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MG Midget and Sprite Technical - 1971 Midget diff ratio`s

Hello I'm looking at a midget with a Frontline five speed gear box ,it states the diff is a 3.7:1 high speed.

Can anyone explain regarding the diff ,Is it standard ratio.

DCP Parker

According to Terry Horler's book, from December '68 the ratio was changed from 4.22 to 3.7. Then in August '77 on the 1500 Midget it was changed to 3.9.

b higginson

Bernie: that's not right! In 1968 the ratio was changed to 3.9 not 3.7. The 1500 had the 3.7
(Terry had got it right, you have just got the numbers the wrong way around)
G Williams (Graeme)

Thanks ,so it has a standard diff ,what is a "high speed"

3.7 does not seem a very high geared diff ..

Any idea how the standard Sierra gear box will effect the gearing.
DCP Parker

standard (not 2.8/2.9 V6)

first 3.65:1
second 1.97:1
third 1.37:1
fourth 1.00:1
fifth 0.82:1
Nigel Atkins

3.7 is the highest available diff ratio for Spridgets. It was only fitted to 1500s from August 1977. It's easy enough to nock up a spreadsheet using the Sierra box ratios with various diff ratios, and a set tyre size (turns per mile) to work out RPM at say 70 mph.
David Smith

Aftermarket 3.5 diffs are available.
Trevor Jessie

FWIW from Moss's catalogue

R W Bowers

Graeme is of course correct. I got my 9s and 7s mixed up. Well it was Sunday, lunchtime at the pub and all that! Sorry.
b higginson


Where can you get 3.5 diffs from?

Eddie Brown

After being unobtanium for a few years it appears Moss at least have them in stock, no doubt others do.
Paul Walbran

And ther is some heritage to the 3.5 diffs
Healey used them on their le Mans specials to go where no spridget had gone before!
Onno K

I wonder what the quality is like. I have a 3.7 diff fitted to mine. Frontline rebuilt an old one for me and it is excellent, completely silent. They attempted to obtain an original 3.5 but with no success. They have had problems with new ones. The metal quality being suspect.

I will speak to Moss.

Eddie Brown

I know a guy that bought a 3.5 back in the late 1980s when Special Tuning were having a clearout, I may have to enquire if he still has it. Best I can do is a NOS 3.9.
David Billington

Re discussion of Type 9 gearbox and other diff ratios.

If you start off with the 4 pot Ford type 9 gearbox ratios of

First 3.65:1
Second 1.97:1
Third 1.37:1
Fourth 1.00:1
Fifth 0.82:1

and A type gearbox ratios of

First 3.628
Second 2.172
Third 1.412
Fourth 1.0

First 3.2
Second 21.916
Third 11.357
Fourth 1.0
apart from the 5th gear, driving a 1275 with Type 9 gearbox would be more like having a 1098 gearbox fitted to your 1275 engine but with a bigger hole between 1st and 2nd.

Assuming your 1275 originally had a 3.9 diff, if you now fitted a 3.5 diff, the Type 9 ratios would "appear" that your gearbox had the following gear ratios with an "apparent" 3.9 diff.

First 3.3:1
Second 1.8:1
Third 1.2:1
Fourth 0.90:1
Fifth 0.74:1

the ratios are now "closer" to 1275 gearbox ratios and your car would accelerate at about the same rate as with a 1275 gearbox but you may struggle in 5th because Fourth and Fifth now "appear" to be overdrive ratios. You would need the engine tuned more for low down torque to pull these gears but it would be a great motorway driver.
MG Moneypit

There must be a point where the little 1275 engine will not pull very well .surely your better having the 3.7 diff and a higher fifth for cruising .

My old 1500cc needed another gear .

So what would be the rpm at 70mph with the 3.7 and standard type 9 gearbox in fifth ..opposed to a four speed with the 1275 engine
DCP Parker

Assuming 16.5 mph/1000 rpm (I'm not sure if that is correct but it's close enough) in top gear(1:1) with a 3.9 diff, the speeds in intermediate gears would be.

First 4.55 mph/1000 rpm
Second 7.6 mph/1000 rpm
Third 11.7 mph/1000 rpm
Fourth 16.5 mph/1000 rpm

First 5.16
Second 8.6
Third 12.2
Fourth 16.5

Ford Type 9
First 4.52
Second 8.4
Third 12.0
Fourth 16.5
Fifth 20.1 mph/1000 rpm

If you fitted a 3.7 diff the above figures would be 3.9/3.7 times higher (x 1.05)

Fitting a 3.5 would be x 1.11 those figures.

I noticed a typo or two in my previous post.
They should be
A type gearbox ratios of

First 3.2
Second 1.916
Third 1.357
Fourth 1.0

MG Moneypit

So what would be the rpm at 70mph with the 3.7 and standard type 9 gearbox in fifth?
David Smith

From those figures MG M has provided, it works out as 3325 rpm @ 70mph for a 5th gear Type 9 and 3.7 dif. Depending on your tyre size of course. ;-)
Guy W

Yes, 70 mph in Type 9 5th would be

3482 for 3.9
3325 for 3.7
3137 for 3.5

Tyre sizes would have some influence but sticking to 13" wheels would make a few % difference.

3.5 probably over doing the gearing unless you are into long distance motorway driving.

MG Moneypit

find out who fitted the FL gearbox if it was FL themselves in the last few years then I can recommend at great financial cost and hassle that you be very wary of it
Nigel Atkins

I have a 3.5 in my 1500 and could still do with higher gearing! But that is just me of course.
JB Anderson

This thread was discussed between 11/08/2013 and 12/08/2013

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