The Stovebolt Site Forums The Shop Area Driveline Double check my driveline math

Been keeping an eye out for a 60s truck 12 bolt to replace the well worn H033 in my 1951 3100. Found one I think I can buy cheap. Axle stamping and GM charts suggest it’s a 3.40 ratio.

Current driveline is an SM420. I have an S10 T5 waiting in the wings. Not in a hurry to install. Probably going to wait until it’s engine rebuild and/or clutch time. It has the 0.72 OD. I run 215/85r16.

I’m thinking this particular rear is a bit of a poor match to my tall tire and steep OD combo. OD wouldn’t be of any use until somewhere past 60 mph. At least that’s what it looks like if I’m using the Spicer online calculator correctly.

Without doing much math, I would agree. Your overall 5th gear ratio looks like it would be below 2.50 and that is going to be difficult unless you have a pretty strong engine. A 235 in good nick can and will work with an overall ratio in the mid to higher 2.6 range (mine is 2.65, however I have 15 inch wheels and tires with a bit smaller diameter than yours). On an ideal day without steep hills and wind, I'm just fine. With steep hills and wind I just drop back to 4th and my overall ratio is 3.07.

It wouldn’t be a bad match to the SM420 though, would it?

Perhaps I could install the rear for now and just downsize tires when I install the T5.

As for the engine condition, it’s stronger than I’ve given it credit. I had my son running the GPS speedometer app. I was able to accelerate up a fairly steep hill today, in 4th gear. I let out of it at 65 mph, because that was well in excess of the posted speed limit. But it was still picking up speed when I backed off.

If you're able to run 65 with the current setup, is there a good reason to change what's already there? There's bound to be a better condition HO 33 out there somewhere that would be a bolt-in swap, right?
Jerry

Fair question. I need to actually verify what gears are in mine. I believe the standard issue 55-62 truck rear was 3.90?

Using the same calculator, I don’t think I was running nearly the RPM it would take to pull 65 mph with my rear/tire combo.

I guess if I could find a better one that actually has the 3.90 gears, that might not be a bad match to the T5.

There's very little difference in RPM between the older torque tube 4.10 and a 3.90. I don't know if anyone has ever mated up a torque tube axle with a T-5, but it seems like it wouldn't be terribly difficult to accomplish. IMHO, the torque tube rears were usually 4.10, weren't they? Drop me a PM about the 58 GMC axle I've got that's looking for a home. I believe it's a 3.90.
Jerry

I always try to have my cruise rpm to be right at the max torque rpm as I think it's the most efficient BUT, Ron (We Be Old) has run his 54 for years with an insane (3.08??) rear ratio and overdrive so I guess the old tractor engines are pretty flexible.

I think you’re right about the torque tube ratio. I strongly suspect I have “numerically lower” than 3.90 gears currently. Need to just get under the truck and verify.

Will do on the PM. I was wondering if you still had it.

The passenger car torque tube axle behind the Powerglide automatic trans was a 3.55 ratio. I think the complete setup can be adapted to a pickup with a little bit of tinkering. I've also got one of them stashed away.
Jerry

And I run mine with a 3.07 rear end ratio. But hey...I'm a member of the Old Geezers Club so I get some special privileges, don't I? Kidding aside, I pitch in with Evan completely here (as I mentioned to another poster asking about this recently)...figure your cruise rpm to be around around your max torque rpm for efficiency. Once you eclipse efficiency and pass over to the other side, you can watch your gas consumption increase. 50+ years of driving these trucks has taught me that and it never varies.

As for the 3.07 and the T5, I've mentioned all this before but here is why it works: T5 with the gear ratio chain beginning with 4.03 first gear gives an overall ratio of 12.37 (a great first gear for a 1/2 ton pickup with an inline 6 and very close to the 12.08 OEM AD first gear ratio). Second and third gear overall ratios in this T5 will also match OEM within hairs, so in the first 3 speeds, your truck will seem just like a stock AD truck with a synchronized first gear. Then when you get to 4th, you get your first "overdrive" gear...a 25% drop in the overall ratio to 3.07. Is this reasonable? Yes, it is. Let's look: The old 3 speed with OD always came from the factory with the 4.11 rear end and then the overdrive dropped things 30%, so you're fine. If you had the old overdrive they mated with the 235, your final ratio would have been 2.87 (which is about 7% more liberal than my 4th gear ratio). When I go to fifth gear (a .86 reduction), my overall drops to 2.64...a wee bit more generous than the 3 speed with OD would have given me, but...I have 15 inch wheels and tires with a 26.8 inch diameter, so we need to divide the 2.64 by .95 to get the real ratio...2.78 (very close to the old OD final gear ratio). Admittedly if I'm in very hilly terrain or headwind, I will just leave it in 4th, but when things even out, I'm back in 5th. It is a see-saw...if you lug the 235 you'll also wreck the gas mileage and you won't be doing the engine any good, either.

If you have a 3.07 or 3.08 rear end and the T5 with the right gear ratio chain, I think you'll be very happy with the combination. You can load 800 pounds of firewood in the back and drive home without a problem. And you can zip along on the highway at 55 or 60 mph all day long. At 60, my engine is running about 2000 rpm. The 1959 version of the 235 (up through 1962) had a torque curve that started getting flat around 1600 rpm and stayed that way until nearly 2800 rpm.

How about an extra 3/8" of stroke and a 60 overbore for 270 cubic inches in that "235"?
Jerry

I was a little flabbergasted the way my truck pulled that hill last night. It’s a 1962 model engine by the numbers. And I don’t know many miles, but nothing about it suggests it’s “fresh.” Rocker shaft was worn out when I got it, insides were sludgy, has some blow by, idle oil pressure is nothing special. Makes me wonder what a really good 235 runs like. Or maybe this one is better than I previously thought.

I'm hoping to have a couple of prototypes of the stroker engine running this year. It would have been sooner, but some health issues got in the way! 258 cubic inch "216"- - - -270 cubic inch "235"- - - - -300 cubic inch "261"!
Jerry

JW51 The first vehicle I ever drove for real had overdrive,the ton truck size load of factory,home built,combinations of other stuff NEVER got any economy for me. The .7 overdrive ratio always was too much. I had piles of calculations just like is above. Not until modern GM equipment(2019) did I see much economy a turbowed 1.5 litre mountain climber with cooling,handles like a dirt car,is safe,it's even guaranteed,is the best i've seen. Easily goes by gas pumps at 31.5 mpg. that is with a suitcase handle for the DRIVER to give his worn out shoulder some therapy. Also made in America... um Mexico IS in America right?? bought it for a song too right before everything went over the mountain !!

Still thinking about this.

3.40 is a bit too tall of a gear for my current setup, but I will need tires “someday.” Current setup is 215/85r16 which which a lot of folks recommended here.

Will a shorter “215” tire still fit the stock AD rims just as well? Or do you need that talk sidewall to be able to squeeze into the narrow rim?

I used 225/70 R16 instead of the 215/85 simply because they were less expensive. They're about an inch shorter than the 215/85s. I like the look of 215/85 better, though

With .72 overdrive, and 215/85 R16 tires (30.5 inch diameter) you will want at least a 3.90 rear end gear. You can go quite a bit higher numerically on the rear end ratio. 3.40 would definitely be a tired dog in first gear and 5th gear wouldn't be useful until 60-65 MPH.

I think I misread the stamping that originated this thread. I now believe it’s 3.73 gears. That wouldn’t be too bad. OD still useful once you get to 55ish.

I ran a 59 Suburban for several years as my daily driver. It had a 307 V8, a SM 420, and a Dana type 60 3.73 rear axle from a GMC 3/4 ton pickup. It had no problem keeping up with traffic on the interstate, and pulling any trailer I chose to back up to! It got about 12MPG, regardless of operating conditions, loaded or empty!
Jerry

I’ve had that thought too. If I ever change my mind about the T5, and stick with the sm420, 3.73 would still be a pretty highway friendly setup at 1:1

I have been playing around on drive train for some time and now that I am retired I will be deciding between
SM420 with tall pizza cutter tires using a 1955 rear end with 3.9 gears
OR
T5 -- with 0.73 OD gear and a 1955 open rear end with 3.90 gears.

I am leaning towards keeping the SM420

Since I am a closet nerd, I set up Google Sheets to compare drive train configurations.

I have updated it to what may help you (you can play with it) ...

Drive Line 1 is setup for T5 w/ 0.73 OD and 3.40 gears on 215/85R16
Drive line 2 is setup for SM420 w/ 3.55 gears on 215/85R16 (I am not sure if you have lower rear end gears in your tube rear end.


Stovebolt Question[/url]https://docs.google.com/spreadsheets/d/1WdXVQvGFr0QUxVdHPDmwGrMwW9m7wMH0m8WvfxcIRUY/edit?usp=sharing

Kilted54,

Unfortunately your link requires folks to sign in to a "Google" account to further access and view the spreadsheets you tried to share.

Perhaps there is another way for you to share those documents???

Dan

I trust everyone that tells me the T5 is the cat’s pajamas. I own a T5 that could be checked over and installed any time.

But for some reason, my current lean is to stick with the SM420. Find a rear that is less worn out and a good match.

Lets Try this link

Driveline Comparison

I am faced with what some would say is a simple choice as I have 0.73 OD T5 and a 1955 Axle with 3.90 Gears. Why wouldn't I switch from my SM420 with 3.55 rear end?

On my drive wheels I will be running 235/85 R16s

I wanted to see the RPMs across MPH for these two options - The spreadsheet I cooked up plots this ... Solid lines are the t5 driveline option and the dash lines are the SM420

[img]https://photos.app.goo.gl/3qKfS6HTVCJy8dCr5[/img]


A comparison of top gear for each driveline makes it clear that the T5 is hard to beat .. In this chart a negative number means the engine for the T5 is spinning that many Rpms slower.

[img]https://photos.app.goo.gl/9yyt5SMLfg6NXeNk7[/img]


I am not looking for interstate cruiser but it would be nice if I could do 65 now and then. Keeping the SM420, with the tall tires, at 65 mph, the engine will be spinning just above 2500 rpms. At 70, it will be spinning at about 2800 rpms, which I have seen often as a nice cursing RPM. Thus, at the moment, I am not going to swap in a T5.... The Granny gear is great for parades and sentimental reasons as my school buses when I was a kid must have used SM420 -- brings back memories of a simpler time.

The real beauty of the T-5 is how it brings new life to a 235 or a 216. With the two extra gears, every time you shift, you will be in the optimal power range of these low horsepower engines. Now more revving up the engine until it screams before upshifting only to have the engine lugging along until the RPM come back up to the optimal power range. This feature is absolutely remarkable.