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MG TD TF 1500 - %$ TF Tach Reduction Gearbox

I am totally confused on the Tach Reduction Gearbox. I have a 54 TF wit the 1250. I recently converted to the 4.3 rear axle. My question relates to the Tach Reduction gear box. I have searched the archive, but cannot find an answer. My tach reads high. The tach reduction gearbox 2.25 ratio. The tach reads about 18% high. Do I need the 2.66 ratio gearbox? If so, is there a source for the 2.66 or does anyone have one laying around? The Dynamo pulley is 3.25 inches.

thanks for any help.


Rick - When you say that your tach reads high, have you checked the actual engine RPM with a diagnostic tach? I believe that 2.25 ratio is the correct ration for our cars. The rear axle ratio has no bearing on the tach reading. Have you checked that the pulley on your generator is the correct size? We are presently in Alaska, so I am not able to measure the generator pulley, but I am sure that one of the other people on this forum can measure that for you. Cheers - Dave
DW DuBois

Dave where in Alaska are you. I am in Fairbanks putting a Subaru engine in my daughter's Vanagon. Call 651-717-8636 if you have time for a beer or coffee.
S E Bryan


With the 4.3 rear end, I should be running about 2926 rpm at 50 mph. My tach is reading 3500. I have checked the gearbox and it is a 2.25. The pulley is 3.25 inches. If I use the 2.66, and apply 2.25/2.66 times the 3500, I come close to the correct RPM reading. Is this the correct math?


If you sit in your driveway in neutral your tach is going to be reading engine speed, your differential has nothing to do with it.

Now if you're talking about your SPEEDOMETER, that's a different animal. It's readings are going to be affected by your differential...are you sure your just going 50mph?

Gene Gillam



I understand the rear end has no bearing on the rpm reading. With the 4.3 rear end, the available information is that the RPMs at 50 should be about 2926. My tach is reading 3500 at 50 mph. Seems like either the tach needs to be calibrated, the gearbox is wrong or the pulley is incorrect.


Rick, if you've made no other changes than the 4.3 conversion, then your reading of 50 mph is actually lower than the speed you are traveling. Might be you're going around 60 mph. I have a discussion of this on my website,, and would give you more information in this message, but if I click out of this I'll loose what I have just written. I have one posting that is under a heading like, "report from trip from Fruita Colorado, or something like that where I report mph per rpm. Hope this helps.
John Brickell

I should have included the MPH is from my GPS and not the speedometer. Really confused on the RPM readings.

Internally if the magnet is dragging on the cup you can get a high reading displayed. Usually in a worse case scenario,the needle reading will fluctuate.

Confirm that grease/oil from the cable has not migrated into the instrument.

Before dismantling follow Dave's advise and measure the engine RPM with another instrument.

G Evans

Rick - The very first thing that you should do is to check the tachometer as I stated above. Once you have verified that the tach is reading correctly or incorrectly, you can proceed from there. Cheers - Dave
DW DuBois

From a 4.8 I put a 4.1 rear in my TF and the speedometer was reading 11 mph low. At 3500 rpm, it is doing 63 mph, speedometer was indicating 51. Re calibrated the speedometer to -.5 mph indicated, couldn't get it to 0 but close enough. PJ


Any suggestions on an inexpensive Dwell and tach meter?

The speedometer was calibrated to the 4.3 and is very close to what the GPS indicates. Just seems like the tach is way off. I will check the tach with an external meter.


Rick I've got a very similar problem and asked one of the local experts. i haven't had time to carry out these suggestions but here they are...
The first thing I would do is to check the zero setting of your rev counter.
Unfortunately, to do this you must first remove it from the car.
Take off the glass by rotating the metal cover until the ears line up with slots in the casing.
Look carefully at the dial. On the wrong side of the needle stop you will see a small white dot.
You must spring the needle over the stop and see where it lands.
My guess is it will not point at the little dot.
Pull the needle from its spindle and press it back on again so that it does line up with that dot.
Then, spring the needle over the stop and re-assemble.
E I Buckley

"Any suggestions on an inexpensive Dwell and tach meter?

There are free phone apps. They use the LED as a strobe.

The one I use for my iPhone is "Strobe Light"

Works well, resolution is 10 RPM however.

Jim B.
JA Benjamin

I am still trying to figure out how to check the tach / generator / gearbox. I bought an electronic mutimeter to read RPM (bosch FIX 7677). Attached as indicated in the instruction manual - common to ground and RPM to distributo side of coil, only reversing for the positive ground, but cannot get a reading. Jumps al over the place.


Hook up meter as direction say. Itís a digital meter may not work on positive Gnd. Try to find a analog meter.

ECS Stanfield


Start watching the garage sales. My 1970s tach/dwell meter works well with positive ground. Should be cheap when you find one. As to the zero calibration be careful. The tach pointer may be stuck on the shaft after many years in one position. The tip of the shaft in the tach is easy to break. Do not ask how I know. A clock hand puller can do the job safely if you can borrow one.


Jim Haskins 1953 TD
J M Haskins

Your meter is probably getting interference from the 'noise' from the copper spark plug wires.
Gene Gillam

This thread was discussed between 19/06/2016 and 27/06/2016

MG TD TF 1500 index

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