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MG MGA - Gear and diff ratios

As part of my 1800cc conversion i am going to fit a type 9 ford gearbox. Now on paper there is not much difference on the first gear ratio between the 4 and 5 speed boxes its next to useless its so short and with plenty of torque its almost redundant,so is anybody running the closer 2.8 ratios with the higher first gear?
For those with 1800cc engines are you running a 5 speed with standard 4.1 rear end or will it be possible to run with a 3.9 as well? this will make Ist gear more usable.
Now as i see it if i can run a 5 speed and a 3.9 maybe the close ratio with a high first will be too high and will cause problems with hill starts etc.
Any thoughts?
R Mcknight


I run the 1800 with the type 9 and a 3.9 diff. Previously I had the standard diff and found the 1st gear a considerable improvement. Obviously with the 3.9 it is much longer. One great advantage that you did not mention is that it is synchronised. The ratio gives me 70mph at 3000rpm if my memory serves me right.

I found the 3.9 very useful for long motorway type driving. Now that I have retired and moved to a more hilly area (Surrey Hills type of countryside) I find the 3.9 gearing a bit high for those roads and queuing traffic etc. I think 4.1 would be suitable.

You are only just down the road. Why not come over and have a drive?

Steve Gyles

Another option you may wish to consider is the alternative taller first gear for the Ford T9. (2.98 vs 3.65)
I chose this for my MGA 1600 Mk II. So, I'm running the standard Mk II 4.1 diff along with the 1622 engine. I'm very pleased; 1st gear is much more useful, and the car pulls it without fuss. Your 1800 would manage it even better.
Do be aware that with a 3.9 diff ratio, 0.82 Vth gear and your 15" wheels your car will be geared quite tall, probably enough to blunt its responsiveness. An MGB with the 3.9 diff and 14" wheels is already geared marginally shorter than my MGA with its 4.1 diff.

T Aczel

I wouldn't dismiss the low first gear as an undesirable feature. I'm guessing you are aiming for a ratio closer to the standard second gear. This will make traffic queues a nightmare, you will be constantly on the clutch and holding high revs to keep the engine from stalling. This will make it much more likely to overheat.

I know this because I tried the same trick on an Austin A35. I fitted a 3.909:1 on a stock gearbox. It was an abysmal failure and I restored the 4.55:1 ratio diff and accepted the low first as a bonus in traffic.

I now run a Magnette with an engine built up as a 1588cc stage II with a T9 gearbox and a 4.3:1 diff. Allowing for the extra weight in the Magnette doubt you will be happy with anything higher ratio than a 4.1:1 diff.

If it is any help, I put together a spreadsheet to calculate the RPM at any speed and the speedo gearing for all T9 and standard boxes with various tyre choices. I can't post it on here but if you would like a copy please send me a suitable Email address.

Final piece of useless information. There is a Magnette speedo which almost exactly matches the T9 speedo drive ratio. The working parts can be fitted directly into an MGA housing removing the need to go for the expesive recalibration or ratio gearbox to get a usable speedo. The one you need is the 1160 TPM version, they come up on Ebay quite frequently.

Best of luck with the conversion.

P Reardon

I have the Standard ford type 9 fitted to my Mk2 Coupe with the 4.1 diff my engine is the 1622 mildly tuned(improved camshaft and with higher Compression - not sure of the exact spec as was done by previous owner when the engine was rebuilt) at a recent Alden rollimg road it developed 90 bhp which is not far off the MGB engine.
Having done some 12000 miles since adding the ford box including some long continental trips I find the ratios ideal- can cruise happily at 70-80 without stress- have hit 100 mph when conditions allowed!! and yet in heavy traffic the second gear allows one to creep and then accelerate away when clear.has made my motoring a pleasure. Speedy cables recalibrated my speedo came back like new
I would recomend having a drain plug fitted to the gearbox to enable the oil to be changed easily- It is supposed to be filled for life butI would rather have the option of renewing the oil. I regret I did not specify this when the box was fitted - but so far no problems !
P D Camp

Thanks Steve , may well take you up on your offer .
And Paul if your first gear was higher would you not be able to creep along in first and then accelerate away ( could you pull away from a standing start in second gear if you could then isnt first gear redundant ) Thats why i am thinking about the box with the higher first gear and closer ratios.
R Mcknight


Just a thought: while you are at this preliminary stage you might want to consider also fitting the MGB clutch. I like it; smoother and more robust.

Steve Gyles

Second gear from standstill is a bit sluggish and needs plenty of revs and I always use first I must check the speed that I change up to second! It seems OK and quite natural?
I also have the MGB clutch
P D Camp

I agree with Steves comments regarding the gearing.
I think the 4.1 diff would be ideal with the standard Ford T9 gearbox ratios.

My car has a very similar set up to Steves with an MGB engine (1850cc 3 bearing, slightly tuned- 106bhp)
It is fitted with the standard 4.3 diff and I used to think that a higher 1st gear would be useful.

Now having recently driven it on both long continental journeys and over high alpine passes I also think that it would also benefit from slightly higher 2nd and 3rd gears.

Fitting a 4.1 diff would probably achieve this maybe at the expense of a slightly high 5th gear but as I use 5th as a long distance motorway cruising gear, I think it would be perfect.
For me the 3.9 may be a bit high.

All I have to do now is to try and find a 4.1 diff!

Paddy, I would really appreciate a copy of your gear ratios spreadsheet if you dont mind.


Colyn Firth


Yet another thought, although, if you have been watching past threads on the Type 9 you will know that everyone recommends fitting the right angle drive speedo adapter to the box. It's a much neater arrangement and reduces the likelihood of speedo swing that you can get with the other arrangement. I guess this also falls within the title of gear ratios, because if you get the appropriate gear ratio in the adapter you may not need to have the speedo recalibrated.

Steve Gyles

Colin, if you find two 4:1 diffs that would be great, as I would also like to try one

J Bray

My Spec will be .. Mgb engine 1966 using MGA H4 carburettors. Type 9 5 speed box with bellhousing for 5 Main bearing MGB 3 syncro engine, this will allow me to use the B diaphram clutch in the A. Will lighten flywheel and also fit oil cooler. Also remove engine fan and fit electric fan. Fit Alternator and lightweight starter from cambridge motorsport.

R Mcknight

This thread was discussed between 07/11/2012 and 09/11/2012

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