Speedo calibration - 3.7 diff 195/65/15 tyres

[jake_a]

A long shot but has anyone got the figures for the number of turns of the inner speedo cable their car makes with ten turns of the rear wheel?

Your car needs to be identical to how mine will be;

3.7 diff

25% J type OD

195/65/15 tyres

cheers,

Jake

[DRD]

Jake,

I used to run those size tyres with the 3.7 diff and the speedo over-read by about 7% at 70mph.

I went back to the standard size tyre in the end because the lower profile tyres meant the exhaust kept hitting the ground.

Daz

[Charlie D]

Jake,

I think you also need to specify what gearbox you are using. I guess you have a “Standard” TR6 box, (If there is such a thing) but if it has been fitted with a saloon box I “Think” the pinion gear ratio of the speedo drive is different.

It doesn’t really matter if the box has an overdrive as the take off for the speedo is behind it, directly driven from the output shaft that goes to the prop flange.

Charlie D

[john.r.davies]

Roll the car for one wheel revolution, chalk line on tyre and set square to make sure its vertical at beginning and end.

5280/distance in feet = revs /mile

If you have enough space, then ten wheel revolutions and use 528/distance - lessens the error.

JOhn

[jake_a]

3 hours ago, john.r.davies said:

Roll the car for one wheel revolution, chalk line on tyre and set square to make sure its vertical at beginning and end.

5280/distance in feet = revs /mile

If you have enough space, then ten wheel revolutions and use 528/distance - lessens the error.

JOhn

Hi John, the car is on old (wrong size) tyres at present.

[Charlie D]

Jake,

Far too cold to go pushing a car up and down the road. (And who can put their hand on a piece of chalk when they need one?)
Sit in your favorite armchair, by the fire, and try this:

https://www.tyresizecalculator.com/tyre-wheel-calculators/tire-size-calculator-tire-dimensions

(John,
Your method may be more accurate, but the calc. is a lot easier.)

However, that does not answer your question:

“…the number of turns of the inner speedo cable their car makes with ten turns of the rear wheel…”

With a 3.7 : 1 diff, the prop will have to turn 37 times to get the wheels to turn 10 times.
The pinion drive to the speedo cable will be something like 2:1 ratio (but you do need to find out accurately.)

This is where I’ve forgotten which way around the ratio is…
The speedo cable will either turn
 37 times 2
Or is it
37 divided by 2

Easiest way to find out is to disconnect the prop shaft and the speedo cable and turn the prop, noting how many turns the speedo cable moves.

Hope this makes sense, (and I’ve got it right.)

Charlie

[ntc]

http://www.jdo1.com/calculation_page_1.html

[BlueTR3A-5EKT]

78.5" is the rolling circumference  of 195/65/15 tyre

 

https://www.tyresizecalculator.com/tyre-wheel-calculators/tire-size-calculator-tire-dimensions

Edited February 5, 2020 by BlueTR3A-5EKT

[john.r.davies]

2 hours ago, jake_a said:

Hi John, the car is on old (wrong size) tyres at present.

No problem, jake!

Do that, then the no,of revs with new tyres will be given by the ratio between the rolling circumferences of the old and new tyres.

Simples!

JOhn

[John McCormack]

19 hours ago, Charlie D said:

Jake,

I think you also need to specify what gearbox you are using. I guess you have a “Standard” TR6 box, (If there is such a thing) but if it has been fitted with a saloon box I “Think” the pinion gear ratio of the speedo drive is different.

It doesn’t really matter if the box has an overdrive as the take off for the speedo is behind it, directly driven from the output shaft that goes to the prop flange.

Charlie D

Yep. My 1970 PI has the 3.7 diff and a saloon box. The speedo is way out but it just so happens the mph speedo (we Aussies changed to km in 1975 or thereabouts) reads almost exactly half the speed in kmh, which we now use.

When the speedo is reading just under 30mph I'm doing 60kmh, just under 40mph = 80kmh, 48 mph = 100kmh etc. 

[Charlie D]

All,

I do think there is some confusion going on here.

The number of revs of the speedo cable for a specific number of revs of the rear wheels has nothing to do with the size of the tyres.

It is simply a factor of the diff ratio and the speedo drive pinion drive ratio.

The only time you need to know the tyre size is if you need to know how far the car has traveled in relationship to the number of turns of the speedo cable.

(That, I believe, is the number that is printed on the speedo face---The number of speedo cable revolutions to go one mile.[Or Kilometer if it is a Kmph speedo])

A TR3 will have a number like 1180 (1180 cable revs when traveled one mile). BUT this is only valid if the back axle, gearbox, (specifically speedo pinion ratio) and tyres are the same as when the car came out of the factory.

Charlie.

[john.r.davies]

Sure, Charlie, "The number of revs of the speedo cable for a specific number of revs of the rear wheels has nothing to do with the size of the tyres."

But it has everything to do with speed, and speedometer, when different rolling circumferences of those tyres are applied.

JOhn

[Mk2 Chopper]

And how could you bodge calibrate the speedo to read more accurately, mine is also reading around 10mph less than the actually speed I'm doing. 

Gareth

[john.r.davies]

The cable drives  a magnet on a rotor , that drives the the speedo needle.    The speed of the rotor governs how far round the dial the needle goes.

I believe that American speedos have interchangable cogs between the cable and the rotor, but that in the Uk this must be done by a specialist technician, like 'Speedy Cables'.

Score one to the Yanks.

J.

[Rob Salisbury]

The speedo needle sits on a tapered spindle and remains there simply by friction (much like the hands of a watch or small clock), you can remove complete speedo head from the case (bezel and glass off first, then screws at the back), then carefully hold the cup that is driven by the spinning magnet and move the position of the needle (in your case Gareth advance it approx 10mph) ... as a check allow the needle to return to the stop (the mechanism is governed by a hairspring, once again similar to a clock or watch) and then carefully move it past the stop and see if it comes to rest next to the little silver dot on the speedo face, if you are using standard sized tyres then you will be in the right ballpark, ... reassemble the speedo and check with a satnav or similar on a flat level road and re-adjust as necessary.

Cheers Rob

[Charlie D]

Rob,

The problem (I think) using that method is that the result will only end up accurate at one speed. If you set it to work at, say, 60mph, it may well be good enough to be legal at points above and below, but it will never be 100% accurate throughout the entire range.

What you need is a  “Sun speedo calibration Unit 210” like this:

https://www.worthpoint.com/worthopedia/vintage-sun-speedometer-tester-1722098390

I bet 1000s were made

And 99.9% have now been melted down…

Good luck if anyone tries to find one.

 

Charlie

[Mk2 Chopper]

3 hours ago, Rob Salisbury said:

The speedo needle sits on a tapered spindle and remains there simply by friction (much like the hands of a watch or small clock), you can remove complete speedo head from the case (bezel and glass off first, then screws at the back), then carefully hold the cup that is driven by the spinning magnet and move the position of the needle (in your case Gareth advance it approx 10mph) ... as a check allow the needle to return to the stop (the mechanism is governed by a hairspring, once again similar to a clock or watch) and then carefully move it past the stop and see if it comes to rest next to the little silver dot on the speedo face, if you are using standard sized tyres then you will be in the right ballpark, ... reassemble the speedo and check with a satnav or similar on a flat level road and re-adjust as necessary.

Cheers Rob

I should give this a go, and seeing as the speedo has been apart the glass and bezel can be taken off quite easily. 

I guess the correct way is somehow lessening or increasing how the magnet affects the rotor. 

Gareth

[matttnz]

Whilst I respect and love the attempts to do this "old school", why don't you just GPS your speed with a smartphone to convert?

[Rob Salisbury]

The thing is the mechanical nature of these old speedos means that there is a balance between the force applied by the magnet and the returning force applied by the hairspring, so long as the needle is in the correct position then the speedos accuracy will be maintained, .... provided your wheels and tyres offer the original rolling diameter then once corrected to the satnav/GPS's given speed then the speedo will be correct (within original tolerance) throughout it's range.

My brother once had a V8 MGB GT that had significantly lower profile tyres fitted, the speedo was reading 30% lower than the actual speed, we removed the hairspring, accurately weighed it and then with constant checking removed 30% of its weight by dissolving it in Nitric Acid, .... we got the speedo to well within the 10% tolerance,  ...  I have to add here that my Dad was a watchmaker and jeweller, and my brother took over the business when Dad retired.     

[john.r.davies]

Wow!   Impressive craft skills!     Never ever thought of dissolving a spring to weaken it!

JOhn

[dingle]

Here's an article on re calibrating the speedo by changing the strength of the magnet.  http://bullfire.net/TR6/TR6-40/TR6-40.html 

Berry

[jake_a]

16 hours ago, matttnz said:

Whilst I respect and love the attempts to do this "old school", why don't you just GPS your speed with a smartphone to convert?

Would love to, would they be able to do that but my old speedo went “POP!” last week!!