Lt77 speedo drive woes


New Member
Ok folks i am the owner of a 1972 Australian 3500 that has speedo problems.

now this is going to get complicated, so i will try to keep it brief.

when i got this car, the speedo was not connected and it runs off a GPS tachometer, and obviously i would like the speedo to be working...... now the fun begins

it was an Auto and had a LT77 fitted, so i duly got the right cable and connected it up, to gat a whopping 24 mph at 40.......

no problem says i, its obviously an Aussie box from after 1974 when they went metric, so if i just get the right gear i can convert it.....

now we get to the meat of the problem, i searched through the forums as you do and got a gear with the right number of teeth, fitted it and .....nothing.

so i bought a second one, and fitted that, nothing, then i had the presence of mind to measure the diameter of the two gears....

my original comes in at 25 mm and all the others come in as 21mm.... is there any way that i can get the correct diameter gear or am i going to have to go the converter in the cable route?
24mph at 40 sounds close to the mph/kph ratio.
its reading 24 at 340 and it just gets worse from there its reading 40 at 70 mph.

the problem i have is that repacement gears don't fir the gearbox, they are too small and don't engage with the main gear in the box.
Rather than trying to find gears that fit and read accurately, I'd put a wanted ad on here for a Speedo head , get that recalibrated and swap them over, or buy a cheap GPS Speedo and use that while you send your head away.
The driving gear inside on the output shaft has 2 options - TKC1273 (black 8 teeth) and 1274 (white 7 teeth) - available from Rimmers. The driven gear comes in 5 colours with different no of teeth - see LR speedo Gears - not sure their numbers are correct, as I have a 20 teeth gear in orange. Currently driving a box I havent opened, so dont know which driving gear it has, and a 20 tooth driven gear my speedo shows 50kph at gps indicated 70, but the proportion at this speed does not seem consistent at other speeds. The speedo is theoretically accurate based on the cost of its overhaul. As far as fixing the problem with gears, I think you are up the well known creek without a paddle. Working on things like this sometimes I wish I could follow Harvey's advice, and send my HEAD away to be fixed.
Note that the speedo face has a small number at the bottom that gives the cable turns per mile - 1248 IIRC, or km as appropriate.
Testing by eye I make the original 4 speed box speedo reduction 2->1, ie cable rotates at half prop shaft rate.
3/6/24 Corrected teeth on driving gears Black 8, white 7.
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When I did my LT77 swap I installed an electronic programmable speedo, it was not hard to do, and worked perfectly, looking good in the dash.
The gauge size was 80mm, it fitted perfectly into the circular hole in the printed circuit board behind the gauges. All was needed was a thick (5mm or so O ring) to close the gap to the trim.
The gauge was triggered by a magic eye looking at the propshaft at the rear of the g box.
A little sub harness was an extra addition to fit the push button to set the instrument, I put this in the right hand trim panel which made it invisible.
I think you can get the speedo without the push button below MPH, but mine came with it, it fitted behind the Rover plexiglass no problem. 1717227104365.png
I fitted a speedo head from an SD1, it has the same construction and cluster mounting holes so is an easy swap. The only thing is the milometer and trip are transposed and appear in the opposite 'window' so a little adjustment to the size of the window is needed if you swap the speedo face. You then have a head that matches the 'box.

Try doing a milage check on the odometer. If the dial needle has been removed it may have been replaced with too much spring tension causing it to read low. changing gears won't fix that....though you might get close but the odometer will then be out. Also there are two different speedo gear ratios, written at the very bottom of the dial face; below/under the cover. The correct one depends on whether the car is a 2000/2200 or 3500 diff. Metric dials use the same gears as Imperial Dials its just the face thats different
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Fitting an sd1 speedo to go with an LT77 will only be correct if the LT77 came from an SD1, not necessarily one of the other makes that used them - TR7, 8, TVR etc. What was involved in your swap please?

re mike's msg - the std speedo revs/mile is 1248, for kms its 780.
Fitting an sd1 speedo to go with an LT77 will only be correct if the LT77 came from an SD1, not necessarily one of the other makes that used them - TR7, 8, TVR etc. What was involved in your swap please?

re mike's msg - the std speedo revs/mile is 1248, for kms its 780.

The gear box and speedo came from the same car. It was a while ago so is a little hazy. I removed the front 'glass', two screws that hold the head in from the back, once that was out I removed the needle and then the face dial. The needle is lightly loaded down onto the stop so cater for that when replacing. Then the same for the SD1 speedo, but as said milometer and trip are transposed, the trip reset is a push button through the face so I decided I would forgo that luxury and cut it off, readjusted the P6 trip window for the SD1 milometer and then "reassembly is the reverse of dismantling". I used an early SD1 speedo and 70 position is in the same position as the P6. SD1 speedo's are set to 1000 revs per mile so the gears at the back of the 'box are chosen for the relevant diff and wheel choice, the same as Ford practice.

This was 20 odd years ago, so the car was my daily and I wanted a working speedo.

do you know the pulse rate for the electronic SD1 speedos please? Is it a rate per propshaft turn or ?? The 1000revs/mile (assume you mean cable?) you quote above seems odd, considering normal P6B S2 rate is 1248 for miles, 780 for kms.
No sorry no idea on the electronic ones. The 1000 is written on the bottom of the face of the SD1 speedo. The electronic speedo on the later cars work from a transducer on the speedo gears and the speedo display is quarter scale.


SD1 later speedo.jpg