MGA Forum

If anyone is interested...

I did some comparisons of changing the differential gearing and/or replacing the transmission. This is applicable to my Silver '57 and also the yet to be restored Red '59. For the Silver '57, I recently swapped the 4:3:1 pumpkin with a 3.9:1. I wanted to see how much of a difference it would make in the cruising speed and rpm (which can be a bit frenetic when trying to keep up with highway traffic). I was shooting for a target cruise rpm of 3000, which I thought would be 'comfortable' and 3300 would be an 'acceptable' level. I achieved my goal I think with just the 3:9:1 swap.

The attached *.pdf is a copy of my spreadsheet, can't post the *.xls file. It compares a standard MGA transmission with a 4:3:1 and 3.9:1 differential, a MGB OD transmission, a Miata (basis of the Vitese 5spd), a T5 5spd transmission from a '83 V6 Camaro (P/N 1532-228, you can find other T5 options here https://www.britishv8.org/Articles/Borg-Warner-T5-ID-Tags.htm) and a Datsun Z transmission (260Z, I think) that is popular with the Rivergate adapters. I'm using the tires from the 2nd row 165/80 R15 for the comparisons shown, which I'm using on the '57.

What is interesting is comparing the speed at 3000rpm using a 3.9:1 and the standard MGA transmission with the transmission only swaps. With the tires I'm using, I am within 6 mph at 3000 if I had only replaced the transmission with a MGB OD or Vitese and kept the 4.3:1 differential. The Camaro is the 'best' as it has a very low OD (0.76) which would end up at 69 mph. But I've found they are getting pretty pricey and rare in as-is (not reconditioned) condition and even pricier in reconditioned form.

To me the ultimate would be a MGB OD transmission and a 3.9:1 differential, that would put me at 71 mph at 3000. This (to me) seems the easiest and likely economical modification. That is what I'm shooting for in the '59, I've got the pumpkin already and the transmission is on my wish list .

There are several similar online resources for gearing calculators. If you want to play with the spreadsheet, send me a PM with your email address and I'll email you a copy.

Cheers,
-Eric



Edited 1 time(s). Last edit at 2025-04-21 06:33 PM by ohmite.

Very cool Eric.

In people with experience… how is the 3.9 around town?

Asking because I have no idea how this translates.

Thank you Todd.

From my point of view I see/feel no difference in town driving. It still has the same pickup from stopped. I do notice the rpm is down slightly at 30mph, but it doesn’t give you the feel that it’s lugging or that you have to downshift if you have to accelerate a bit. An example is sitting in 3rd at 3000 with the 4.3 you’d be at 38, with the 3.9 you’d be at 42, not much different. The real noticeable improvement is cruising down the highway and keeping up with traffic and not feel like you are flogging the horses while you are doing it .

I’m pretty pleased with how it’s worked out.

Cheers,
Eric

I first did a 3.9 B diff conversion and noticed little to no difference with my strong 1600. A few years later when the British pound was down I bought a Hi Gear kit directly from Peter Gamble and installed it. It was half of what Moss was selling it for at the time. I left the 3.9 in the car as the standard gearing on the T9 is about exactly the same as the stock MGA transmission. I also had West Valley Instruments re calibrate the speedo. I’ve thought about re installing the 4.3, but as it originally made so little difference I just left it in. Unless I’m up in high speed 70 mph plus freeway traffic, it hardly sees fifth.
Since doing this I bought an MG TC. I’m so used to touring in that car now at 60-65 MPH at 3500-3800 rpm (higher diff ratio) that I probably would have just left the MGA with just the 3.9 and not done the five speed. It is nice though. Part of this is we’re just so used to low RPMs in our moderns that a free revving MG engine seems like it’s on the edge when it isn’t.

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"Time flies like an arrow......Fruit flies like a banana"

I’ve got to put the on my list then… sounds like a great upgrade

I swap and change diffs at about the same frequency as I change my underwear
I do this depending on the tracks I'm competing on. A 3.9 should carry you (almost) down a 1-mile straight to > 100 mph.
If you have a good strong engine, then should be no problem off the line and cruise nicely on the highway. If you want all-round performance, then stick with the 4.3.
For Hillclimbs, Autocross and sprints, then 4.55 or 4.875
3.9 is sweet for highways and freeways.
Cheers
Greg



Edited 1 time(s). Last edit at 2025-04-22 02:54 AM by MandoG.

I switched to a 3.9 diff a couple of years ago on the suggestion of a fellow MGA owner. Shoulda done it 20 yrs ago. I don’t notice any low speed issues and it reduce revs on the highway by about 10%. May not sound like much but it’s noticeable.

John

I've located a 3.9. While the lump is out for the spider assessment, I might as well.

Funny, though..... When I owned my TD, I swapped in an MGA R/P to get a bit more speed on the highway!

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The Flat Earth Society has members from all corners of the globe.


'53 XK120SE OTS
'58 MGA roadster
'66 Series 1, 4.2 Jag E-Type OTS

Mitch,
That's great to hear, I saw you were in search of one. Definitely a 'might as well'.
Cheers,
Eric

What’s the FMV of a 3.9 these days? Or, does it all depend on how motivated the buyer is?

John

Hi John
I bought one last year (really good condition) for AU$500 (not too different to CAD I guess).
I would have paid more for one in the same condition.... I thought I 'stole' it.
I then bought some Spider gears (10-spline) from Cecelia ... these were from an old diff rather than repro...again ..perfect condition...
So you need to add the cost of retro-fitting to 10-spline or if you have 25-spline half-shafts already ...sweet
Cheers
Greg

John,
I think the most I’ve paid for an unrestored pumpkin but in very good shape was $350 plus shipping. The least was essentially free but it turned out the ring and pinion had excessive corrosion. Most of the parts (bearings, shims, etc) are replaceable with new, but if the ring and/or pinion (the expensive bits) are damaged or worn they both need to be replaced. Without a detailed inspection you don’t know, so don’t be ‘too’ motivated unless you have high confidence those are in good shape.

Keep an eye out on BST, they come up periodically. Or put a wanted ad up like Mitch did and you may get one sooner from someone who just hasn’t gotten around to clearing out their spares.

Cheers,
Eric

That T-5 is out of a 2.8 engines GM and so doesn't have quite as good ratios ast the V8 version but it should be decent.

There are other ways to go about it. I transplanted an early MGB close ratio trans with OD into my race MGA car and have run from 4.1 through 5.125 (the latter exclusively for hill climbs). First is just a starting gear anyway and I don't use 2nd OD, so I ran in effect a 5 speed - 2nd, 3rd, 3rd OD, 4th, 4th OD. It worked really well. 3rd OD is very close to 4th so you can also skip that if you aren't racing. although on a dining road being able to essentially do a 3rd-4th shift and back with the flick of a switch is rather nice.

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Bill Spohn www.rhodo.citymax.com/carstuff.html
Current: 1958 MGA Twincam (race car (170 bhp)),1962 MGA Deluxe Coupe (98 bhp)
1957 Jamaican MGA (200 bhp)1965 1971 Jensen Interceptor (350 bhp)
2009 Pontiac Solstice GXP Coupe (375 bhp)
2007 BMW Z4M coupe (340 bhp)
Recent: 1969 MGC roadster (175 bhp),Jensen CV8 (375 bhp),
1969 Lamborghini Islero S (350 bhp), 1988 Fiero GT turbo (300 bhp)
North Vancouver BC

Bill,
I picked that one because, when I was doing this comparison (before making a choice what to do) it was for sale locally and I was seriously considering buying a bell housing adapter kit that was also for sale. So it became part of my mix. There are lots of options for T5’s I learned (the BritishV8 link), some better than others. The breaker who was selling it also had a couple of V8 transmissions, but he wanted top dollar for any of them and they were all of unknown history and condition inside. So I passed.

I like your MGB OD transmission solution, likely what I’ll be doing on the ‘59. But I’m not going to be racing it so I’ll be going a bit more sedate . All in the future, I need to get the MGiata complete (soft top, etc) and take some road trips .

Cheers,
Eric

I swapped in a 3.9:1 diff in my MGA Coupe a few years ago and it is great for the highway, BUT going up a mountain pass, if you get stuck behind someone poking along one has to shift down. The engine likes to rev and if you keep the revs up it will pull the hill, but if you let them drop down, you have to shift down. This is on a 1600 that is pretty much stock except it has a better breathing MGB head on it.