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MG MGA - speedo problem

I have just got round to replacing my speedo after it broke last year with one that i found on ebay from an early mgb, the problem i have is that ita about 20 mph out, with 40 mph showing on my sat nav my clock shows 59, im sure its my own fault i have a type 9 5 speed box and i have recently replaced a 3.9 diff with a 4.3 but what if anything can i do about it, any thoughts please, Vin
Vin Rafter


I got Speedograph to recalibrate for me. It was accurate to 1mph when I got it back.

The attached link includes the data you need to supply when you send off the instrument.

Steve Gyles

You're not even close. The original speedometer was either 1450 or 1440 turns per mile. If you only replaced the 4.3 final drive with 3.9, the speedometer and odometer would read 10% low. The T9 gearbox also has a different drive ratio for the speedometer cable, I think people have been mentioning 1.4:1 (slower). Then the MGB speedometer also has an entirely different internal drive. Depending on which year of MGB it might be 1200, 1080, or 1000 turns per mile.

Look on the face of the instrument just below the main odometer window. The last 4 digits after the model number should be turns per mile. Then test the speedometer drive cable to see how many turns that does.

Disconnect the cable from back of instrument, put a masking tape flag on it so you can count the turns, and let it hang below the dash where you can see it. Mark two lines on the pavement 52-ft 9-5/8-in apart (1/100 mile). Start at one line, push or drive the car to the other line, count turns of the cable, and multiply by 100.

The difference between the cable turns per mile and the number in the instrument should correlate with the speedometer error you observe.
Barney Gaylord

You don't need to push the car 52 feet, just follow the speedograph instructions and you will be fine.

There's also a guy in your neck of the woods, I'll dig his card out and send you the details

dominic clancy

That'll probably be John Ostick
Lindsay Sampford

Hi Vin
let me know if you need a hand in sorting out your speedo, it sounds like it is a two man job to check out the turns of the cable whist spinning the tyre round.

My speedo reads 10mph too slow and sometimes can get me in trouble on those average speed cameras they seem to be using more these days.
I had driven at great speed through a section of average speed camera controlled A road near Doncaster unwittingly last year. I had a bad feeling about my average speed and so I decided to pull into a layby before the final camera and read the newspaper for 5 minutes. The stop probably made my average speed one of the slowest of the day but I still enjoyed the drive and I never received the fine I expected!

So I will get my speedo sorted out this winter.

I have heard good things about JDO instruments in Keithley. email Tel 01535 672125
They give you two alternatives to help recalibrate.

The first is as mentioned above in counting the turns on the cable

The second is to drive your car approx 10 miles and record the distance on the odometer trip.
Then repeat the identical journey in another car which has an accurate speedo.(VW?) You could double check the distance with a sat nav I suppose.

They can use the difference to recalibrate.

I think there is a limit to how much they can recalibrate the speedo and if yours is beyond this then you may have to fit a gadget to the speedo cable to change the gearing.

They say they can give you a good idea if a recalibration is possible if you call them and tell them which speedo you have etc.

Best of luck


Just noticed that Lyndsay has already posted this, I must speed up my typing!
Colyn Firth

Colyn, did you start typing just before 11 last night or is Yorkshire in another time zone?!
Lindsay Sampford

My spedo bounces all over the place. I replaced the cable because somone said that was the problem but it still bounces about a forty mile range. Is there anybody in the US that repairs spedos?
GK George

I like MoMa in Albuquerque, NM, for instrument repairs.
Barney Gaylord

I've had two speedo's and two tachs done by Nissonger and like the result. A bit pricey, but good.
G T Foster

GK, a bouncing speedo needle is sometimes caused by worn teeth on the odometer causing it to jam intermittantly. Eventually the odometer siezes up completely breaking the cable. It would be a good idea to get the speedo fixed before it takes out your nice new cable!
Lindsay Sampford

I may be missing something here but Vin has a GPS (sat nav ) then I would suggest he gets the ratio between actual road speed at a high speed and his speedo reading and then buys the one of the small gear boxes for this ratio and fits it to the output on the gear box and attaches the cable to it. I have done this to my mga after fitting the 5 speed conversion and I recollect it costs about 80 quid. No need to play around with the speedo or measure revs of cable and wheel etc etc. there are a few companies selling these has been referenced in previous posts on this site.
Neil Ferguson

Come to think of is the same speedograph company Steve included in his post ....great company to-deal with. I got one of these wee gearboxes with my conversion but they changed it free of charge to one I calibrated more precisely using the gps as above....
Neil Ferguson

I have used John Ostik, as mentioned by Lindsay above - was very happy with the service
Graham M V


That's the guy I was thinking of too. Buying the speedograph adapter is the same price as having your speedo redone (I would get the original one done), and it's a lot easier to fit the speedo than the right angle adapter under the car. I have a right angle adapter in my car, and ended up making an inspection cover on the side if the gearbox tunnel to be able to get my (fairly small) hands in and get the thing to seat properly. I'd definitely get the speedo fixed and recalibrated, then you can sell the one you just bought on ebay again!

dominic clancy

I have the type 9 FORD 5-speed, together with a 3.9 diff, and an MGB speedo. It reads exact over the complete range! The speedo is a Smiths but has no numbers on its face. How lucky is that?
P. Tilbury

Maybe not luck. Sounds like it was recalibrated, and numbers on the face were painted over. Check to see if there are some numbers printed on the back of the instrument, specifically a non-standard number for Turns Per Mile. This is exactly what MoMa did for my speedo when I changed to 3.909 final drive.
Barney Gaylord

This speedo was in the '58 1500 Coupe when I bought it. This car at that time was fitted with the MGA gearbox and a 4.3 diff. So after I changed the gearbox and the diff, it now reads spot on. Some luck involved in that result, I feel.
Face is original, ie, not painted over. It shows "Made in England", and "Smiths" on the face but definitley no numbers. Next time I have it out I will check the rear for any ID numbers.
P. Tilbury

I run the identical transmission gearing as Peter, but with the Speedograph right angle adapter and electronic ignition. I use the Smiths RVC2415/00AF. Other than the name and number change from the original MGA Jaeger instrument it has the correct MGA face. I chose to have the instrument recalibrated rather than sort out a suitable right-angle drive reduction gearing. Reads spot on over the entire range.

Steve Gyles

Sorry. Ignore my last. Total rubbish. I got confused with the tachometer!! Too much wine again.

Steve Gyles

Steve, I cant comment on the accuracy of your information on the speedometer / tachometer subject but I really take issue with your last sentence.

Can there really be any such thing as "Too much wine?!!!!

Colyn Hic!

Colyn Firth

re. my previous email.. Dominic is correct. The choice over whether to reset the speedo using the wee gearbox or a speedo recalibrate when installing a 5speed box etc is in practice determined before the gearbox is put in as the position of the cable take off point on the box makes it impractical to change after installation. The 5 speed kit comes with a 1:1 angle adapter and if you know the speedo issue and calculate the ratio before installation then you can get an angle adaptor with a ratio to correct. Luckily by reading this site I had the chance to do this and Speedograph Ritchfield changed the units without charge. Speedo is spot on!!
Neil Ferguson

The angle adaptor is not standard in the kit, which comes with a long cable that fits directly to the gearbox and then snakes under the carpet on the RH side of the car.
dominic clancy came with mine..but then perhaps I read these pages and used the grey cells...
Neil Ferguson

This thread was discussed between 01/12/2011 and 06/12/2011

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