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MG Midget and Sprite Technical - Midget diff ratio

Hi I have a '78 midget fitted with 155/80/13 tyres and wondered if anyone can tell me what speed I should be getting at 4,000 revs. The speedo reads around 60 but on a sat nav it is showing around 56. I have had a look at the diff and can't really make sense of the markings. I suspect it is 9/41. What numbers should I be seeing on the top of the diff. Many thanks
m aldridge

A 1978 car should have a 3.7:1 ratio and the axle stamped 11/41

The 3.7 (11/41) will give 17.3 mph / 1000 rpm so it would be 69 mph at 4000 rpm.

A 4.22 (9/38) early axle will give 15.4 mph / 1000 rpm and 61 mph at 4000 rpm.

A 4.55 ( 9/41) axle from a Morris minor etc will give 14.2 mph /1000 rpm and 57 mph.

So it looks like yours is a non original swap at some time.
Chris at Octarine Services

Thanks Chris that is just the info I needed I suspected it had the wrong one as it struggled to keep up with my Peugeot 204 diesel. Would the minor complete axle fit on a midget as it looks like the diff has been in the casing a long time rather than having been recently robbed. Anyone have a decent diff for sale
m aldridge

I have no idea, not a Minor expert!

But my guess is that only the diff was swapped as it is an easy, in the car, job to do.
Chris at Octarine Services

Also check your rev counter if it's an original one, mine was over reading by about 700 rpm at higher revs.
You can adjust them I think, a little screw on the back but obviously you need something to calibrate it to - like an automotive multimeter etc.
John

A minor axle is about 5" wider than a Midget, so won't fit.

A way to check the diff ratio is as follows:-

Jack up the rear of the car (safely) and lock one wheel so it can't turn.

Mark the propshaft so that you can count the number of turns.

Mark the wheel which isn't locked, then rotate it two complete turns, while counting the propshaft turns.

The number of propshaft turns will give you the ratio.

If your diff has been swapped for a different one, the speedo and rev counter will still correlate, as the speedo drive is taken BEFORE the diff, so it gives a theoretical reading rather than a true one.
Dave O'Neill 2

M,
Chris was working on standard 145/80/13 tyres with your bigger tyres of 155/80/13 on a standard '78 1500 should show 2.85% more so around 71mph.

Tyre calculator here -
http://www.carbibles.com/tyre_size_calculators.html#

Wheel & Tyre Bible -
http://www.carbibles.com/tyre_bible.html

The following site will be useful to you if you discover your axle (and/or gearbox) ratio(s) but bear in mind with this calculator page it seems to round the figures up more so not exact figures but enough to give you a good idea -
fisc-racing.com
http://www.fisc-racing.com/misc/ratios.html
N Atkins

ETA: Dave has put me right - it won't show the increase on the car but on the SatNav.
N Atkins

Apart from John we are assuming that the rev counter is accurate - this ain't necessarily so. The difference between the speedo and the sat nav is common, even on modern cars.
Mike Howlett

That's a good point.

Though I think normally the electric rev counters at least aren't that far out (normally, usually, often, etc.) and it wouldn't pay to work things out to too many decimal places with all the variances and knowing the figures as they (about) should be should get 'm' to see what might be very inaccurate.

My rev counter is out by different amounts at different points in the range but not enough to make any real difference.
N Atkins

The 3.7 ratio was from car GAN6-200001 - Aug 77 so worth checking as car may still be '77 build which would have the 3.9 ratio.

Markings are usually on the flange which butts against the axle casing.

What part of Zumerset are you in ?
richard b

The markings on the diff are not very clear which is why I have asked the question. It does appear that it is running the Minor diff as the engine is sceaming at anything over 60 measured on the sat nav. The car belongs to my lad who is in Weston
m aldridge

An easy way to decide perhaps is to meet up with a standard late 1500 and compare, more chance now as more will soon be coming out of hibernation.
Nigel Atkins

Or check the diff ratio, as I described earlier.
Dave O'Neill 2

Dave,
your previous post has prompted me to see about getting my speedo recalibrated after the g/box conversion and they need the rear wheel turning and circumference of tyre (as I don't have a reliable trip on other car or SatNav), yet another lot put not to jack the car up (I know about the rolling circumference difference).

My question for you please is why when the speedo drive is on the gearbox do they want this info?

I know my speedo is 25% slow (or is it 33.3%?) throughout the range from '000's of miles of driving with it, e.g. it shows 30mph when my road speed is 40mph.

Cheers.
Nigel Atkins

because the speedo drive is driven by the output shaft, which is directly connected via the diff and the wheels to the road.
davidsmith

David,
thank you, still not quite sure but I'll look it up later

- but - to save me looking please, you quoted a person and phone number, if he does recalibration and refurb (my trip twist stick is missing) could you post the details again please.

I'm just about to start the biannual cleaning of the Midget ready for next week's rides at NEC show but will keep popping backs for mugs of tea, beautiful day and I'm cleaning the car instead of driving it.
Nigel Atkins

Sure, it's RICHARD JENKINS, UNIT 49H, PIPERS ROAD,
PARK FARM INDUSTRIAL ESTATE, REDDITCH, B98 OHU
email woodykeys67@msn.com
phone 07747 357044
davidsmith

Thanks David, that's excellent.
Nigel Atkins

I used Richard for exactly that job. He did a complete recalibration and repaired the trip. Spot on now!
Graeme Williams

Nigel: if you have a smartphone there are apps you can down load to give you gps speed. If you can do that Richard can recalibrate if you give him a list of indicated speeds and gps speeds. Much easier than all the rolling diameter stuff as long as you take the gps on a level road.
Graeme Williams

Hi Graeme,
I've got a set of (?)20+ year old cordless home phones and a top up basic mobile that used more or less only for the rides all others time it sits on the desk top.

I use to deal will mobiles in the 80s, "hello, can you hear me", "hello", "yes sorry I lost you can you hear me now", and things haven't improved that much when I get calls from mates nowadays, "no you fade out, say again", "can you hear me".

With the speedo I'm happy to accept plus 33.3%, i.e. 48mph shown on my speedo is 64mph, 30mph dial is 40mph road speed, etc..
Nigel Atkins

This thread was discussed between 18/03/2017 and 26/03/2017

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