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MG TD TF 1500 - TF Speedo Calibration

I'm getting ready to send out the instruments for repair/refurbishment. The repair shop has a step by step process to determine "speedometer ratio" which I believe is to set the proper indicated speed by the number of revs the cable turns over a given distance. Anyway...they emphasize checking tire pressure (3 times) to match the factory specs. Well...since I don't have the original Dunlop 5.50/15 which the owners manual says is 18 PST, I intend to set the tire pressure at ~25PSI (we discussed this in an earlier thread)for the radial tires I do have (Michelin ZX 175/15), run the indicated distance (52 ft. 9 1/2 inches) and provide the cable rev count as the basis to set the speedo accuracy. I think that will work just fine. But...am I missing anything here?

The other question...does tire pressure really have an impact on calibrating the speedo? Besides rolling resistance, is the tire likely to cover a different distance (however small) due to varying tire pressures?

I'm planning to do the speedo cable rev count tomorrow so any input as soon as possible would be appreciated,

Thanks!

Jim
Jim Rice

Jim
All being equal, If you usually run your tyres at 25psi then that would be the pressure to use for the rollout then if you change tyre pressures later it'd only be a couple of pound either way, it won't make much difference-
My money's on use the pressure you usually run and your speedo should be spot on when you get it back--
Cheers
willy
William Revit

When I design my speedo corrector gearboxes I ask for a comparison of the speedo read out with a GPS at the correct tire pressure and use the GPS value to design the gearbox. I don't bother with counting TPM. That went out with the dinosaurs.
Regards
Declan
Declan Burns

Thanks for the quick responses. Maybe I'll do a quick GPS run before the TMP (that's what they asked for), then another after I get the speedo back for comparison. As the speedo does float, it's hard to nail exactly what the indicated speed actually is. I recall following a friend in their car at an agreed 50 MPH. Mine was reading around 60 MPH. No wonder other cars were rather impatiently trying to pass!

Jim
Jim Rice

Jim,
I would do the GPS comparison before sending them the speedo and send them the results. The GPS method is far more accurate. I usuallly ask for four measurements-say 20, 30, 40 and 50MPH. 30, 40, 50 and 60 is better. Input the values on a spreadsheet and look at the curve. These speedos are not linear over the whole range and at low speeds they can be quite a bit off. I would also use 50MPH as the main design speed.

Regards
Declan
Declan Burns

And FYI, when Nissonger was doing speedo repairs they had a process for the owner to measure out 52' 9" and roll the car forward this distance and count the number of turns and fractions of turns when moving the car this distance. Do it three times for accuracy. 52'9" is close to 100th of a mile.
John Quilter (TD8986)

John:

I guess West Valley read from Nissonger's playbook (or perhaps the other way round) as that is the same set of instructions West Valley recommends.

Jim
Jim Rice

Pushing the car 52' 9" will certainly keep you fit! Absolutely unnecessary in my opinion.
Regards
Declan
Declan Burns

I would caution about using any free phone Apps for GPS speedo, my mate used two on his i-can't phone and I knew neither was correct by looking at my rev counter and, at the time incorrect, speedo. Why he couldn't use a satnav setting on the device I don't know.

The Apple stuff is supposed to be the best for security but I bet the Chinese, Russians and Mossad have a good laugh at that.

I borrowed another mate's satnav and that was fine.

When my speedo was calibrated, very successfully and accurate in the end, I think we pushed the car so many wheel turns, about 13 feet IIRC, three times and took the average.
Nigel Atkins

Nigel, I used a couple of free Apps and an actual GPS and got very similar results from all 3. Then I bought a digital tacho that had a cable around one spark plug wire. Checked it against my auto tester.

Then I found my tacho was incorrect, by a lot.

Going to have my tacho rebuilt, gotta save my pence first.

Peter
P G Gilvarry

I did the TPM process as recommended. Spot on with all three times at 15 3/4 revs over the stated distance. I too used a free BPS app based on the reviews. At a minimum, it confirmed that my actual speed was less than indicated. It did vary (as Declan alluded to) based on speed, with the max differential of 8 MPH at 60. I'll provide those results as a reference set of additional data points to West Valley. Whether they use the info or not.... who knows. We have some running around to do today, so I'll also check the GPS read-out against the other cars modern speedometer. Now comes the "fun" part....removing the instruments from the dash. I already feel a backache coming on!

Jim Rice

Peter,
that's good then. I've no idea what free Apps my mate used but he said when I shouted the speedo readings when I was able to drive at different steady speeds that they were about right and I knew from driving off my tacho for a few years that it couldn't be and I knew the minor discrepancies of the tacho from a rolling road check. But my mate isn't the most tech savvy and I wasn't looking at his phone.

The satnav check was done with my usual 'navigator' on a long straight quiet road near to where I borrowed the satnav and it confirmed the error and later how accurate the speedo amendment was on a second borrowing and run.

Until I change the gearbox (to a Ford Type 9) the speedometer was very accurate, more so than my wife's new modern car at the time.
Nigel Atkins

This thread was discussed between 02/07/2022 and 04/07/2022

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