Speedo calibration and/or axle ratio change

[Brettfcars]

Hi,
Taking a just finished car out (it’s an Ulsteroid.) for some shake down tests.


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I found a few tweaks required, adjusted dampers, Tick over etc

A few tweaks still to do.

1. Speedo reads 20mph when GPS reads 45.  60mph max Speedo.

   

2. 1st gear not really required, pulls Away fine in 2nd.

3. 4th gear top speed about 45mph via gps.

Car is on 17” wheels 4speed synchro box, 37 ruby axle believe standard.


4. Temporary fitted a triumph gear selector to an adaptor plate and it works well, very direct and in right place. But fells a bit to triumph..

   


I have a 8.43 5.375 marked pinion and Ring gear in garage, would this be preferable?
I have a spare rear axle I could build it into....

So a couple of questions.
1. Would the change of rear axle improve things. Top speed and indicated speed ?
2. Is there different Speedo gearbox output gears that may improve things?

Thanks

Brett

[Howard Wright]

Hi Brett

Can’t really help with your question but that is a lovely looking Ulster  .  I particularly like the colour 

Cheers

Howard

[John Cornforth]

Hi Brettfcars

Assuming your rear axle is a late Ruby 5.125 to 1, and the tyres are 4.00 - 17 with 803 revs per mile, the engine turns 4115 revs per mile in direct top gear. At a mile per minute (60 mph) the engine would be doing 4115 RPM. Or put another way, the gearing in top will be 14.6 mph per 1000 RPM.

45mph top speed sounds rather low to me, that's only 3080 RPM. I doubt if changing the gearing by 5% by going to your other diff would alter it much. Maybe the engine still needs some tuning to help it develop more power at higher RPM.

The speedo is a long way out, I'll leave it to others to suggest a cure.

[Stuart Giles]

If your speedo came off a car with a three speed gearbox it will be a long way out. The ratio of the speedo drive gears is different in the two 'boxes. Ratio is 4:1 in the four speed IIRC, but much higher in the 3 speed box AFAIK, possibly a relic of when the speedo drive was provided by a belt off the propshaft on early cars.

[Robert Leigh]

I think your speedo problem is 4 speed box with early speedo. On mine I took the easy way out, making a plate the size of the vintage speedo and fitting a late Ruby speedo. Not very vintage but gives a nearer indication of correct speed. If you change the internals on the early speedo to give correct speed you will need to do a conversion on the mileage figure: you can't win!

[Alan]

What you need is a corrector box from someone like Speedograph Richfield. You need to know the turns per mile coming out of the speedo drive on the gearbox, and the turns per mile the speedo head needs. They do a set of gears which fastens to the back of the speedo. Costs about a hundred quid. Sounds a lot but it will be a lot more to have the speedo recalibrated properly.

[Brettfcars]

Hi all,
Thanks for replies.
Colour is Jaguar BRG, old version as there’s more than one.

The engine require’s running in, and the carb is unproved.
I did rebuild the Speedo, and rev counter. Some confirmation of correct speed rev’s per rotation may be required !

Very noticeable is how the 1st gear seams to be a bit pointless. But my dad says the same of a MG TD he owns.

I will check the rev counter As I may be holding the engine back.

And Do a bit of engine tuning, not sure I have the timing where it should be !

I will check the axel is Standard ruby, and investigate the spare parts I have. Interesting it’s Only 5% different. 

Thanks

Brett

[Jamie]

Good morning.

I cannot help with the questions, but I do like the Triumph gear shift. It looks very neat and tidy. It is the same as the one which I fitted to my Standard 10 donkeys years ago. I planned to make a remote shift for my car, but I may keep an eye out on ebay for one of these instead.

Jamie.

[Mike Costigan]

Pre-war cars (and indeed many cars of the fifties and sixties) usually had an 'emergency' first gear which would only be used for hill-starting with a full load, so normal driving practice was to start in second gear. The Ruby was no different, so even standard Seven mechanics in a light-weight body will render first gear redundant in most circumstances.

The remote gear linkage looks to be standard Triumph Herald, so should be readily available for very few pennies; don't be tempted to pay extra because it's a Spitfire or GT6 change, they are all identical!

[Neil Morrison]

Brett, 
You may find this website useful:-  www.magnetic-speedometer-repair.com - .Don't be put off by "magnetic" he covers governor types as well. ( I am assuming that yours is the governor type).  If your drive train is as John Cornforth suggests then I would be reasonably certain that your speedo drive on the gearbox gives 1040 turns per mile.   The website gives specific instructions plus a formula to check this.   However you also need to know the TPM of the speedometer itself which is presumably higher.   Some  speedos have the TPM marked on the face but the Smith's governor type seem to use  letters .  I have a Smiths governor type marked TN which turned out to be 3360 TPM.  As my gearbox is as yours,  Speedy Cables ( same sort of work as Richfield Speedograph ) supplied me with a conversion gearbox to bring the 1040 up to 3360.  Interestingly they insisted that the converter should not be more than 10ins away from the gearbox drive so the cable is in two sections ( I needed a new cable anyway). Something to do with potential cable breakage? 

In the not too distant past there was a thread - 'Early speedometer and 4 speed gearbox' on this very subject.

Neil.