Correct speedo for GT6 Mk3 with overdrive

[Philbry]

I've recently bought a Mk3 with overdrive and noticed using my phone GPS that the speedo is significantly over reading. When I'm doing 32/33 on the GPS, the speedo shows 40. Unfortunately because the previous owner fitted a 123 dizzy I currently don't have a tacho so I can't tell you the rpm in a particular gear to check that tallies. The distance shown on the trip also seems to be out by the same proportion. The TPI on the dial is 980. I'm guessing the car has had overdrive added at some point and the original speedo left in? What's the right TPI number for a Mk3

with overdrive? Is my best option to get the speedo recalibrated? Thanks.

[mishmosh]

Lots of things affect it. My Herald was similarand over read by 10 percent. My Bond Under read by ten percent. I put the herald wheels on the bond and vice versa. The Bond was shod with 185 /13 tyres so a high rolling circumferance. Now both are almost spot on  ( and the Bond accellerates like a demon ! )

[68vitesse]

Overdrive will not affect it, overdrive reduces the engine and gearbox rpm not the prop shaft which the speedo uses, output overdrive shaft. There are different speedo drive gears available for some overdrives, a change of diff ratio will affect speedo.

 

Regards

 

Paul

[Pete Lewis]

well certain sites had photos you could read the tpm on the dials so someone sneaky has removed them all ... helpfull ???

 

the easy way to get the tpm is measure 52.8ft   down the road ,  disconnect the speedo cable attach a card needle to the inner.

 

push car exactly the length of the test track  count the turns of the cable

 

multiply by 100 and thats the tpm youre looking for 

 

if the roads short then 26.4 ft x 200

 

chris witor spells it out well but thats big saloon            http://www.chriswitor.com/cw_technical/speedo_data.pdf

 

here's some useful and repair   clues on

 

 http://tidentenn.com/pdf/Speedometer%20Repairs.pdf

[thescrapman]

I think 980 goes with a 3.27 diff.

 

Do you have a D or J type overdrive and what diff ratio?

[Pete Lewis]

bear in mind the speedo tolerance new,  was 10% +- 4mph  but cant read slow  so if it reads 70 at  true 60 you're  just  in tolerance.

 

Pete

[Philbry]

Not sure what O/D I have, but when I was on the motorway today doing 60, the speedo was around to 75-80 I saw a Mk3 at Leatherhead with O/D and the TPM on that speedo was 1152 which makes more sense than 980. I accept some difference in tolerance but over 20% is a bit much. I guess I'll have to get it calibrated as finding another one will be almost impossible.

Thanks for the feedback everyone.

[Pete Lewis]

If you go that route there are

ciagauge.com

speedycables.com

jdo1.com

 

pete

[Gully]

1152 TPM was for the overdrive cars with 3.89 diff ratio. 980 TPM was non-OD with a 3.27 diff.

 

I had my 1152 TPM recalibrated by Speedy Cables to 1075 TPM as I run a 3.63 diff - having checked it against the speedo app on my phone, it reads about 4-5mph over at 60mph, so much in line with modern speedo readings. I paid £80 + £12 delivery + VAT.

 

Gully

[Pete Lewis]

begin to wish i can saved our Smiths drive and calibration machine from being dumped ,, but is was a bit huge for what it did

places like speedy etc could well use the same 

it had some humongous electro magnets , that would have dulled the lights round here.

 

Pete

[Philbry]

Update - I managed to get hold of a different speedo with 1152 tpm on the dial although it's a newer one than mine so has a different font and needle. But having tested it, it's close to being spot on when compared to the GPS on my phone and the trip distance was also accurate too. So I guess I must have a stock diff and O\D. Just need to see if there's any way to swap the face needle without messing up the calibration.

[Pete Lewis]

if its the same brand on instrument then you can  twist off tp remove the rim

undo the two case screws and drop the innards out.

 

hold firmly the alloy drive disc ,  marl its rim against its frame to keep its position if you like.

 

with needle at its stop  pull it off and press on the replacement .. without allowing the drive disc to move 

 

then all remains unchanged ,  if you let the disc free then the hair spring will unwind and pretty much impossible to refit correctly ever again, 

 

so easy but care required 

 

Pete

[dave.vitesse]

Before removing the pointer. With the pointer on zero mph mark the aluminium driving disc with a pencil. Then place a corresponding mark on the metal frame so they line up.

 

Remove the pointer.

 

Now line up the marks by carefully holding the disc and fit replacement pointer at zero mph.

 

Dave

[Philbry]

Final update. Just in case others want to do this. I. swapped out the innards of my older Speedo and replaced it with the newer one with the 1152 tpm setting. I managed to move the mileage wheels gently to adjust the distance to match the original one by pulling the individual wheels away from the adjacent ones without dismantling it which was a bit of a result. I fitted the old dial and pointer (which actually seems to be a heavier mass than the newer one) so I was wondering if that would affect the damper spring behaviour. Anyhow having reassembled it and taken it for a drive it seems to be pretty close to the right speed reading slightly over by 5-10% compared to my phone GPS and the pointer is pretty stable. The references earlier to marking the wheel at the rear to the case are worth keeping in mind if you do this. The other thing that seemed to help is that the pointer without tension seemed to point to a small dot (mark) on the dial at around -5 mph. If you pull the wire stop behind the dial face whilst you set it up, you can get the pointer to move to this.

Anyway thanks for all the comments/advice.