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MG MGB Technical - Rear end gears

Hello all, I did not find anything on this subject in the archive. Some time ago I changed the rear end gears in my TD to ones from an MGA. No five-speed gearbox needed!

I now have a 1973 B that I would love to an upgrade.
Does anyone know if a suitable gear set is available? Thanks, Tom?
Thomas McNamara

I believe that the MGB automatic had a 3.7:1 ratio and later MGCs were 3.07
Dave O'Neill 2

Tom
Dave noted that MGB autos had 3.7 gears, I've heard this before from others as well over the years and have checked a few and all that i've checked have the 3.9----3.7 wouldn't be a large enough step to notice much difference really anyway
The 3.07 from MGC/v8--if you can find one, "won't fit" your crownwheel carrier without machining .215" off the flange of the carrier
BUT
There is an answer though.
There's a cw/pinion set available that's used for v8 conversions that has the correct offset to go straight into your diff---It's advertised as 3.07 but in fact it's slightly different, closer to 3.1
An MGB with 3.9 in overdrive is 3.135 so one of these sets will give you a gearing very close to a normal MGB in overdrive
3.9 =---- 17.9 mph/1000rpm
3.135(od)=22.3mph/1000rpm

https://www.mgv8parts.com/product/v8-3-07-x-1-c-w-pinion/

If you go to get one of these make sure they understand you're fitting it to the 4cyl housing as they sell sets for both the 4 cyl --and the v8/mgc housing(which won't fit your car)

I fitted one up earlier this year and they look good quality and fitted up good and runs quiet
William Revit

You would be better off with OD if you don't have that, usually several available from the UK.

If you change the rear end gearing to match OD 4th you will effectively have OD on all gears with consequent loss of acceleration and pulling-power.
paulh4

To be honest Paul you can hardly tell the difference in power and cruising at speed is much better, the MGB 1800 engine pulls this ratio fine and the bonus is 3rd gear is more useable out on the road ,as well as 1st and second giving a longer legged going somewhere feeling from a standing start---I did one as the first part of a v8 conversion and drove it for a couple of weeks in 4cyl form and was pleasantly surprised how well it suited the 1800, much better than the 3.9 all up
willy
William Revit

Thanks to all for the answers and information. I have contacted the folks at MGV8parts to quote on a gear set plus shipping. Tom
Thomas McNamara

This thread was discussed between 06/10/2023 and 07/10/2023

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