Speedo accuracy question tyre size ?

[trabant]

Hi I have a newly acquired early series 2 car with the series mm type dashboard ( separate gauges) the cars speedo seems to constantly read a good deal slower than the car is actually travelling. For example on driving the car home the speedo was reading roughly 50 mph however my friend following in his shiny new Kia claims he was doing 60 to keep up.

My car is running on 145/14 tyres which look a tad small my 1000 was on 145/80/14 tyres sooooooo could this be the reason though I should have thought smaller wheels would make the speedo read higher than the actual speed .

[liammonty]

You're right - those tyres would make the speedo read a little fast compared with the original cross plies, so there is something else going on. Most likely, given that you said your friend was having to do 60 to keep up, is that the final drive, and likely the engine, have been replaced with later versions, as a standard Series II wouldn't be able to 'cruise' at that speed. I suspect you've got the final drive from a later car, meaning that the car travels further for every revolution of the speedo drive in the gearbox, resulting in an under-reading speedo. Be very happy that you can reach 60 in your Series II!!! I did it a couple of times in my car, and it resulted in destroyed big ends as the engine is so weak and the standard car is so low geared!

[mike.perry]

What is the TPM on the speedo? It should be 1000 for a Series II. Has the gearbox/diff been changed?

[liammonty]

What is the TPM on the speedo? It should be 1000 for a Series II. Has the gearbox/diff been changed?

TPM could be a confounding factor, but it won't be the only problem, as the only other option is the MM speedo (right Mike?) which has a TPM of 1600 - this would make the speedo in an otherwise standard SII car under read by around 50%, which is far more than the OP suggests his is doing. It could, of course, be combining to the problem though!

I think key is first to establish which final drive the car has. Assuming it has the later Austin-type final drive, the easiest giveaway that it has been changed would be if the filler plug is in the rear casing of the axle, as opposed to on the front of the diff assembly. That would indicate a 4.22, 3.9 or 3.7 final drive, which would account for the speedo inaccuracy. If the filler plug is in the standard (for a Series II) position, it could of course still be a 4.55 final drive from an early Minor 1000, but in my experience, that isn't enough to throw the speedo out far (bearing in mind they tend to over read wildly on the early cars anyway).

[mike.perry]

The speedo drive gear to pinion ratio for a Series II gearbox is I believe, 2:9. For the rest of the A Series boxes it is 5:13 causing a conciderable difference in readings if the speedos or boxes are swapped independently. I do not know difference the wrong gear to pinion ratio would make, perhaps someone with a calculator could work it out

[liammonty]

Mike - I did it myself, and it causes a massive over-read of at least 50% My SII could reach an indicated 90 mph!!!!! That won't be the problem here (I guess if it was, it could be rectified by a swap to an MM speedo which would more or less iron out the difference).

[mike.perry]

The original tyres would have been 5.20/88/24 crossplies or possibly 5.00/88/14. modern tyres are 145/80/14 or 155/80/14

[trabant]

I can tell you its on the original engine as I have virtually all the paperwork for the car from new having moved from a 1968 1000 to this wee beasty I can't believe how horrendously underpowered it is. The current plan is to stick an 1100 in it as I use the cat as everyday transport and keep the original lump to put back in when required . I'm assuming there wont be much difference fitting wise other than the flywheel ?

[bmcecosse]

And the gearbox - unless you keep the original flywheel and small clutch.

[mike.perry]

If you fit a 1098 you will quickly find out how bad the 803 gearbox is with it's poor set of ratios and it's weak construction

[liammonty]

A 948 is an easier swap, as the clutch and flywheel are the same as yours. Brakes are also the same size, so might make things easier for insurance purposes, I.e. Without having to change them. A 948 in an SII is a nice combination in my experience. Check what final drive you've got at the moment, as if you're changing the engine, the 5.375 diff, if still fitted, will be a nightmare!

[smithskids]

I had a low ratio diff in my mm for a while when I rebuilt my original diff and boy did it go up hills fast and had good acceleration, but it only did 50 mph flat out.

[mike1864]

And don't forget your mate's shiny Kia's speedo probably over-reads. These days cars are often set to +5-7%, presumably to cover the manufacturer's bottoms. And tyre wear can add another 2%. I'd suggest checking against a SatNav.