MG Sports Cars

engine swaps and other performance upgrades, plus "factory" and Costello V8s

Go to Thread: PreviousNext
Go to: Forum ListMessage ListNew TopicLog In


Dave Underhill
Dave Underhill

(19 posts)

Registered:
09/05/2017 07:30AM

Main British Car:
1972 MGB GT 2.8 Chevy

Rear diff ratios
Posted by: Dave Underhill
Date: May 09, 2020 09:12AM

What rear diff ratios did MGB's have? I have a GT with a 2.8 chevy and was told it had a 3.45 ratio in it.


mgb260
Jim Nichols
Sequim,WA
(2058 posts)

Registered:
02/29/2008 08:29PM

Main British Car:
1973 MGB roadster 260 Ford V8

Re: Rear diff ratios
Posted by: mgb260
Date: May 09, 2020 07:00PM

3.91 in MGB. MGBV8 had 3.07. MGC had 3.07, 3.31 and 3.70.


rficalora
Rob Ficalora
Willis, TX
(2686 posts)

Registered:
10/24/2007 02:46PM

Main British Car:
'76 MGB w/CB front, Sebring rear, early metal dash Ford 302

authors avatar
Re: Rear diff ratios
Posted by: rficalora
Date: May 09, 2020 09:44PM

Quote:
I have a GT with a 2.8 chevy and was told it had a 3.45 ratio in it.

I don't know about 2.8L swaps, but it's not uncommon to change the rear axle to a GM or other make when the motor is swapped. Are you sure it's an MG rear?

Either way, if you want to know what ratio you have, this article tells you how to determine: [shiftsst.com]

And, if you aren't sure what axle you have, post a couple of pictures here and most likely someone will be able to quickly identify it. Be sure to get the front pinion area and the back showing whether it has a cover or not and as much of the bolts on the cover as you can get.



Edited 1 time(s). Last edit at 05/09/2020 09:45PM by rficalora.


Dave Underhill
Dave Underhill

(19 posts)

Registered:
09/05/2017 07:30AM

Main British Car:
1972 MGB GT 2.8 Chevy

Re: Rear diff ratios
Posted by: Dave Underhill
Date: May 10, 2020 06:54AM

It is a MGB rear axle. The paper work I got with the car said 3.45 out of a V8 car. It was done back in 1986 and may be wrong.
I will try and figure out what the ratio really is.
The reason I'm asking is that I feel it is geared too tall and would like to try a stock MGB rear diff.



Edited 1 time(s). Last edit at 05/10/2020 07:01AM by Dave Underhill.


MGBV8
Carl Floyd
Kingsport, TN
(3980 posts)

Registered:
10/23/2007 11:32PM

Main British Car:
1979 MGB, Buick 215

authors avatar
Re: Rear diff ratios
Posted by: MGBV8
Date: May 10, 2020 10:48AM

I am not aware of a 3.45 that can be swapped into an MGB rearend. If it truly came from a OEM MGB/GT V8, maybe it is a 3.31. That would be the closest. If the 2.8 really feels like a dog, maybe it is a 3.07. Best cure is a 3.4 upgrade. ;)


Dave Underhill
Dave Underhill

(19 posts)

Registered:
09/05/2017 07:30AM

Main British Car:
1972 MGB GT 2.8 Chevy

Re: Rear diff ratios
Posted by: Dave Underhill
Date: May 10, 2020 05:31PM

I went back through the paper work and it is a 3.07. Sorry for the confusion guys. I feel that is too tall as I don't shift into 5th until 60mph.
I wonder what a 3.90 would be like as that's all I have.


Dave Underhill
Dave Underhill

(19 posts)

Registered:
09/05/2017 07:30AM

Main British Car:
1972 MGB GT 2.8 Chevy

Re: Rear diff ratios
Posted by: Dave Underhill
Date: May 10, 2020 05:43PM

My TR6 has a 3.45 in it. That must have what confused me.



88v8
Ivor Duarte
Gloucestershire UK
(801 posts)

Registered:
02/11/2010 04:29AM

Main British Car:
1974 Land Rover Lightweight V8

Re: Rear diff ratios
Posted by: 88v8
Date: May 11, 2020 04:18AM

When I had an MGC it was routine not to use overdrive top - equivalent to 5th - below 60mph.

Likewise in my TR6, but there I did undertake some diff swapping. The car when I bought it had the stock 3.45 diff with 215/70 tyres. The size of the tyres obviously has an effect on overall gearing., and it was waaay overgeared.

So I tried a 3.07 and that gave me the stock factory gearing, about 27mph/1000 rpm in overdrive top.
But that was gearing designed for the days when you could cruse all day at 80/90mph without being bothered by Plod. So it was still on the tall side.
Then I changed to a 4.11. On those huge fat tyres that gave me 24mph/1000 which seemed a good compromise between cruise and acceleration, given that the TR6 engine is a long-stroker that doesn't like sustained high revs.

It depends on the engine, how it likes revs, the torque band, and how you want to use the car.
A low ratio could turn it intro a light-to-light car. Good fun locally but a fuel guzzler and a PITA on longer journeys..

So what RPM is your B doing at 60mph in 5th?

Oh, and changing the diff will need the speedo recalibrating. Small point to bear in mind.

Ivor


Dave Underhill
Dave Underhill

(19 posts)

Registered:
09/05/2017 07:30AM

Main British Car:
1972 MGB GT 2.8 Chevy

Re: Rear diff ratios
Posted by: Dave Underhill
Date: May 11, 2020 09:29AM

I drive it locally. Not on the highway much at all. It doesnt have a working tach right now but guessing it doing 1500 to 2000 at 60.


MGBV8
Carl Floyd
Kingsport, TN
(3980 posts)

Registered:
10/23/2007 11:32PM

Main British Car:
1979 MGB, Buick 215

authors avatar
Re: Rear diff ratios
Posted by: MGBV8
Date: May 11, 2020 09:54AM

You in the USA or UK, Dave?

The USA TR6 only came with the 3.7 rear gear. The 3.45 was for the UK market. We did use a 3.45 in the TR6 race car.

Living with the stock 3.9 gear in an MGB conversion, mainly has to do with useage & the 5th gear ratio. I have found that the 3.9 is quite acceptable when paired with a .63 5th gear.


Dave Underhill
Dave Underhill

(19 posts)

Registered:
09/05/2017 07:30AM

Main British Car:
1972 MGB GT 2.8 Chevy

Re: Rear diff ratios
Posted by: Dave Underhill
Date: May 11, 2020 05:25PM

I live in Canada. I have a TR6 that came from the Bahamas and is fuel injected and left hand drive.
What would an honest opinion of what the 3.07 diff would be worth.


88v8
Ivor Duarte
Gloucestershire UK
(801 posts)

Registered:
02/11/2010 04:29AM

Main British Car:
1974 Land Rover Lightweight V8

Re: Rear diff ratios
Posted by: 88v8
Date: May 12, 2020 04:17AM

If it's really doing 30mph/1000 on a 3.07 then I can understand that it's struggling, as that engine in stock form doesn't look as if it's going to pull down any barns.
With the 3.9 diff you'd have 24mph/1000 which I think would be fine.

Before you do anything however I'd rig up an electronic tach so you can actually see what's going on. Either that or jack up the car, put it in 5th, rotate the engine by hand and count the wheel turns then do some arithmetic.
Rolling tyre diameters can be found here [tiresize.com]

Ivor


Dave Underhill
Dave Underhill

(19 posts)

Registered:
09/05/2017 07:30AM

Main British Car:
1972 MGB GT 2.8 Chevy

Re: Rear diff ratios
Posted by: Dave Underhill
Date: May 12, 2020 07:00AM

Thanks Ivor, I think getting a tach hooked up is a good idea.
The motor is stock, 128 hp. I wouldn't say it struggles but I don't think it works as well as it could.


88v8
Ivor Duarte
Gloucestershire UK
(801 posts)

Registered:
02/11/2010 04:29AM

Main British Car:
1974 Land Rover Lightweight V8

Re: Rear diff ratios
Posted by: 88v8
Date: May 14, 2020 04:05AM

128 hp even in the flush of youth, wouldn't be happy pulling 30/1000 anywhere but downhill.

I had an 86 Renault 5 GT Turbo, 120 hp, smaller frontal area than the MGB. Pocket rocket. Don't recall what the gearing was, but even up motorway hills, it sometimes struggled. Then I had the 91 version, same engine but the gearing was a bit lower. Made all the difference.

Gear ratios can completely change the character of a car. As you'll know if you've ever tried doing much highway mileage in your TR6 with the o/d inop.

Ivor


Dave Underhill
Dave Underhill

(19 posts)

Registered:
09/05/2017 07:30AM

Main British Car:
1972 MGB GT 2.8 Chevy

Re: Rear diff ratios
Posted by: Dave Underhill
Date: May 14, 2020 08:20AM

I have a couple 3.9 diffs around so I think I will try one.



wspohn
bill spohn

(29 posts)

Registered:
10/23/2007 09:14AM

Main British Car:


authors avatar
Re: Rear diff ratios
Posted by: wspohn
Date: June 14, 2020 04:29PM

Depends on which trans you are using. Many people go with the V6 gear sets because they got the trans with the engine. Bad choice - lower gear ratios are way too low unless you are a truck. The V8 T5 makes a good combination with a 3.9 diff and 15" tires - might be a tad low geared for an MGB on 14" wheels.


Sorry, only registered users may post in this forum.