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MG TD TF 1500 - TD Road Speed Expectations

Guys, I finally got my '53 TD on the road late last year after a 4 year restoration. What a blast! But never having driving one before, I'm wondering what to expect from it on the road. The speedometer is obviously optimistic, as I brought a GPS with me on one ride and while the speedo reads 70mph, the GPS reads 45mph. At this speed the tachometer is reading around 3500 RPMs in 4th gear. Discounting the speedo, does this sound correct for a stock set-up? If so, and assuming everything is in good shape, what's the best speed (or RPM) I should be able to maintain without being concerned about damaging something, like blowing up the engine?

Thanks,
Mark
MJ Rotsky

Yep. That's about right. You have to get up to about 5,000 rpm do do 65 mph with a 5.125 rear end if I remember right. There is a chart somewhere in the archives that gives the exact speeds but it varies by tire size etc.

This is the reason that so many convert their TD's to 4.78 or even 4.3 ratios to keep the RPM's down to 3,500 or 3,000 even at freeway speeds.

I feel comfortable driving my car at 4,000 RPM and that's about as fast I care to go. But to be honest I don't drive my car much or certainly on long trips. If I did I would get a 4.3 for sure.
Christopher Couper

I have the 4.3:1 gearing but the engine loves to run between 3500 and 4000 RPM. Oil Pressure is good and valve timing is good at those RPM's. I just "Hums" along.
C.R. Tyrell

4200 RPM is about 60 MPH with a stock 5.125 rear axle. We have twice taken our TD on 5000 mile trips around the western US and most of the time we cruised at 4200 all day long. The TD loved it. I would suggest getting a diagnostic tachometer from your local NAPA and attach it to the coil to get an accurate rpm reading. Cheers - Dave
D W DuBois

Mark,
Could your speedo be KPH? 70KPH is about 45MPH. They could both be right.
45 to 70 sounds like a big difference.
I get very good correlation between my GPS and speedo.
Mort
Mort 50 TD

Mark, the stock TD figure is 14.4 mph/1000 rpm. So, 3500 rpm in 4th is about 50 mph. My TD, with a 4.3 rear end gets about 16.5 mph/krpm, so 4,000 rpm is just about 65 mph. Where abouts in MA are you. I'm in Plymouth. Bud
Bud Krueger

Mark, if you've not read Carl Cederstrand's treatise called "Ring & Pinion Interchanges for the TD/TF" I would recommend you get hold of a copy. It is a detailed & technical article which will answer all of your questions & pose a couple more. During the restoration of my TD I became aware that it was possible to replace the 5.125 with the MGA 4.3, housed in the same centre. On page 2 of that booklet Carl makes some astonishing statements one of which I'll include here.

" I will categorically state that the operation of any T series engine @ a steady 4,500 rpm (this is 64.8 mph with the original 5.125/1 rear end) amounts to little more than an exercise in engine destruction. At about 10,000 miles a newly installed set of rod bearings will have developed fatigue cracks, a newly installed timing chain will have become noticeably slack, & the seating of new exhaust valves will have generally deterioated to the point where a gentle "pist" is heard from the exhaust pipe. I have seen this occur over twenty years without exception to every T series that has crossed the southwestern United States at normal slow lane highway speeds, with its engine flogging 5.125/1 rear end."

Because I always wanted to be able to keep up with freeway traffic & intended to take the car on extended club runs & as Carl is considered something of a legend in the MG fraternity, his statement above, along with "the 4.300/1 is absolutely without peer for cross country travel", meant that the 5.125 had to go. If you google "Virgin Rear End Conversion" there is a great "how to" article on the conversion. There is some loss of 4th gear performance but I believe the higher top speed @ lower revs more than make up for it. Fitting the Chet Herbert cam with the roller lifters will see a significant improvement in performance & I expect to regain much more than I have lost. Moss sell the booklet but it should also be available @ your library. Cheers
Peter TD 5801
P Hehir

I guess I tend to baby old engines too much. I'll have to force myself into bringing the revs up and get him to sing. I did speak to one friend previously about this who has a '52 TD and he said he had his at 86mpg using a GPS. He just runs until the valves float. Sorry, that's just nuts for many reasons.

Didn't think about KPH. That could be about right.

Bud,
I'm not that far away. I'm in Somerset.

Thanks for all your responses. I'll be working to get my courage up for this weekend and give it a try.

Mark
Mark Rotsky

Peter, I do remember seeing that article a while ago. Pretty ambitious stuff. There is actually a guy close by who is a retired machinist and does this conversion and sells the complete axle for around $1000.00US with trade in. Probably a fair deal, but I guess I'd have to see if I could live with occasional over-reving vs coming up with that amount of money. Lots of trade-offs to consider. Thanks for the info.
Mark
Mark Rotsky

Mark, I would suggest forgetting Carl's description of the conversion..the Atalanta MG group was doing these at a steady pace and put out a very easy to read and accomplish set of instructions. An easy weekend job after all parts are collected if you are reasonably mechanically inclined . Regards, Tom
tm peterson

Tom. That's the "Virgin Rear End Conversion" article I mentioned. It describes the Atlanta group converting about 6 diffs at the same time & was published in a sports car magazine in the States. I do agree that Carl's methods are highly technical. Cheers
Peter TD 5801.
P Hehir

My TF 1500 with a 4.3 rear is a nice highway cruiser and the engine seems quite happy between 3500 and 4000 RPM. I have calculated around 17 MPH per thousand RPM using a GPS.
Jack Long

We've got a chap here in the Rockburn area who does it for $400 (Cdn). Does it in an afternoon. Don't know how, but he does. I do know he has a box of shims and when the conversion is complete, he makes new replacements to keep his inventory up.

The problem of course is finding the 4.3 CWP. They're getting harder 'n harder to find.

I don't know how I ever drove to FL, and CA and back in the 1950s with that 4.875!! The 4.3 is an absolute must if you're going anywhere.

Gord Clark
Rockburn, Qué.
Gordon A Clark

Mark, see http://www.ttalk.info/RearEndConversion.htm from my website. Bud
Bud Krueger

Gordon,

I suspect your FL and CA journeys were accomplished before the completion of the interstate highway system.

How many miles and how many overhauls on your engine?

Jim
JE Carroll

This thread was discussed between 09/06/2014 and 10/06/2014

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