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Speedo / Overdrive Gearing


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A slightly different one from the norm this time.

 

I finally have a working speedo of the correct ratio to match the GT6, it came all the way from the states!

It's in lovely condition and works perfectly.

 

However, it hasn't been very easy to calibrate the speed, and also it would appear that the odometer is over reading by about 3 miles to every 7 (as in, it says 10, I've only driven 7...).

 

I haven't had a proper working one in the car for years so I've never been sure what has been going on.

 

The speedo is a 980 tpm unit, which matches the old one. I changed the overdrive not so long ago, which is the part that drives the speedo. This is a Laycock D-Type unit. Are the Spitfire MKIV ones different to the GT6 MK3 ones at all, other than the output flange? I'm wondering if the speedo drive ratio could be wrong...

 

The diff is a 3.89:1 as standard. The tyres are slightly different 175 70 13, instead of 155 80 13 or whatever the standard size is, haven't done any maths to see if that makes much difference... I'll do that now! I reckon that's negligible.

 

Cheers,

 

Henry

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the best diy way to get your TPM is 

mark 52.8ft down the road 

stick a card needle on the now detatched inner cable

drive car accurately the exact lenght of the test track you measured out.

count the number of turns of the card needle

multiply the turns by 100 thats the tpm you are looking for.

 

if the oddometer is that far out whats the speed reading , can you check it against a sat nav ??

whilst the drive to the oddometer rolls is seperate from the speed needle its all driven from the cable

if one is wrong then both should be wrong

 

having said that if the speed reading  is good it may be the small pawl and ratchet on the oddo drive is stiff with age 

a dash of lube on the lever that prods the pawl can be stiff /tired seized 

its held on with a small sprung clip so easy to pop it off or just apply some lite oil/grease 

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The 980 TPM speedo is for a non overdrive car with the 3.27 ratio diff. For a 3.89 diff you need the 1152 TPM speedo. A 980 will over-read considerably on an overdrive car (3.89 diff) as the prop is turning faster for each revolution of the rear wheels. Minty Lamb will cover off the tyre size variance percentage, but the ones you've cited are pretty close.

 

Speedy Cables can re-calibrate - they did mine when I changed to a 3.63 diff.

 

Gully

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Hmm, that's interesting stuff.

 

Pete - I'll have to run the car down the road an measure it all up when I get a chance.

 

Gully - That sounds like the most obvious problem there then. I guess that means the car has been upgraded to overdrive at some point and has never had the speedo changed... When you say re-calibrate though, that works for the needle, but the odometer can't be recalibrated, it needs gears replacing with different ones to change the speed, will they do that too? I might have a dig and just make sure I don't have any gears lying about that might work.

 

John - The reason I mention the overdrive is because the worm and pinion gears that set the ratio for the speedo are contained in the overdrive, so wasn't sure if they were different between cars as they are changeable. Looking at part numbers on canleys would suggest that isn't the case. I was quietly hoping that would be a neat little fix there!

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John - The reason I mention the overdrive is because the worm and pinion gears that set the ratio for the speedo are contained in the overdrive, so wasn't sure if they were different between cars as they are changeable. Looking at part numbers on canleys would suggest that isn't the case. I was quietly hoping that would be a neat little fix there!

 

Pete's advice is most interesting and helpful!

 

My point is that the speed of rotation of the prop shaft, plus the diff ratio (plus wheel/tyre size but let's not go there!)  determines the input to the speedo.    It doesn't matter if there is an O/drive or not.

" the worm and pinion gears that set the ratio for the speedo are contained in the overdrive" isn't in fact the case, the worm is on the output shaft.

 

Of course, cars that left the factory with an O/d may have had a different diff ratio, but not according to JohnThomason's Originality book.  Certainly it may have been changed since then - all our cars are Bitzas now! - and a different speedo gear would be needed for each final drive,  differential, ratio.   Unless the gearing was in the speedo, which I believe is the US practice.   Which I got completely wrong, and was corrected, in this Sideways thread: http://sideways-technologies.co.uk/forums/index.php/topic/7080-speedometer-calibration-after-differential-change/

 

John

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The output shaft / Annulus on the overdrive is what the worm gear is attached to, so the worm gear is running at propshaft speed, which in my case is 3.89 times wheel speed, however the ratio of the worm and the pinion in the overdrive could be changed, so without changing the speed of the Annulus, the output for the speedo could be changed if an alternative gear set were available. Of course, there doesn't appear to be any alternatives, so that would have to be custom made.

 

Does anyone know what the ratio on the speedo drive is? Without having to dismantle it, I can't count the teeth... Is it just 1:1 (Annulus:Speedo Cable)?

 

John, reading what you said, suggests that the ratio would be 1:1, agreed?

 

Henry

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Just did some maths.

 

So my wheels do 918 turns per mile.

That means the prop does 3,571.02 turns per mile.

 

My speedo requires 980 turns per mile.

Gully suggests it should require 1152 turns per mile.

 

If Gully is correct that means the speedo drive is approx 1/3 prop speed. 1:3 (Speedo:Prop) if I've got that correct.

 

If that's correct and I was to have the perfect speedo, it would need to be 1271 TPM unit...

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If you call Overdrive Repair Services near Sheffield they should be able to advise which one you need and they sell them too?

 

I got one off them for my Vitesse with 3.89 Diff and 'J' Type Over-drive and the speedo was spot on.

 

I'm sure they could supply a correct one for a 'D' Type Over-drive too.

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was going to say those are j types

 

on trucks we had a wild variety of pinions housed in an eccentric housing so you could rotate the hsg to suit the pinion diameter all with the same worm........if only 

 

from distant memory d type worm is a  5 start  dont hold me to that

 

ring dave twigger at odspares 

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I think I'm right in saying that if it's a 5 start worm, for me, the pinion would need to have 18 teeth to read correctly... Correct me if I'm wrong.

 

I spoke to Overdrive Repair Services, but he wasn't too sure, he suggested the pinion would usually have 15 teeth, but sounded vague... I'll try Dave Twigger.

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Pete's advice is most interesting and helpful!

 

My point is that the speed of rotation of the prop shaft, plus the diff ratio (plus wheel/tyre size but let's not go there!)  determines the input to the speedo.    It doesn't matter if there is an O/drive or not.

" the worm and pinion gears that set the ratio for the speedo are contained in the overdrive" isn't in fact the case, the worm is on the output shaft.

 

Of course, cars that left the factory with an O/d may have had a different diff ratio, but not according to JohnThomason's Originality book.  Certainly it may have been changed since then - all our cars are Bitzas now! - and a different speedo gear would be needed for each final drive,  differential, ratio.   Unless the gearing was in the speedo, which I believe is the US practice.   Which I got completely wrong, and was corrected, in this Sideways thread: http://sideways-technologies.co.uk/forums/index.php/topic/7080-speedometer-calibration-after-differential-change/

 

John

The GT6 Mk 3 parts book is very clear on the factory fit overdrive cars having different diff ratios (except for German market cars) and correspondingly have different speedos - Plates 05-41 and 06-04 refer. The latter specifically notes the overdrive German market cars as having the same as other market non-overdrive speedometers.

 

The original figments were 3.89 diff ratio 1152 TPM, 3.27 diff ratio 980 TPM. If you do the maths, these ratios assume the speedo drive ratio is the same.

 

I believe on the Mk 2 GT6 the 3.27 diff was fitted irrespective of the factory fitting the overdrive option - according to Richard Dredge's complete story book.

 

Gully

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Making progress here. So I have taken my new speedo and my old speedo to bits, done measurements and maths and have found the fault.
 
I said earlier that my speedo ideally needed to be 1190 TPM.
 
Took the new speedo apart to find this;
1 start on the worm.
20 teeth on the pinion.
That's a 20:1 ratio.
So every 20 turns of the input gives one push on the clock gear.
That has 49 teeth, so 20 * 49 = 980 TPM as expected.
 
The old speedo;
1 start on the worm.
20 teeth on the pinion.
That's a 20:1 ratio also.
Every 20 turns of the input gives one push on the clock gear.
However this one has a 59 tooth gear, so 20 * 59 = 1180 TPM which explains why that used to work.
 
It is obvious now too as this gear is made of brass and clearly isn't designed for this gauge but fits fine and works.
I'm going to pop that gear in my car and see what happens now!
 
Henry
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Gear worked, odometer is now correct.

 

Balance wheel didn't work. But I got the soldering iron, power drill, laser tachometer on it and now it is bang on over the whole speed range!

 

Success!

Well done! Never had the courage to take one apart :-)

 

Out of interest, on the one you calculated as 1180 TPM from its innards, what does it say it is on the casing? The TPM is written in small numbers below the mileage reading.

 

Gully

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Haha, I just thought I had two gauges, one was knackered and one was immaculate, so I took the balance wheel out of the immaculate gauge and preserved it so that one can be made original again if needs be, and modified the bits that didn't matter instead!

 

Both gauges are 980 TPM units, or at least, they were when new! I believe the old one was the cars original gauge but it must have been modified when the overdrive and differential were done assumably many years before I had the car. I have an extensive history file for the car, so I imagine I'll have some paperwork to confirm all of the above if I dig deep enough!

 

Gully - To confirm, it said 980 on the housing, but had been tampered with internally.

 

Henry

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