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MG MGB GT V8 Factory Originals Technical - Rear End Gears

With my 74 MGB Rover 3.5 conversion I am still running the original rear end. I believe its 3.9:1. I am considering changing the rear end. I suppose it would be a Ford 8 with Limited Slip/Posi track and disk bakes
What is everyone else using and why?


Bruce Mills

Others will disagree, but I like the 3.9 ratio just fine. I can take off from 2nd gear and be at the speed limit before changing (straight to 4th or 5th). Or since the car in front of you in peak hour is probably a beetle or something, first then third, which gives plenty of torque for getting past obstructing traffic. In fifth (0.78 ratio) at the speed limit I'm doing about 26 or 2700 RPM which is a really nice part of the rpm/power curve to be in. In fact I've read accounts of people who've put in a three to one diff' and found the car became less economical because the engine was reving at a lower and less efficient speed i.e. they were having to press the accelerator harder. Engines have a best/most efficient rpm I guess.
Also I like a low first gear for pratical manovering reasons. Parking , peak hour etc.
I would keep the ratio the same which makes for a zippy car, but I would very definitely like to have a limited slip diff' for rainy days.

I just don't like the 3.9's at all. Loss of 1st gear, high rpm's on the freeway, and the rare times I head to trackI have to use OD just to keep from blowing the motor.

I have a few gearsets and will just swap them out until I get what I want from her, but 2.73 to 3.08 will probably be what I use. The mustang I got had 2.73's and weighed 1000lbs more and smaller diameter tires, so it should work great.
Larry Embrey

Ideally, common/cruising speed should be right at the peak torque(curve).


I can't help you with your gear choice, but maybe this
MS Excel spreadsheet will be of help:

Right-click on the above link and select the "save as" option. I have re-written this to make it applicable for 6-speeds as well. I'll e-mail a copy to anyone who would like to have it.
Dan Masters

Dan, Thanks for the very usefull spreadsheet, I was currently working out the options on final drive ratios and your spreadsheet confirms my own calculations and saves a lot of brainwork.
I have a 3.5 modified to 200+BHP mated to a late SD1 gearbox which I believe has a .782 5TH gear. With the current standard 3.9 ratio this only gives 24.37 MPH/1000RPM in 5th which is to low in my opinion. I have a SD1 Rover axle which I was going to have narrowed and was of the opinion that the ratio was 3.31 which would appear to be about right(for me) approx 28.73MPH/1000RPM but have seen information that suggests that it is 3.08 which I feel is too high and if this is the case it would also be much easier to swap in an original V8 or MGC crown wheel and pinion. Anybody Know for certain the actual ratio of the Rover SDI diff?.I am running 195/65 x 15 tyre so the rolling radius is almost identical to your 215/60 x 15 set up.

Kevin Jackson.
K.R Jackson

I have MGB V8 conversion with 3.5 rover slightly modified. I have LT77 gearbox with close ratio gear 1: 2.33/1, 2: 1.61/1, 3: 1.21/1, 4: 1.00/1, 5: 0.83/1. I have Quaife ATB diff with 3.7 final drive ratio (MGB automatic).Wheels are Compomotive ML shod with 205/55x15 Yokohama A032R. It's above all track day car.

I am presently running a 3.4:1 axle in my V6 midget with 13" tires. It's a little on the low side. With the larger diameter tires on a B I would think that something around a 3.3 would be ideal for both acceleration and cruising rpm. I plan on changing to a 3.27 when I install my posi differential.
Bill Young


The ratio of the SD1 axle depends on which engine was fitted. Four ratios were used (but not 3.31:1 as far as I know).

2.84:1 V8 Efi VDP and auto Vitesse (possibly some manual Vitesse).
3.08:1 All carb V8s, manual Vitesse and 2.6 auto.
3.45:1 2000 auto, 2.3 auto and manual, 2.6 manual.
3.90:1 2000 manual and 2.4 TD.

Geoff King


Just a quick thought on SD1 axles, i was under the impression that the TR7 used the same axle as the SD1 only narrower!! In fact the more i think about it the more i'm sure its the case. You would need to confirm it with a TR7 specilaist though.. Could save a lot of hard work and cash.
John Maynard

I am using the GM rear end as supplied by D & D with a 3.08 posi ratio .This is thru a T5 WC Ford box from a 302 Ford V 8. I have done over 12 K miles this summer and find it just to my liking. Highway cruising is about 2200 rpm at 70 mph and running a stretch at that averaged 27 mpg .
Iam not dragging the car but take off is adequate to surprise most takers .
If needs be no problem taking off in second gear--lots of torque-and 1st gear I use mostly to just get rolling.
Just my 2 cents --

Gil Price

This thread was discussed between 25/09/2004 and 28/09/2004

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