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MG MGA - 5-Speed Conversion

I am doing a body off restoration of a MGA 1600. Standard engine has been rebuilt and I am putting a 5-speed conversion kit in. While I have it this way I am trying to decide which will be the best rear end to use. Has anybody had any experience with the different rear ends, 4.3, 4.1, 3.9 ??
J Butler

I initially ran with the standard ratio. Lovely feel and probably the best for local driving. I went to 3.9 because I was doing a lot of motorway (freeway) driving. This gave me long distance/low rev/low noise cruising and comfortably keeping up with the traffic flow. Slight loss in low speed, low gear acceleration. Probably most noticeable when starting off from stationary. However, with the T9 box you can hold the gears much longer which makes up for it.

Steve Gyles

I have a t9 5-speed and a 4.3 diff which is probably the best compromise for mixed terrain.
I recently did a 2000 mile trip to the Pyrenees and back, a 3.9 diff would have made the long motorway runs even better by reducing the revs, reducing noise and saving fuel.

We drove over all the high passes in the pyrenees, many of them up to 7000 ft and on most of the long climbs I was constantly changing between 2nd and 3rd gears as 3rd gear was a little too high to pull and 2nd a little low.
A 3.9 diff would have made 3rd gear too high to use at all.

The ideal diff for the mountains would have been a 4.55, it would have made 3rd gear perfect for climbing those long mountain passes but the engine would have been revving its head off on the 800 mile motorway run down from the uk.

I have an torquey 1850cc mgb engine fitted (approx 105 bhp) so there is enough power.

So if you live in a region with lots of wide open plains, the 3.9 would be fine but if you regularly climb lots of hills, you are probably
better off with a 4.3.
I haven't mentioned the 4.1 as they are now so difficult to find.

My probable solution to getting my car to pull 3rd up the mountains is going to be to fit a supercharger!
c firth

Just drove our '56 A roadster 6,500 miles to Ottawa and back to Vancouver, in July.

Have the FORD 5 speed, 1800 engine, and 3.9 diff. Cruising speed was 60 - 65 mph, and we did average of about 30mpg. drove 300 miles per day for the first 10 days, and 400 miles per day for the return trip. Revs at 60 miles an hour is just shy of 3000 rpm.

Very comfortable, very quiet, and plenty of power for the Rockies and The Continental Divide. One morning, in Wyoming going to Utah, we crossed the CD 4 times, each time over 8,000 feet!

P. Tilbury

I suppose it depends on the type of roads you are driving on, the roads through the Pyrenees were often single carriageway with just enough space for 2 cars to pass side by side, very twisty with very few straightaways and really steep. So 2nd and 3rd gears were all we could use most of the time.
A couple of years ago we did a superb run around the Mont Blanc region of the Alps and the roads were much more open and the bends more gentle so we could use 3rd and 4th gears more. (still a bit too steep for the engine to pull 5th gear though)

I have uploaded some video via YouTube of our recent drive across the Pyrenees and if you log onto my new blog you should be able to see the sort of roads we were driving on.

c firth

This thread was discussed between 20/09/2014 and 23/09/2014

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