Diff ratio

[peters3103]

Hi,
My Speedo (SN 6322/00), which I believe is for a 3:31 diff ratio, is reading fast. I suspect my diff ratio has been changed at some time before my ownership. How best do I determine my diff ratio (no tag fitted) so I can have my speedo recalibrated. Incidentally I’m running correct size Michelin XVS 185VR15 tyres.

Pete

[Philk]

Hi Pete

You are right that that speedometer reference number should be for a 3.31 ratio (for a 3.8). What I would highlight though is that the Speedometer would not have had the Jaguar cat's head logo on it (see photo below) which has replaced the Smith's name.



As such, it could be that your rear-diff ratio is correct but the speedometer has been altered at some point in its life. To the best of my knowledge, you would expect a 3.07 rear diff ratio to be reading around 2750 RPM at 70mph and a 3.31 ratio to be reading around 3000 RPM at the same speed. A 3.54 ratio would read correspondingly higher on the rev counter. All of these carry the caveat that your rev counter generator and rev counter are producing an accurate reading!

[mgcjag]

Hi Pete...jack up the rear end.....turn tbe propshaft by hand......count the turns of prop and wheel to calculate....Steve

[peters3103]

Thanks for replies gents. Some checks to carry out.
Interestingly the rev counter has the Jaguar cats head in place of the Smiths name also. Something I’ve never noticed after 14 years of ownership!
I wonder how that came about.

[angelw]

Hello Peter,
You're better off taking a larger sample than only tail-shaft rotations to rotate the rear wheels once. Ten full turns of the tail-shaft will equal very close to three rotations of the wheel if the diff ratio is 3.31:1. If the result is not close to three turns of the rear wheel, then 3.31 can be eliminated. Nine turns of the tail shaft will result in the rear wheel turning close to 3 turns of the rear wheel if the diff ratio is 3.07:1. This procedure will give more definitive results than trying to judge small parts of a rotation of either the tail-shaft, or rear wheels.

Regards,

Bill

[abowie]

My approach is to put 3 white paint marks on the speedo at 60kph, 100kph and 110kph. None of my speedos read accurately.

If you do want to get yours calibrated send it to Speedograph Richfields in the UK. It'll cost 1/3 of what you'd be charged here and they have all the correct parts to set the odometer correctly.

[angelw]

Hello Andrew,
There's an old fellow in Clunes, Victoria, not far from Ballarat, that has been working in the field of gauge calibration for all his working life. His workshop is like working into a time capsule; he has all the original Smith calibration instruments.

He charged me $50.00 for the last speedo I had calibrated.

Regards,

Bill

[peters3103]

Thanks again for more useful information gents.
I would like to get my speedo as accurate as I can get it. Bill, could you PM me with contact details of your chap in Clunes please? I did contact an instrument repairer in Heidelberg. They quoted $330 for recalibration, a service and new ratchet mechanism for the odometer.

Rgds,
Pete

[abowie]

. Bill, could you PM me with contact details of your chap in Clunes please?

Rgds,
Pete

Me too please!

He's going to be busy...