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MG MG Y Type - Speedometer

the speedometer in Y4002 was very erratic making keeping to legal limits quite difficult, (so far nil points on licence ever! since 1965).

Cable replaced with a new old stock suitably oiled, no significant change.

So I overhauled speedo and recalibrated against 1600 RPM equates to 60MPH. Refitted and compared to Tom-Tom now within 1 MPH.

Noticed that after about 20 miles, reading slowly dropped resulting in actual speed of car being above indicated speed. After stopping at a junction, indicated speed returned to accurate, but slowly declined until next stop.

So, I have temporarily fitted an electronic speedometer from a bicycle, then I noted that the milometer was reading about 25% lower than actual distance, (both trip distance and odometer agree) this was confirmed on a long run with true distance being worked out from Ordinance Survey.

I'm running on New 5.00/5.25 - 16 Waymaster Premium tyres, are these bigger than original? I was running on Avon H.M Tourist.

Now, are ther different heads i.e. YA YB TF? mine has all the right numbers for an early ish YA and it's been in the car for at least 38 years,

Is the warmth from the engine compartment having an effect on the magnet in the speedometer?

Do I suspect the speedometer drive in the gearbox? although it looks satisfactory from an in-situ visual inspection.

Any advice or info would be greatly appreciated, this is now bugging me, as I did some training as an instrument mechanic and pride myself on being able to overhaul/repair most type of instrument, but this has got me stumped.

D P Jones


Firstly the cable should be run DRY. There is no need to lubricate or even grease the cable. Your problem is directly related to the fact that you are running it wet. Indeed originally much effort was made to ensure that the cable ran dry. At the gear box end there should be a small felt washer/seal specifically designed to catch and trap any oil or grease getting out of the gear box and being spirally (archamedianly) up the cable onto the speedometer head.

The fluctuations you are experiencing are primarily due to the fact that you now have oil contaminating the faces of the speedometer. The only solution, remove the cable, thoroughly flush out ALL the oil contaminents from both the sheath and the wire (alternately and quicker and more sure fire - throw it away and buy a new one from Speedograph Richfield [see the Links page at]) and then strip down the speedometer and clean up all the magnetic faces and all the inner parts of the speedometer with brake cleaning fluid, then reassemble.

Are the Waymaster tires you are using Bias/cross ply or radials? Radials are slightly larger I beileve in profile (but I stand to be corrected as tires arent my forte) and will give you false readings.

You then go on to talk about heads - are you referring to speedometer heads or cylinder heads? The best answer to this kind of question can be found by consulting either the book "Let there be Ys" or the CD-book if you do not have the paper one. The great advantage of the CD-book is that you can search it by typing "speedometer" into the find/search box. At only $35 plus shipping ($7.10 currently) to the UK it represents excellent value even if I say so myself! The CD-book can be bought by clicking on the gif at the home page of along with 3 other great MG Y titles. With Christmas coming up soon it is a great time to "drop the hint" to your other half for stocking fillers!

Paul Barrow


Paul's explanation sounds good, but speedometers and their cables aren't my forte. With tyres I'm much more comfortable.

Paul: compared to a bias/cross ply tyre with the same width, a radial tyre SHOULD have a smaller diameter.
A crossply has a width/height ratio of at least 100/100 and for a normal radial that is 100/85 at the most.

According to the suppliers the diameters of the 5.00/5.25 cross ply tyres are:
Waymaster 685mm
Avon 678
Dunlop 680

So the Waymaster is only slightly bigger, not big enough to cause a big difference with the Avon?
A low tyre pressure will cause a lower height and thus a smaller diameter. This will influence the speedometer reading, too.

If you think that through the following (admittedly far fetched) theory results: with low pressure the tyre will get warmer through the friction of the bending rubber. This warmth raises the airtemperature in the tyre, which will expand, raising the tyre pressure. This will enlarge the the diameter and that lowers the revolutions of the drive train at a given (TomTom) speed, resulting in a lower speedoreading.
Raising the cold tyre pressure would give a more constant reading.

The obviously gaping hole in my theory is that a tyre wouldn't cool down enough during a short stop to explain your declining readings, so I'd stick to Paul's
theory first....
Willem vd Veer

Thank you both for your suggestions,
The cable is actually dry, it was wiped clean after oiling, there is no evidence of oil getting into the speedometer head (just put it back together again)
I meant speedometer heads, with the different wheel size and axle ratio between the YA & YB I presume the take off in the gearbox may be different.
The Waymaster tyres are cross ply.
I took the car out on a measured mile course today, the cycle sppedometer and the car trip indicator were within 0.05 of a mile over 4 seperate runs in either direction (Y type being slightly down) The cycle speedometer has its sensor on the drivers side rear brake drum and is calibrated to 1600 revolutions per mile. Tyre pressures are correct at 25 psi cold.
Thanks for the info on the books, I do have both, (and signed by the authors!)
Many thanks for your assistance.
D P Jones

If you want any easy way to refine your revs per mile just follow these simple steps:-

1/-Adjust your tyres pressures as desired
2/-Take the car for a 30min drive allowing the tyres to reach operating temperature
3/-Park the car on a level smooth surface
4/-Draw a chalk line across the face of the tyre
5/-Roll the car slowly forward for over one full rotation and you will see the chalk has left two lines on the ground
6/-Measure the distance between the 2 lines
7/-Using the circumference measured, you can now calculate the rolling diameter
8/-Divide a mile by the measurement and you will have the revs/mile.

Yes the tyres do grow a little diameter at speed, but do the sums and you'll work out just how small this growth is.


Could you also not use D ( x diameter = 2 x x R) of the tire where or pye is 3.1415926 if my high school math is correct?

Since 1 inch is 1/63,360th of an mile if you Knowing the diameter you could then divide that into 63,360 to get the number of turns per mile.

If you do not want to use , and do not have any chalk to hand you can also use the tire valve. Just put the tire valve on one tire (assuming you have all the same tires all the way around the car and all correctly inflated) at the top or bottom of the turn and then roll the car forward and measure the distance travelled.

We used to do this as a driver/car skill test on rallies to pass the time/an agility knowledge test requiring the driver to actually drive the car forward 3 full wheel rotations and then back 2 full wheel rotations. The final position of the valve should then be at the same place it was when started. The judges measured the angle of the tire valve was off top/bottom at the end of the 3 forward turns and added that to the angle it was off at the end of the 2 rear turns. If you were over or under a full turn at either point then that was an additional 360 penalty points per turn you are off count. The driver with the lowest score wins.

Try that at your next natter, it is good fun. The person with the highest score can buy in the next round!

Corry Grainger

Hello David

Many years ago I had a speedometer/oddometer problem. All it turned out to be was the fact that I hadn't tightened the fastener holding the drive cable at the back of the speedometer sufficiently so the squared off end of the drive cable wasn't engaging properly. It's awkward to do it properly lying on your back on the driver's seat, so it's easy to get it wrong.

Some months ago Ed winters had a similar problem. He thought it might be the speedometer itself but all it waas the fastening at the back of the speedo not done up tightly enough. (see archived correspondence on the bulletin board.)

Hope this fixes it.

Clive Evrall
J.C. Evrall

This thread was discussed between 11/08/2011 and 14/08/2011

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