MGB & GT Forum

I have a 1976 mgb with a non o/d transmission.

From what I can tell from some research on the forum I should have
a speedometer rated at 1000 TPM. Instead it has one rated a 1280 TPM (SN5230/085).


Can anyone tell me if that means my speedometer should read high or low?

It reads about 8 mph high at 50, which seems counterintuitive.

Yes, a 1000 tpm box connected to a 1280tpm speedo will read low. The 1280 should read a little under 80% of the actual speed-i.e. at 65 it should read 51. I can confirm that from running this exact combo for little while. The opposite will read 1.28x higher than the actual speed-50 will read as 64mph.

So, based on that, this isn't your problem.

Is your speedometer error proportional to the speed or is it constant? As an example, my speedo reads 5mph fast regardless of whether I'm going 15mph or 100mph. This is an easy fix that I just haven't done-I just need to remove the needle and replace it 5mph back.

If the error is proportional, I'd look at your tire size. The "original" size was either 155r14 or 165r14. These both have roughly an 80 series sidewall, and a modern 185/70R14 has almost the same diameter. In later years, 185/70R14 became the factory size-I don't know exactly when but I know it was the correct size in 1980(the only late year I've bothered to research).

If you are running a 175/70R14-a common alternative size-you will see roughly a 2mph hour in the 55mph range(i.e. the speedo will read 2mph too slow). Drop that down to a more extreme size-like a 175/60-and the error becomes 5mph in the 50mph range(10mph at 100 and 2mph at 25mph).

I have the 1280tpm on an OD gearbox and when it broke I got a spare 1000tpm to bridge the repair period. It gave a 28% higher speed reading as was to be expected.
Your situation is the reverse and should read too low, NOT too high. The difference of 8 mph to 50mph nominal means a difference of +16%, almost impossible to achieve by larger tires IMO. Even so we should even compensate the indication to be expected by the wrong speedo by its 20% lower indication, meaning the overall difference would become >36%.

There must be something completely wrong with this wrong type of speedo you are having in now. I would suggest you find a 1000tpm type first (or lend one just for a testdrive)

PS I do have the very 1000tpm speedo from a RB car you need, still lying in my garage as a spare.
But sending it over would be like bringing water to the ocean: there must be lots of them around in breaker yards in the USA. Look and you will find

Thanks guys.

Turns out tires are smaller then original and I didn't check the simplest thing.....speedo needle is at 5 MPH when car is at rest.

I'll adjust the needle. Think with the smaller tires its only going to be about 3 over at 45 mph.

Close enough for now. Plus, a few quirks in a 40 year old car just make it more interesting!

Usually tell speed from Tach anyway which seems to be right around 9 MPH per 500 RPM.

Thanks again!

I do wonder about your speedo then, maybe the PO had it converted to 1000tpm internally, then such a diversion on smaller tires would be logical.
There are people around who can convert any speedo from 1000 to 1280 or kmh to mph and vice versa, basically there are just more types covering all cars (rare automatics excepted)

Guess how I know from meeting them on part fairs