Incorrect Speedo Reading

[JonnyP]

Hi all, when I bought it TC2190 had a replica speedo fitted, which read approx 30% too low for a given speed. I have recently installed an original chronometric speedo which has been restored by Vintage Restorations and had expected that to resolve the inaccurate reading, but the same degree of error has carried across to the new speedo. I also fitted a new cable.
Would this group agree that this must mean an incorrect speedo pinion is fitted, or could it be something else?
I know there are several different pinions for T-Types, but are they all compatible with the drive gear on the gearbox output flange?

Many thanks for any guidance you can provide
Jono

[Steve Simmons]

I would first suspect that you have non-stock rear end gearing. Possibly TA gears (4.875:1) or high ratio 4.625:1. That would throw it off quite a bit. Non-stock tires also have a notable effect.

Rather than pay for a rebuild and calibration, you could fit a conversion adapter between the cable and speedometer. Contact Butch Taras (Butch T on this forum). He builds the conversion boxes.

[JonnyP]

Thanks for the help Steve. Tyres are definitely standard size. Is there an easy way to work out what dif is fitted? I have records to show a reconditioned one was fitted years ago, but nothing that lists the ratio.

[Steve Simmons]

You can raise one rear wheel (leave the other on the ground). Mark the driveshaft or diff flange with tape, paint, whatever. Put the tire valve stem at the bottom (as a marker), then spin the wheel, counting how many times the wheel turns per one turn of the driveshaft. Then divide that number by 2. That will give you the ratio, i.e. 5.125 is stock, 4.875 is TA, 4.625 is high speed, etc.

Someone please correct me if I screwed up the math...

[timberstone]

From the Website of West Valley Instruments:

Determine Your Vehicle’s Ratio
Do you know how to determine the actual ratio of your vehicle? We outlined a list of detailed steps that will give you an accurate figure:

1 Make sure that the tire pressure matches the exact factory specifications.
2 Make sure that the tire pressure matches the exact factory specifications.
3 Make sure that the tire pressure matches the exact factory specifications.
4 Measure a distance of 52 feet and 9 1/2 inches using tape marks or chalk.
5 Unscrew the speedometer head’s cable.
6 Place a mark on the inner core. You can use a hairpin, paper flag, or paper clip for this.
7 Drive or push your vehicle across the entire distance. Take note of the partial and full cable revolutions as you do this.
8 In order to remove gear clearance, roll your vehicle toward the first marking and stop when the inner core starts to turn. Take note of which vehicle part is at the start of the line before you start counting. When the marked part goes beyond the line, stop counting.
9 Perform the test three times to confirm that there is no gear slippage.
10 For accurate calibration, send this test form together with the gauge.
11 Please include contact information

[Steve Simmons]

Those instructions will determine the speedo radio, but not the differential ratio!

[Ray White]

Those instructions will determine the speedo radio, but not the differential ratio!

Quite right!

I have had very good service from John Marks at Vintage Restorations so if it was me I would establish the correct differential ratio and return the instrument to him for re calibration.

Perhaps the cost of buying an add on gear box from America (when something similar is available from Richfield Speedograph here in the U.K.) should be considered... but in my opinion re calibration is the better option.

[cdrolshagen]

Hi,
John Marks retiered a decade ago, shop is closed.
I can recommend Patrick Henry, he follows John Marks.
vintageinstruments@hotmail.co.uk
All Chronometric Speedos have a input rev of 1600 TPM (turns per Mile)
You can´t calibrate a Chronometric Speedo a special diff ratio.
Either you can adjust the gearbox output rev, if not, you need a correction box,
to get 1600 TPM
cheers Carl

[Ray White]

Hi,
John Marks retiered a decade ago, shop is closed.
I can recommend Patrick Henry, he follows John Marks.
vintageinstruments@hotmail.co.uk
All Chronometric Speedos have a input rev of 1600 TPM (turns per Mile)
You can´t calibrate a Chronometric Speedo a special diff ratio.
Either you can adjust the gearbox output rev, if not, you need a correction box,
to get 1600 TPM
cheers Carl

Not so fast! I spoke to John recently and he has reduced - rather than stopped completely - restoring MG instruments. He did a complete set for me last year and I am very please with the results.

[Steve Simmons]

To calibrate the instrument, it has to be completely rebuilt again. There is no quick gear change or anything. So you may get some resistance if you ask for the ratio to be changed for free!

[Ray White]

To calibrate the instrument, it has to be completely rebuilt again. There is no quick gear change or anything. So you may get some resistance if you ask for the ratio to be changed for free!

You need a reliable instrument so probably worth spending the money on.

[JonnyP]

Thanks for the steer on checking the diff ratio. You were correct, a TA CW&P has been fitted. Now I just need to figure out what solution to follow to get the speedo corrected…..