Spitfire Speedometer, differential and speedo gear issues

[mmcglynn264]

Quick rundown.
Just purchased a 1976 Triumph spitfire w/OD J-type, single rail. Tires 155/80/13

Speedometer tested again pace car and multiple GPS phone apps to verify.

Speedometer is 1000rev mechanical and reads 50mph when actual GPS speed is 60mph. Odometer is also lower than actual.

At GPS speed, electric tachometer reads 3000 rpm.

OD has blue sealant at flange telling me it was serviced at some point.

Speedo drive gear is orange.

I don't know what differential is in it. I'm guessing not the factory 3.89.

What Speedo gear do I need since orange is not the correct one?

 

[philstr6]

count the teeth on the speedo gear,you have to look at the drive gear as well and thats inside the overdrive to see whar speedo gear you need
phil

 

[mmcglynn264]

20 teeth

 

[philstr6]

that tells me that you have a tr6 speedo gear and and a 8 start drive gear
you need a 6 start 18 teeth
to fix this you have to take your trannie out,pull the overdrive apart and change the drive gear in the overdrive
Phil

 

[mmcglynn264]

There's no other way to correct the issue besides that? I'm OK with mechanical work to an extent. That's a bit outside my wheel house.

 

[philstr6]

speedo repair shop and see if thet can caliabrate your speedo and tac
Phil

 

[mmcglynn264]

count the teeth on the speedo gear,you have to look at the drive gear as well and thats inside the overdrive to see whar speedo gear you need
phil

Pulled speedometer gear and actually counted teeth instead of relying on color. It has 18 teeth. Included better picture.

 

[philstr6]

ok,then you have the correct speedo gear for spitefire,find a speedo shop that can or maybe calibrate your speedo for your problem
Phil

 

[CJD]

A speedo shop can calibrate the speedo, if you have the correct parts. It is common for the magnet to lose strength, making the speedo read slow. The odometer cannot be calibrated, though. It is fixed to the number of turns of the cable.

When diagnosing, I always check the odometer accuracy first, and that will tell you if it is a gear issue or a calibration issue.

 

[Kaybass66]

There's no other way to correct the issue besides that? I'm OK with mechanical work to an extent. That's a bit outside my wheel house.

Yes there is another way and it’s easy. Contact Powl Speedometer service in Lancaster Pa at 717-898-2552. Send them your speedometer cable along with the actual GPS speed and reading on your speedometer and they will make the necessary calculations and make you a dandy little gearbox that splices into your cable and sits just outside the firewall inside the engine compartment next to the wiper motor (TR6) You will need to measure the distance from the back of your speedo into the engine compartment. In my TR6 it’s about 10-12 inches. That’s where they will cut the cable and splice in the gearbox . It’s very cool.

There's no other way to correct the issue besides that? I'm OK with mechanical work to an extent. That's a bit outside my wheel house.