Speedo inaccuracy

[54woody]

The speedo in my series II traveller was working well but not the odometer. Taking the casing off, I discovered that several teeth were missing from the gear that drives the odometer. Easiest fix, I thought, was just to replace the whole unit with another used, known to be working one.

I purchased one which the seller assured me had a working odometer. It 's n my car now. Having just returned from a 1500 mile trip, I can attest to the fact that the odometer on the replacement unit is near bang on. Measured over a 20 mile stretch on the highway, the odometer was off (on the high side) by less than 5%. That's close enough for me. However, the speedo is wildly inaccurate. When travelling at 50 mph (according to my GPS/sat nav), the speedo was reading about 72 mph. At 55, the speedo showed about 80 mph.

So my question is, how can the speedo be so inaccurate when the odometer is almost bang on? I thought about just adjusting the position of the needle, but the inaccuracy of the speedo reading is not linear. It seems to be further off the faster I travel. Thoughts?

[mike.perry]

What diff on car and what TPM on speedo?

[54woody]

145x14 tires, 3.9:1 rear end, Sierra gearbox. Measured 10.5 turns of the speedo cable in 1/100th of a mile. Speedo is 1000 tpm unit. This all jives with my measurement of the odometer being out by about 5%. My question is how the speedo reading can be so far off when it's driven by the same tpm cable that's driving the odometer at the (almost) correct rate.

[bmcecosse]

All you can do is reposition the needle for best average reading.

[liammonty]

Sorry, but I disagree with the previous! Obviously you've done your homework regarding what TPM you need, as the odometer is near enough bang on. The reason the speedo isn't, I believe, is because it isn't 'direct drive' as the odometer is, that is, he needle is dependant on a spring that prevents it from turning too far at a given speed. With time, the spring weakens and the needle on the speedo is able to read high. Of course, the odometer is unaffected, as it relies on a direct drive. So, in principle your speedo is the right unit, but needs a bit of an overhaul to get the speedo part of the unit correct. I think you'll find this is the problem, although I'm the first to admit I don't have first-hand experience of stripping one of these. BTW, I assume from the age of your car and the TPM of 1000 there is a chance that this is the old-style speedo - if so, these are generally even less accurate than the later centrally-mounted ones anyway.

[bmcecosse]

All it needs is the needle to be repositioned - to tighten the spring slightly !!!!!!

[liammonty]

Useful to explain it though

[bmcecosse]

Seemed extremely obvious to me.....

[liammonty]

Good for you! I'm only trying to help and it may not be obvious for someone who is asking about how it's possible the speedo is over reading! Indeed had it had seemed obvious to me, I wouldn't have taken the time to explain it.

[54woody]

Thank you for taking the time to explain it. It wasn't obvious to me either, or I wouldn't have asked the question.

[bmcecosse]

I thought I had made it pretty clear when I suggested repositioning the needle.... To suggest an expensive (£100+??) 'overhaul' is required is surely way OTT !!!

[liammonty]

Thank you for taking the time to explain it. It wasn't obvious to me either, or I wouldn't have asked the question.

You're welcome. I only understood how they worked after I saw one in bits. Incidentally, I wasn't suggesting that you necessarily pay someone to overhaul it - I'm sure if you have the time and inclination to fiddle around with it, it is doable at home. Good luck with it, and at least you know you've got the unit that is closest to what you need with your final drive and transmission set up!

[mike.perry]

Is it possible to swap the speedos and mileometers around so that the working speedo has the working mileometer?
A 1000 tpm speedo is normally used on a Series II with 803 box and 5.375 diff