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MG MGA - 4.1 diff

Hi All.
To reduce the revs whilst cruising I was thinking of fitting a 4.1 diff rather than my 4.3. Are there any 4.1 diffs out there for sale?
Thanks. Simon
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gearbox five speed conversion instead. A used 4.1 will be had to find, and although you can buy the gear sets as a set they are HORRIBLY expensive at
dominic clancy

if you fit a 4.1 diff your gearing in 4th gear will be 17.8 mph per 1000 rpm instead of the 17 mph per 1000 you get from the 4.33 diff.
So not very much difference.

A 3.9 diff would give you 18.7 mph per 1000 rpm and so you would definitely notice the reduction in revs with that one.
If I had a 4 speed box and was just going to change the diff then I would go with the 3.9, but unless you increase the power of your engine you would also notice the reduction in performance too.

As Dominic says, the 5 speed is the perfect answer as, with a 4.33 diff, gives you virtually the same ratios in 1st, 2nd, 3rd and 4th.
This plus the advantage of synchromesh on 1st and an overdriven 5th gear which with a 4.3 diff gives 20.7 mph per 1000 rpm.

More expensive though!


Have you looked at Mick Andersons brilliant excel spreadsheet which lets you input the various diff options to see how this affects your gearing.
You also have to input your tyres diameter, for accuracy, measure the circumference of the tyre and calculate the diam using D = C/3.142

Colyn Firth


Just a word of warning if you use the tyre circumference and pi to calculate the diameter.
Remember the old joke "my tyre is only flat on the bottom".
This is true when using the tyre circumference in accurate calculations. I have this problem when setting up traction control on a race car where you have different size wheels on the front and rear.
At slow speeds the depressed bottom of the tyre is at the greatest and the tyre becomes more circular at higher speeds.
The can be ignored for normal use but be aware of it.

M F Anderson

Thanks for that Mick,
funnily enough, the subject of tyre distortion came up some time ago and I since then i have wondered if the flat "contact patch" at the bottom actually made any significant difference the rolling circumference?

If you consider an under-inflated tyre for example, although the radius from wheel centre to the road is much less than normal, if you actually put a tape-measure around the circumference, would it not still be the same as a fully inflated tyre?
And therefore, the tyre would move the same distance forwards with each revolution?

A little like the caterpillar track on a bulldozer I suppose.

Or have I missed something important here? Like does the tyre buckle and distort at the contact patch? ☺


Colyn Firth


Your last comment first.
Yes, the tyre does move forward the same distance at each revolution, at a given speed, but that is not the problem.
The question is what do you use as the diameter of the wheel in the spreadsheet calculation? Twice the loaded radius, twice the unloaded radius, or the average of the two.
You can see the problem if you draw a circle and then a chord at the bottom. What is the circumference?
By simple trial and error I used the height of the tyre tread at the unloaded top down to the ground.
This in effect gives the average of a loaded and unloaded tyre.
When doing Traction Control and Launch Control it really does not matter as you are just comparing wheels with each other.
I suggest you use the same method for road use as you average the low speed and high speed tyre effects.

M F Anderson

I would love to discuss this further with you Mick but I don't want to sidetrack Simons thread.
Email me if get the chance and maybe we can continue this and put me straight ☺
Colyn Firth

Hi All,
Many thanks for your comments. I didn't realise the difference per 1000 revs was so small. I am gathering parts together for a swap to tuned 1860cc engine and 5 speed conversion next winter. I thought the 3.9 diff would cause me to lose some of the extra zip the new engine will give me which is why I was enquiring about the 4.1.
I have the base engine and the 5 speed box, and will soon empty the piggy bank for the HiGear kit.

Regards. Simon
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Did you find a backplate?
Dave O'Neill 2

if you do go ahead and fit the 5-speed box, stay with the standard 4.3 diff.

I had the 1850cc (105 bhp)engine with the above and it was a great combination. It gave pretty good accelleration combined with quieter cruising.

I did think about fitting a 3.9 diff with this but I decided against it because the car already struggled a little to pull 5th gear on long hills and the 3.9 would only have made made this worse.

Colyn Firth

I swapped in a 3.9 for a 4.1 and didn't notice anything...not even a loss of power. It's such a minuscule difference it's not worth the cost or the effort.
Gene Gillam

Hi all.
Thanks for your comments . I will stick with the 4.3.Yes Dave I have a early 3 bwaring 1800 backplate and front plate and a 1.8 Marina flywheel which I think is the same diameter as a 1600 A one and certainly smaller than the 5 bearing B one that came with the engine. Higear here I come.
l will let youknow how it all goes together.
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This thread was discussed between 23/05/2016 and 26/05/2016

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