Wrong diff on my new Minor?

[Dryad]

I've just bought a very nice early '63 Minor fitted with a 948cc engine - probably a car that had been sitting in a car showroom for a few months because they changed to 1098cc engines in Sept '62. When I drove it home I was going almost flat out, and although the speedometer said I was doing 75mph it was, according to my friend following me home in a modern car, actually 55mph which judging by all the cars streaming past me on the dual carriageway was about right. What I was wondering was if the car has had a rear axle fitted with the wrong diff - it would explain the incorrect speed showing on the speedometer. Any ideas? My old '68 Minor with a 1098cc engine could manage 75mph quite easily.

[bmcecosse]

If the engine is 'ailing' then 55 might be all it can manage! The speedos do tend to over read more and more as they age - and the spring weakens. To check the final drive - you can either jack up one wheel and turn the prop ten times while counting the wheel turns- or pull out the diff and inspect the gears for the ratio. A 948 car should have the 4.55 final drive. Maybe it has been fitted with the earlier 5.3 final drive ! Was the engine revving really hard ? Other option (and more likely really) is that it has the wrong speedo fitted.......... tell us the tpm (= turns per mile) figure on the dial - small 4 digit number.

[Dryad]

The tpm is 1504 and I think the speedo is a replacement as the milage indicated now would mean it had done 45000 miles in 2 years according to old MOTs if it was still the original speedo - unlikely! The engine is in good order and was recently fitted with an unleaded head, and there's no smoke or knocking. At that top speed it really did sound like the engine was at the limit of it's revs.

[Nuffles]

I'm having similar problems to these in Aristotle. The speedo reads 55-60 and the satnav says 45mph this is at what 'feels' to be the optimum engine rpms, verging in too high. Bummer.

[bmcecosse]

1504 is correct for a 4.55 final drive...........which is correct for a 948 car. So - you need to check the final drive ratio now!

[Dryad]

Hmm - I might do that tomorrow.

[Dryad]

I'm having similar problems to these in Aristotle. The speedo reads 55-60 and the satnav says 45mph this is at what 'feels' to be the optimum engine rpms, verging in too high. Bummer.

Sounds familiar. Looks like we'll both be jacking up our Minors tomorrow.

[bmcecosse]

Make sure it's properly supported lads - DO NOT use the standard issue side mount jack - EVER!

[mike.perry]

Isn't the ratio stamped on the diff casing?

[charlie_morris_minor]

i think it might be easier to turn the wheel a few times than try and clean off all the crud on the diff casing looking for some numbers

[bmcecosse]

I haven't found the ratio - counting turns is fool proof.

[BitMoreBlurred]

My speedo also overstates the speed (although my wife quite likes this - she calls it quirky).

My speedo cable is quite slack and tends to swing easily. I suspect that both the speedo spring and cable need replacing. However, as the car was brought back onto the road after a long rest, we had it upgraded from 948 to 1098. Does this mean that the speedo also needs to be changed to match the new engine, or am I misunderstanding?

Have fun,

Mikey

[bmcecosse]

No - engine change - or gearbox change makes no difference. It's only when the final drive ratio is changed that the speedo needs to change to keep things right.

[RobMoore]

Wheel size/tyre size change from the non-standard will also affect your speedo.

[Dryad]

After a few attempts with interruptions (including someone asking me the way to somewhere ) I think for every ten revolutions of the prop it turns the wheel approx 4.4 times. Does that mean it's a 4.55 diff? I know nothing of these matters

[alex_holden]

After a few attempts with interruptions (including someone asking me the way to somewhere ) I think for every ten revolutions of the prop it turns the wheel approx 4.4 times. Does that mean it's a 4.55 diff? I know nothing of these matters

Yes. Because only one of the wheels is turning, it will be turning twice as fast as it normally would on the road, so divide 4.4 by 2 to get a ratio of 10:2.2. 10/2.2 = approximately 4.55.

[bmcecosse]

Well done - you've cracked it!

[Dryad]

Ok, thanks. Looks like I have to stick with 55mph top speed then. Would there be any advantage to changing to a later diff? In Paul Skilleter's book it mentions in the chapter about the 1098 MInor "...the rear axle was ratio was raised from 4.55 to 4.22, giving more relaxed cruising."

[bmcecosse]

Indeed it was - but your car should do much more than 55 mph true speed - there must be a lack of power........ the 4.22 makes hardly any difference - not really worth it unless doing high mileage.

[Dryad]

The engine does sound like it's running at the top of it's limit, but it could be me as it's been three years since I've had a Minor on the road, and I've got used to modern cars with 5 gears. I'll see how it goes when I've sorted the gearbox problem.