. And I used a good machine oil, very sparingly, to lube it. Before the cleaning, I seldom paid much attention to my speed because the needle bounced so erratically that it was pointless. (a pointless needle?) But one thing about it, when it was doing all that bouncing around it was indicating a much higher speed than I was traveling. But I just figured that was because of the bouncing. So, the mark I put on it before I took it apart, assuming it was calibrated, could very well have been bogus. I also noticed when I had it apart that there was a black mark on the disc but I didn't even think to look for another black mark to match it with, since I had put my own mark on it.

I'll will have to read back through some of the notes on how to calibrate the speedo and then I'll probably
take it to Portland Speedometer, as Ed Bell recommended.

Thanks again for all the responses. This is a great site!
P.R. Fahey

Point taken Neil. As a matter of interest the electronic speedo in my boxer six cyl. Subaru is accurate in 60/ 80 kph range but the faster I travel the higher it reads. At an indicated 120 kph my sat nav. reads 110kph. A local police patrol told me that all Subarus read high, where as my wife's current C class Merc is only 1% high. The local G.M 6 cyl Holdens are spot on where as the 6 cyl. Fords read high. Ours is not to reason why. Sean
S Sherry

Sean, yes current legislation still has a positive (higher) reading for upper tolerance, but (the legislation!) is not linear. I think the bottom tolerance is nearer to, or even at, +0% as component accuracy and repeatability (more important)is much higher today.

It's some years since I worked in Quality Assurance in the car assembly business, so maybe someone could give the latest legal requirement for speedometer reading? (I know the Dept. of Transport here in UK do not like this information being given out in public.)

Neil McGurk


It is now all in ECE regulation 39. It takes a bit of getting your head round but Google found this from a reply in the House of Lords.


"The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed.

A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. Her Majesty's Government have no plans to introduce instrument tests."

End of quote

Of course, as our cars pre-date 1986 none of this really applies. Someone can probably correct me but I think there is nothing in the MOT about speedometers.

The EEC regs also say that speedometers used in countries with imperial measures for distance must show speeds in mph. So kilometre only marked speedometers are technically illegal.

Malcolm Asquith

This thread was discussed between 11/03/2009 and 12/03/2009

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