Speedo Type GT6 Mk3 OD Pre KE20000

[Stratton Jimmer]

Interesting article in the May 24 Courier regarding gauge types but the pictures on p31 have an error. The bottom, middle picture is a duplicate of the one on row three and is not a tacho! 

Anyway, my question is simply this, what is the significance of the manufacturers number immediately below the yellow digit of the trip odometer? Mine matches that shown for the top centre picture = 1152 but the car's original speedo which I was given with the vehicle, shows 980 which is, according to the picture's caption the same as the non-overdrive speedo (middle picture, second row) yet, this car had overdrive from new. The article, which appears to have some script missing says this number denotes OD or not.

I remain, as ever, confused of SSM.

Edited October 22 by Stratton Jimmer
Additional information added.

[Pete Lewis]

that is the turns per mile required for a correct speed/distance reading 

the numbers will vary depends on the drive line and tyre rolling radius 

Pete

[Stratton Jimmer]

Thanks Pete. My six is a very early Mk3 and has some aspects of Mk2 about it. I'm wondering if this original speedo was one of the ones Andy referred to in the article that were fitted to Mk2 OD with 3.27 diffs. The speedo now fitted has always read low.

[Pete Lewis]

you can do a check to find what TPM you need 

this is a bit laborious and doing the maths can shorten the plan

remove the speedo cable and attach a card needle to the inner 

measure 52.8 ft down the road ( there being 5280 ft per mile)

push the car over  this distance and count the turns of the cable 

X 100 gives you the TPM    

if you halve the distance  X  the needle count by 2

might need some wheatabix 

pete

[Stratton Jimmer]

Discussing this with my mate Tim in the pub last night we came up with a simple scheme. Drive the car over a measured distance of ten miles and see what the odometer reading is. If it comes up way short then the wrong speedo is fitted. Swap the speedos and repeat. 

[johny]

Are you sure you havent got a 3.27 diff fitted? Seems strange if the speedo supplied with the car originally was right for that and would also explain why you now read low with the replacement unit. Speedos normally read high, to be on the safe side, and I just confirmed this on mine using a sat nav app on my phone...    

[Pete Lewis]

going back some years most local police had a measured mile with black and white posts every 1/4 mile in the verge 

so they could check a speedo with a stop watch  

think most have disappeared now 

Pete

[DVD3500]

36 minutes ago, Pete Lewis said:

going back some years most local police had a measured mile with black and white posts every 1/4 mile in the verge 

so they could check a speedo with a stop watch  

think most have disappeared now 

Pete

Local police or stop watches?

😄

 

[johny]

See police now in Spain have mini rolling road machines (dont know how many mind) for checking max speed that can be achieved by electric scooters and bikes👍 

[Stratton Jimmer]

5 hours ago, johny said:

Are you sure you havent got a 3.27 diff fitted?

Is there an easy way to find out? This is what I suspect but the article in the Courier stated that some OD Mk2s with 3.27 diffs had 980 speedos but mine is a very early Mk3 and has, for example, chrome vents rather than body coloured vents. I think it might be a Mk2 drive train.

[Josef]

The diff casing number should be KC for non OD, KD for OD, at least for a mk2 GT6. Usual caveats about diff internals changes over the years apply. HC is a Vitesse 3.89 I think. It’s easy enough to look. And / or you can mark the wheel and prop, turn them the appropriate number of times and see if it comes out nearer three than four. 

[johny]

Its a big difference in ratio (almost 20%) so easily show up in the revs your doing at a set speed so just stick your cars details in mintylamb.co.uk and give it a test...

[Gully]

On 22/10/2024 at 15:07, Stratton Jimmer said:

Interesting article in the May 24 Courier regarding gauge types but the pictures on p31 have an error. The bottom, middle picture is a duplicate of the one on row three and is not a tacho! 

Anyway, my question is simply this, what is the significance of the manufacturers number immediately below the yellow digit of the trip odometer? Mine matches that shown for the top centre picture = 1152 but the car's original speedo which I was given with the vehicle, shows 980 which is, according to the picture's caption the same as the non-overdrive speedo (middle picture, second row) yet, this car had overdrive from new. The article, which appears to have some script missing says this number denotes OD or not.

I remain, as ever, confused of SSM.

The confusion is people associating the Speedo TPM with whether or not the car has overdrive. It's the diff ratio alone that determines which speedo is correct, as the speedo take-off measures propshaft rotations. The association is understandable, as in the UK market Triumph changed the diff ratio in the GT6 Mk III when overdrive was taken as an option, but if you add overdrive to a non-overdrive car, the speedo will still be correct unless you change the diff... 

I run a 1:3.63 differential in my GT6 - that required a speedo recalibration to around 1080 TPM if I recall correctly. It's written on a box in the shed anyway!

Gully 

[johny]

Thanks yes that adds up: 3.89 to 3.63 diff ratio equals speedo ratio 1152 to 1075👍