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MG TD TF 1500 - OT - Final Gear Ratios

We bicker about the difference between using 4.3 vs. 4.55, vs. 4.875 gears. Ha! Yesterday I flew from Plymouth, Mass. to Ft. Myers, FL to drive a snowbird's car back up North. Got home about an hour ago. 1500+ miles driven since 9:30 yesterday morning, mostly at 75'ish mph in a 2.5 liter 2011 Nissan Sentra. Even with an automatic transmission it has a tachometer. The final drive gearing is such that 2,000 rpm has the car cruising along at 75 mph. 80 mph is about 2100 rpm. Must admit that there isn't very much oomph available when you put your foot down at 80.
I brag about Lazarus, my TD, getting almost 17 mph per 1,000 rpm. This is more than double that. Bud
Bud Krueger

Easy calc- twice the displacement = half the rpm ;-)That's the downside of our small engines. I guess you could add a 6 speed for flat road cruising easily and reach similar rpms on a TD but would still need to shift gears when the wind changes. But maybe we should start searching for a suitable tranny for a conversion.
My Mini (diesel though) reaches similar rpms, less than 3000 at 100 mph and uses a 6 speed tranny, so very smooth on long distance.
Rgds
Mike
Mike Fritsch

My other car, 1991 Corvette Convertible at 75 MPH gets 25 miles per gallon turning over about 1900 RPM's. When you hit the throttle at that speed, there is some oomph there still.
Tom Maine (TD8105)

I've just bought a 2012 Kia Rio5 with a 6-speed manual (also available with 6-speed auto). The 1.6 litre engine is working hard in 6th if you're on anything other than only mild hills, but gas economy is fantastic (4.9 litres/100kms ~ 57½ mpg).

I don't know how the manufacturers select the final drive, as fuel consumption takes precedence to available torque.

Will be interesting to see how it behaves with 4 people on board plus a few bags.

Gord Clark
Rockburn, Qué.
Gordon A Clark

It's like business "You can have it fast, you can have it cheap, you can have high quality, or any two, but you can't have all three."

-or-

Airplanes, "you can fill the seats, you can fill the tanks, you can take off on a scorching hot day, or any two, but you can't do all three."

There are always trade-offs.

Even with the low tire pressures often quoted for the bias ply tires of the 50s, the 165R80 radials must have smaller rolling radius than the originals. I figure with the 4.3 I'm turning just over 16.8 mph per 1000 rpm, should be 17.16... it's no wonder with the 5.125 our cars run out of mph.

Bud, did it feel like a TD once you got used to the cruising? We are still pushing the equivalent of a sheet of plywood through the air with a TD, so the wind noise must have been there, but are the other factors different?

I've been testing the MGB Maggie as we get it ready for the GOF Central... so many things are brand new or fresh on the car, and slowly I'm bringing everything into harmony. It reminds me of shaking down Tommy after his rebuild. But honestly, right now if I had to do long distance in an MG I would take the TD... it just feels so right. I guess it is what I'm used to.

warmly,
dave

Dave Braun

Although OT, wery interesting details.

As I am planing to rebuilt my XPAG engine with a Laystall Lucas head, .060 pistons with 12:1 cr, 1 3/4 SU's, and tubular headers, I am wondering whether the 4.3 conversion is the way to go or to decide for a modified MGB tube axle with the MGB drums and rotors in front.
As I have the choice of both the MGB axles with wire wheels or TR3A wheels, it would not be that hard to do it this way and stock the OEM TD parts.

Who has done this kind of conversion and what experience was made concerning performance and lover rev. at normal cruising?

Thanks,

Ralph
R.S. Ralph Siebenhaar

No, Dave, it never felt like a TD. Especially with the Bose sound system. I did an awful lot of NASCAR style drafting to keep close to the leader. After thinking back, I suspect it was a Nissan Altima, rather than Sentra. BTW, the tach redlined at 6,000. Never saw more than 4,000 (automatic transmission). I did think about LaVerne when I was averaging over 60 mph for 15+ hours. Bud
Bud Krueger

Ah, but Dave-think how much better your left leg will feel after 6 hrs on I-94. Right knee will still be stiff from the bent position. I still haven't found a decent position for long drives in the TD. Crappy knees suck. Dan
Dan Craig

Bud, what does "OT" mean?

tn
Tom Norby

Tom Norby, Oregon, USA:

Bud, what does "OT" mean?

Over here, Tom, it means "Off Topic" and is used to flag-up a thread that some people, who are only looking for information directly related to the Board's purpose, may not want to read.

- Tom.




Tom Bennett - 53TD 24232

Last weekend I drove a 'snowbird's' Toyota Camry down to Sarasota Florida. Gearing is 35 mph/1000 rpm. Rolled along mostly at about 75 mph. Averaged 28.9 mpg. Bud
Bud Krueger

Ralph.
In addition to the MGA 4.3 and 1962 MGA 4.1 the early MGB 3.9 ring and pinion gears fit the TD/TF rear axle. I think this is a better choice than fitting the later MGB entire rear end.

The standard conversion works just the same with the MGB 3.9 as the MGA conversion and you have no problems with brakes and wheels.

I hav a TF 1500, stage 3 tuned and have the MGA 4.1 in it with great success.
Don Harmer

Don, I have sold one 3.9 to a fellow that had a TF (I think) with an later MGA or maybe a B engine in it. I think it would be too much for most situations with our stock engines even with your state of tune. The 4.1 was surely the right choice for you.

There are so many variables. Drag is really different on the TD vs. TF. Terrain where you're driving hills vs. flatland. Then freeway vs. slower roads. Engine type, age and state of tune. There is a very nice section in Carl Cederstrands book about this with all of the minute details his fine engineers mind could come up with :-)

I usually have the 4.55, 4.3, 4.1 and the 3.9 in stock to satisfy most customers needs. The 4.3 is such a great fit for most drivers that I'm only building the others on special order. The 4.875 is really not a good ratio 99% of the time. I get a lot of those back as cores.

Best,
Dave
Dave Clark Arizona

For those even vaguely interested, here's a list of available CWPs for our rear axle.

Note that the CWP for the Y-Types can only be added with serious mods (who would want such a stump-puller, anyway?)

5.44 ~ 9/49 Super stump-puller see AKD834R
5.143 ~ 7/36, standard on Y-Types
5.125 ~ ordinary stump puller and std on ALL TDs
4.875 ~ Std on all TFs
4.55 ~ Morris Minor Std. And Tuning Manual option
4.3 ~ Standard (early) MGA
4.10 ~ Late MGA 1600 Mk II
3.9 ~ Standard MGB
3.7 ~ optional MGB

Gord Clark
Rockburn, Qué.
Gordon A Clark

I have a 1250 TF with less than 5,000 miles on a rebuilt engine. I am contemplating changing the ring and pinion gears from the 4.85 to 4.55. The mechanic at the British specialty shop is trying to convince me that I would be happier with the 4.33 ratio. I would love some advise from any of you that have made these conversions.

Bill
Bill Schroeder

Bill,
My TF-1500 has a 4:55 and I think I would have preferred a 4:33.
1st gear is still only good to get the car rolling and cruising above 60mph is a little unconfortable.
I think with the 4:33 1st gear would be a little more useful and the cruising speed would be a bit higher, maybe 65 to 70 mph.
Just my opinion.
Thanks,
Dave B.
DW Burdette

Ask yourself what you want to accomplish with the gear change Bill. Higher top speed? Lower engine rpm at cruising ? Faster acceleration? Better fuel economy? What kind of driving do you do with it...hills?...strictly flat?...combination?...cross country?...town only?

Any modifications to your engine? Stock gear box?

Here is my take..

After rebuilding the TF engine in stock configuration, a 5 speed transmission and the stock 4.875 rear end I drove it for about a year that way. I drive long distances and in town. Mostly steep grades, when on in state trips. Ok around town but it wouldn't begin to keep up with anything on the highway. In fact the only time I ever passed another vehicle was going to a car show 75 miles away. Just barely got around a stock 40 Ford on the interstate. Got around 32 miles to the gallon.

I added a modern super charger and that allowed me to keep up with modern traffic but the engine rpm was higher than I would have liked at freeway cruising speeds. Dropped MPG to about 22

Last year I swapped out the rear end for a 4.3. My first reaction was to think about putting the 4.875 back in because of the drop in acceleration but in the end Im fairly happy with it. I took the 4.875 and put it in the TD (all stock). I didn't notice a drop off in performance but it will now cruise at 55 without me thinking the engine is going to fly apart. For my altitude (near 5000 ft) and driving I think the 4.55 would be a great gear set for the TD. For your altitude (you get a lot more HP near sea level) I would think the 4.3 would be a good choice.

I had an option of a 4.1 for the TF at the same price as the 4.3. I'm glad I didn't do it. If you have had some engine modifications for some more hp then the 4.1 might be a good choice.

Just my two cents.
L E D LaVerne

Bill,
My TF-1500 has a 4:55 and I think I would have preferred a 4:33.
1st gear is still only good to get the car rolling and cruising above 60mph is a little unconfortable.
I think with the 4:33 1st gear would be a little more useful and the curising spee would be a bit higher, maybe 65 to 70 mph.
Just my opion.
Thanks,
Dave B.
DW Burdette

I changed my TD to 4.55, and wish I had done the 4.3. I do use first gear now, but it still revs too fast at speed. Engine is stock, just bored .060, head slightly skimmed. No hills here either, totally flat. George
George Butz

Thanks L E D, George and Dave. My primary objective is to reduce the RPMs when cruising. My concern is driving around town and up grades. Will I be disappointed with acceleration and/or having to constantly downshift. At this point I am inclined to go with the 4.33.

Bill
Bill Schroeder

Hi gear SierraT9 five speed conversion In my TF 1500. A ratio for any occasion.
Jan Kristoffersen

4.3 with a 5 speed fantastic. Add an Eaton-type blower, in the words of Michael Kay, seeee ya. One of the more important things to do is have your distributor recurved.

Jim
Jim Neel TD28423

Bill, LaVerne, (who has one of my 4.3's), posed the right questions. I'm thinking you need to determine how often you'll be at freeway speeds vs. driving around town. Also in question is the frequency of driving in hills. You'll find that the 4.3 will pull pretty good with your fresh engine and slightly better aerodynamics of the TF. Here in AZ My 1955 1500 with the 4.3 was really nice in the hilly areas at 5000 ft elevation. I expect the 1250 in lower country will make up for some of the hp difference. The most remarkable thing you'll notice right away is that first gear is usable and all the shift points are more in line with what you're used to in normal cars. I guess I don't mind shifting.

Jim Neel has one of my 4.3's on his '53 TD with the perfect setup I'd say. He did drive it with the 4.3 and stock transmission for a bit before the 5 speed went in.

Mort Resnicoff in Monroe Township NJ has just put one of my 4.55 rear ends in his TD perhaps you can get together.

You can give me a call anytime to discuss it further if you like.
360x317x4775
Dave Clark Arizona

Bill,

Dave is correct, I did drive my car with his conversion and the stock transmission which was a definite improvement. Here in upper Westchester County we have lots of hills along with the parkways and the 4.3 worked well on both. I installed the 5 speed because the original tranny needed to be rebuilt and I wanted a lower cruise RPM since if you don't keep up with traffic here you get eaten alive.

Regards,
Jim
Jim Neel TD28423

Hi all --- I have both in my TD, a Skyhook 5 speed and a 4.33. I love it and we have encountered some pretty steep hills going to GOF's, drop back to 4th or 3rd and just keep going. The engine has a few tweaks to get livlier performance, pertronics, shaved head (with some porting), overbore,1 1/2 carbies, Crane cam (stock replacement). Altogether a very satisfactory setup. I prefer it to my stock TF1500 (with 3.9 CWP) altho that goes like a raped ape.
Cheers,
Bob
Bob Jeffers

Bill Schroeder. I run a 1951 MGTD with a 4.55 axle which was fitted over 15 years ago. I am fully satisfied with the round town performance and hill climbing ability but my over 55 MPH highway use is relatively rare and for short distances. I would not want to go with a lower numerical ratio for the use that I put the car to. For longer distance highway use I have other more suitable collector cars.

John Quilter
Eugene, Oregon
John Quilter (TD8986)

Just came in from having driven another Snowbird's car to Florida. Left at 3am Tuesday. This time it was a fairly new Lexus ES350. Virtually identical gearing as the Nissan from a couple of years ago- 2000 rpm had it rolling effortlessly at 74 mph. And, BOY did it have a lot of oomph left at 80. In this car the redline on the tach started at 6200. Tach registered to 8000. No. I did not see how high it wanted to go. Bud
Bud Krueger

I have the same experience when driving my (new) mini. It is a Diesel with 6 gear stick shift, and below 3000 rpms you are well above US highway speeds.

Re. ratios: Having looked around for diffs just recently, I found quite a few 4.22 (38/9) diffs, mostly marked as Morris Minor, Austin Healey Sprite, MG midget. Does anyone know if these would fit the TD axle? They come at very reasonable prices compared to MGA diffs.

rgds, Mike
Mike Fritsch

Mike,

Those won't fit. Barney Gaylord has a great website and the cars that can have donor gears that will fit are found at http://mgaguru.com/mgtech/rearaxle/ra201.htm

Dave
Dave Clark Arizona

Great list, exactly what I was looking for!
Thx a lot.
Mike Fritsch

I may be shot on sight for what I am about to admit but my 52 TD has a 1980 Datsun 210 1.4 l OHV engine and 5 speed transmission installed sometime around 1985 near as I can figure.
It is a wonderful combination but it still has the 5.125 rear axle ratio that gives me 2950 RPM at 50MPH and 3475 RPM at 60 MPH.I don't care for freeway driving with that car so I rarely exceed 55 MPH. Besides I get the dreaded scuttle shake much above 50 MPH until it reaches almost 60 MPH where it disappears.
By the way the Fifth gear ratio is .854.
Overall, I feel that my rear axle ratio is ideal for snappy performance on back roads with an 80 HP engine.
B.J. Facey

This thread was discussed between 02/05/2012 and 09/11/2014

MG TD TF 1500 index

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