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MG MGB Technical - Slow Speedo

I have just acquired a 1972 mgb roadster with overdrive.

I have checked the speedometer with a digital device and it is reading about 8 mph slow at indicated 30 mph.
Speedo is a '1280' which seems to be correct for this car.
Any thoughts as to why it is so far out, and whether it can be recalibrated?

Thanks

Colin
Colin Parkinson

Colin, I think the first thing to do is check what wheels and tyres are fitted to the car This website from Paul Hunt you will find helpful for tyre and wheel sizes..

http://www.mgb-stuff.org.uk/wheelstext.htm

8% slow seems quite a lot, so there may be something wrong with the speedo drive gearing. Lots of info on speedo's at this website.

http://www.mgb-stuff.org.uk/misctext.htm#speedo

Andy
Andy Robinson

Colin. Had a similar issue with my 79. Check your tire size. I had the wrong sized tires (smaller) and my speedo was off. After replacing them with the correct 185 x 70-14 vs. 185 x 65-14 the problem self corrected.

Cheers

Gary
79 MGB
gary hansen

Colin
It would be interesting to see what the error margin is at highway speed
If the percentage of error is the same at that speed (16mph@60) it will be a gearing problem but if it's different the speedo itself is probably dicky and needing an overhaul
willy
William Revit

Do your have the correct gearbox? It may have been fitted with a rubber-bumper gearbox. Does it have the filler/level plug in the side or does it have a dipstick?
Dave O'Neill 2

Colin said it is 8 mph slow at 30 mph, not 8%. His error is actually 26.667% ! If that error is consistent through the range it will show 51 at a true 70 mph. The speedo can be rebuilt by one or two specialists. Speedy Cables and JDO Instruments come to mind.

Could it be an incorrect drive gear in the tail of the gearbox? I know with some cars there are different gears available with different numbers of teeth.
Mike Howlett

Even 8% slow against a sat-nav implies getting-on for 13% out in total, as legally they should be between 0% and 10% fast, i.e. 5% on average, but never slow. That is way more than you would get for anything other than huge tyres - you would have to go to a width of 220 or 18" wheels with an 80 profile tyre to get that sort of error, and I dread to think how big for 27% error.

Check the odometer, if that's OK then it probably requires servicing as it looks like the needle mechanism is dragging. If the odo is similarly out then the gearing is wrong somewhere. Could also be axle as well as gearbox or speedo.

As far as gearbox goes rubber bumper cars had 1000tpm gearing as opposed to 1280. The difference? 280 rpm or 28%! Either you have an RB OD, or someone as installed the incorrect gears. In fact the whole gearbox may be an RB item, easily identifiable from the filler - dipstick for CB, side filler/level plug for RB. If not that and you have the patience you can count turns of the propshaft against turns of the speedo cable and axle (lock one wheel and halve the number of turns on the other).
paulh4


If it's 8mph out at 30mph
and it's a gearing problem causing it
It should be 16mph out at 60mph
Same percentage out
willy
Paul's got it covered
William Revit

Tyres are 165 70 x 14.

I will check out the gearbox / filler plug tomorrow.

Colin
Colin Parkinson

Looks like the gearbox has been changed! Has filler plug and no dipstick.

Colin
Colin Parkinson

I have checked the parts book, and the gearing on the rubber bumper overdrive is different to the chrome bumper overdrive.

How can I now solve that without changing the gearbox?

Can the speedo be recalibrated??

Colin
Colin Parkinson

It can be recalibrated, or you could get a speedo which almost matches the gearbox.

As Paul said, the rubber bumper speedo is 1000 tpm. IIRC there is a chrome bumper speedo which is close to that.
Dave O'Neill 2

RB speedos for the UK were 80mm as opposed to 4" so don't fit, but you may be able to get a later American re-import which were 4" - SN5373,

Mk1 speedos i.e. 3-synch gearbox were variously 1020, 1040 or 1060 tpm, which is pretty close to what you need. Very early ones will be Jaeger (and probably difficult to come by) but later ones were Smiths. Try searching for SN6124, SN6125, or SN6144, some of those will be KPH.
paulh4

I have the opposite problem. My speedo is reading 70mph when my gps is showing 100kph( approx 62 mph). The number on the speedo is 1020. Tacho is reading 5000rpm at this speed which is obviously wrong as it should do 22.3 mph per 1000rpm for a 67B with overdrive. PO changed it to negative earth and I have just fitted a 45 dizzy with lumingition so this could be my problem although it did drop 500rpm to 4000rpm immediately after fitting. It has since gained a 1000rmp. I got the 22.3mph out of a service manual of Leyland Australia. Am not sure what changes POs did to drive train. On the tacho face it looks as though the word positive earth has been crossed out.
R J Pemberton

"PO changed it to negative earth and I have just fitted a 45 dizzy with lumingition so this could be my problem"

No way on earth will this affect speedo gearing, or the relationship between tach and speedo.

WSM states 3-synch top-gear speed per 1000rpm standard is 17.9mph, overdrive is 22.3mph. The speedo drive ratios are 9/28 non-OD, 5/16 OD.

4-synch top-gear per 1000rpm standard is 18mph, OD is 22mph. Speedo ratio is 10:26 standard, 8:21 OD. It's possible the wrong mix of speedo drive gears and pinions has been used.

Reading 70 at 62 is not much outside the legal limits (in the UK at least) of 0% under-reading to 10% over-reading. What gearbox/OD does it have? 3-sync or 4 synch? Top-fill or side-fill? Whilst a side-fill uses a 1000tpm speedo drive which is close to the 3-synch, it should reduce the reading by 2% on a 1020 speedo, not increase it. Is it 8mph fast across the range? Or a consistent percentage fast dependant on speed? What inaccuracy does the ODO have? At the end of the day it could just be an inaccurate speedo.
paulh4

Thanks for your comments Paul. I have confused the issue by writing about 2 problems in 1 post. After all the speedo is mechanical and the tacho is electrical so cant be connected. In NZ you are allowed 10% error in your speedo and mine is above that. The error is the same over all the range ie 35 mph is 50kph. I have a 3-synch box. I would have thought that going for a higher figure in tpm would give you a lower reading on the speedo. A bit like an imput shaft. The smaller that gear the more turns it takes to make the other shaft to complete 1 rev.
Rod
R J Pemberton

Higher TPM means the cable, hence speedo is spinning faster.
Dave O'Neill 2

"35 mph is 50kph"

Now you are really confusing me :o) 50kph is 31mph, so do you mean it shows 35 at 31? If so that is the same percentage error as 70 at 62. I assume so as showing 50mph at 35mph is a far bigger error.

What is the ODO error? If that's within the 10% then just get the speedo serviced and that should correct the calibration. If the ODO is out by the same amount then again that should be correctable by a servicing place.
paulh4

Getting back to my problem, would changing the o/drive for an earlier type solve the problem? I guess it would?
Colin Parkinson

Colin, have you thought about fitting a taxi/truck speedo gearbox. Many moons ago when I worked at Mintex, the car department bought some Jurid data loggers and they fitted one of these gearboxes into the speedo cable to get very accurate speedo readings.
john wright

Don't change the trans OD or the trans unless you want to send it to me for free. :-)

This is not a big problem. Contact a speedo shop. If there's one near to you they will have two lines. They will mark you tires and have you drive to the second line counting the revolutions. Then they will take your speedo and adjust it to be accurate.

If no shop near you they will tell you the distance of the lines and explain how to do this then send in the speedo.
Max71

Colin, I think your easiest option would be to recalibrate your speedo. Speedy cables can help you out.

www.speedycables.com/

Andy
Andy Robinson

Cheers Andy, will try them.

Colin

Colin Parkinson

Colin,
I acquired one of my B's with a slow reading speedo. So I removed it for a look. The "disc" bearings were stuck with grease, as was the odometer gearing. This happens when well meaning PO's grease the cable too liberally and too close to the end.
20 mins with meths and a small paint brush cured it.
Allan Reeling

This thread was discussed between 02/07/2017 and 18/08/2017

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