Different rear end ratios

Hi All,

There is a thread a few spaces down about the pro's and con's of od's and 5 speed conversions. Rather than hijack that thread I thought I would take this opportunity to ask a question that has been rolling around in my head for a long time and that is "Could a change of rear end ratio to higher gear (numerically lower) in conjunction with the stock trans come close to the same effect as adding an overdrive gear of either type?" One of the things that originally brought this to mind was when I found out that there are (at times) a 3.55 gear set available for big Healey's which I believe use a 3.9 set like the B. Now you have a low gear that is acutally useful, your not always fiddling about with the shift lever while in town, and cruising is very relaxed.

I know that the engine in a 'B' will run at 4,000 all day long with no problems but it is tiring to me and I am much more comfortable with about 3,000 rpm at 60 mph which is what my VW's would do. I acutally feel that a lot of todays cars have too much od as I don't care much for turning less than 2,000 at highway speeds although I'm sure it looks good on the EPA drive cycle.

So, are gear sets in the 3.55 range availble, or is there a different rear axle that could be swapped in with no more work than say replacing a sill or a new windshield glass? :) Inquiring minds want to know!

TIA,
Joe

The V8 ratio is 3.07 , and I think the C could be 3.3?

The MGC axle is the only available option that you might find at an even semi-realistic price. It would probably be cheaper to convert to an overdrive transmission though. Gear sets are just not readily available for these cars since they're not a part that goes bad with any frequency.

If you want to perform an axle swap, check the V6/V8 forum, as guys there use the Ford 8", 9", and Chevy S-10 axles but at a price $$$. I hear some old postal jeep axles fit too, but with modifications.

Some C rears used a 3.7 that will fit in the B rear. That isn't a great change, but is about halfway to the one you were thinking about. There is alot of info in the V6/V8 section.
John

The 3.07 gears are available. And they will work in the mgb differential. There is a guy in Canada who sells the ring and pinion. Search the V6 and V8 forum like Jeff said.
Now, I wouldn't think that would be a good set up for a stock 4 banger. You would get great hwy rpm's, but what would you give up on take off from a dead stop? I would think it would be a real dog IMHO.
Ken

The 3.07 is in the V8 which has over twice the torque of the 1800.

There are lots of axles out there that could be easily modified to fit into the MGB. a whole industry exists that does nothing but modify rear ends to fit custom applications. Contact one of them, tell them what it is you want (i.e. better ratio selection, easier swaps, stronger, whatever) the dimensions and they will have a pile of options for you.

Here's something that may be of interest to you on the subject. HTH

http://www.mgcars.org.uk/mgccz/technic/diffs.htm

A lot of people swap gearing in older M.G.s, and I have done so in my TC and MGA. It's always a trade-off, nothing comes for free. You will reduce RPMs at speed but first gear acceleration will suffer.

Another consideration is where your RPMs will sit at the speeds you use most. In traffic, it's a real drag to have to keep downshifting to first because you're lugging it in second. In this case lower gearing is better because it keeps the revs UP rather than down.

A friend has a modified S-10 rear in his V8. He paid about a grand for it. Seems to me that a 5spd or OD could be had for the same money and not lose the low end acceleration.

swapping ratios is easier to do in the early banjo rear end (if only because you can do the whole job on the bench) and there are/were a variety or ratios available--I once had a 4.55 and a 5.12

nobody has mentioned, though, that the effect might be nicer cruising, but a 2CV is likely to give you a hard time at stoplights!<G>

About a year ago I found a site in England that sold new Ring & pinnions in 3:07, 3:30, 3:70, 3:90, and I think a few more. If I remember correctly the price was about 200 pounds per set, which for some ratio's would come out cheaper than you'd pay for used ones (even after installation).... Now If I could just remember the name of the company, I'll post again if I remember, or find them again.

All of those 3.07 and 3.55s are for the tube-type axle only. I thing 3.9 is as low as I've seen for the banjo-type. I have heard that the front diff from a '90's 4WD Toyota truck is an easy conversion but I've never seen one. Basil

MGCs were available in 3.07; 3.31; and 3.7 ratios.

The 3.07 and 3.31 are the most desired....both for MGC owners and conversionists.

I think Tony Barnhill had a line on some 3.7s for banjos. And yes, I think they were coming from the UK.

My '65 has a 3.7 in it. Don't know how that came to be; a previous owner did it.

Coupled with an OD the '65 is quite the highway cruiser, 2500rpm is 68mph, verified by timing legs and a GPS. The motor is loafing along without a load.

I actually think it's a good mod for folks that drive in anything but city traffic constantly. The issue with a B and a 3.7 isn't off the line; 1st on a B transmission is so stinking low it's not a factor. The problem is the gap between 2nd and 3rd is only accentuated.

My '80, desmogged but the original cam, is more comfortable to accelerate in up to about 35mph than the '65. After that a MK 1 set-up, lighter and more power, just overwhelms a RB era motor. I can cruise at '80 in the 80, but it's hard work. The '65 is just loafing along.

I have a 4.3 for the '65 which I keep, just in case I ever get an autocross bug or some such.

The problem you will have is that you're gonna lose the UMPH that the car had down low.

while you will definitely reduce highway revs, the MGb engine just won't have the grunt to move ya like you want down low.

You'll be riding the clutch more to get it going, it'll be like starting out in 3rd gear all the time.

If you could add some torque, well then thats a diferrent story.

B

Joe

"Could a change of rear end ratio to higher gear (numerically lower) in conjunction with the stock trans come close to the same effect as adding an overdrive gear of either type?"

Short answer is YES.
The lower rear axle will lower the cruising RPM for a given road speed as you have said.

The difference will be in every gear throughout the range so all gears will be similarly altered.

Changing from 3.9 to 3.5 or so will give you a drop of around thirteen percent (the percentage differences between the two gear ratios) in your RPM for a given road speed. (60 MPH = 3410 RPM with 3.9 ; and 2966 RPM with 3.55) according to the base numbers from Road and Track’s test of the MGB in July 1968.

For the rest of the gears the differences will be the same and yes the lowest gear does actually become a useable gear as you said.

Changing out the entire rear axle ass’y may be the quickest and most straightforward option as it is basically an afternoon session with the socket set. Unbolt one item and bot in the other.

I am not familiar enough with the situation in the US to give a definite answer on how feasible such changes are there and what the availability of such change out items are.

Good luck , Pete.

I have always thought that a change of rear end ratio would be a good project for a B. I did some calculations once with one of the online gear ratio calculators and I think the best combo turned out to be using the 3.7 with 15 or 16 inch wheels and tall tires.
I have a 3.3 MGC axle that I have been saving for a special project, but from everything I have heard I'm pretty sure that it would be to low for anything like a stock motor.

I would like to thank everyone for their replies

The 3.7 almost seems like it wouldn't be enough of a change to warrant the cost/hassle factor but as Matt pointed out that plus taller tires, which have the same effect as a lower numerically ratio, it might be worthwhile for a stock motor.

3.3's on the other hand would require a real torque monster to get rolling. The 3.5 range might work well with a slightly modded motor, maybe with lower profile tires. After all, nobody wants to get whipped at the stop light by a 2CV!

Joe

I'm thinking along with Matt. The 3.7 may not be bad in a stock engined B. It would make first a little better and reduce the RPM on the highway.
John
I'm told those gears will fit on the B differental carrier.

3.7 = 5% different
3.3 = 15% different
3.1 = 20% different

IF I was going to take this approach, I'd try the 3.3. The 3.7 is a waste of effort, 3.1 perhaps too much. Unless of course you have a 120RWHP engine! ;)

You can get 5% difference by fitting larger tire, perhaps 15". 175/70x14" -> 185/70x15" is 6.16%.

http://www.mintylamb.co.uk/?page=tyre.htm

3.55:1, if it was available? That's about 9%. I think a healthy, desmogged 'B could probably handle it. Let's face it, most of us try and do most of our motoring out on the open road, far from "stop & go" traffic. I know I do! :)

120 RWHP engine? Can't be done! ;)

Personally I feel the 3.3 would be way too tall for a stock B engine unless it would be used almost exclusively on the highway. I swapped an 8/43 for an 8/41 in an older M.G., and while it was a small difference in percentage it did provide relief in RPMs without impacting acceleration TOO much. Going to a 3.3 in a stock B might render 3rd gear useless on a steep climb, and 2nd would be way too low. This is only what I think would happen, no basis on personal experience with this gearing in an MGB. If anyone tries it in real life I would love to hear about the results.

I know it's the wrong way to go, but if you want aceeleration, I have a set of 4:88 to 1 ring an pinion.
Top speed 80 sumthin

Great autox gear, run in 3rd.

LOL