The Stovebolt Site Forums The Parking Lot Big Bolts '51 3800 216 gear ratios

Gear ratii? I thank God for not making me a word nerd. That don't hardly look right, ratii. Darn those old penguins for teaching me Latin. Heh. Okay, here it is: My truck, with its 5.14 diff ratio, is too slow. I want to cruise gently and economically at 50 - 55 mph. My (SM420) transmission's gear ratios are too widely spaced; I am uncomfortable with - when fully loaded starting up a long grade - using my engine's entire powerband and both verges to get up to the next gear. I live in mountainous terrain, that ain't gonna change. I intend, now, to keep the original engine - or one like it - in my truck. That ain't gonna change either. So here's the plan in its current incarnation, subject to change without notice: a 4.10 diff from a '71 3/4-tonner, and a NV 5-speed (overdrive) tranny from a Dodge Cummins. The NV is a much-maligned, but perfectly strong transmission abused by uninformed drivers who believe that the monstrous amounts of torque generated by their overpowered engines makes it okay to stay in overdrive when they should downshift to direct drive to preserve their powertrain.
This combination, with my 235/85R16 tires, will allow me to cruise at 55 mph with an engine speed of barely 2000 rpm. If I downshift to 4th the engine speed will increase to around 2800. This, I think, will give me exactly what I can live with forever in a working truck which may one day be called upon to go transcontinental.
At least, that's my theory, in its current incarnation, subject to, etc.
I want a truck which looks as close to original as possible, but works better. Opinions, anyone?

hotshoe36,

I don't think you will have enough horspower with the original engine to handle the new rear ratio. The original set up was engineered as a matched set. If you were to keep the original rear and just add the OD transmission you would have the best of both worlds, almost. You could still haul a heavy load up hill in a lower gear and out on the flat put it in OD and get a little more speed. It will be hard to have both power and speed at the same time, the engine will just come up short.

Mike B

4.10 and 235/85 will put you at 2400rpm @ 55mph 4th. OD will turn 1750rpm. You can use a gm nv4500. 92-95 have almost the same ratios as your stock sm420, just has overdrive. The mid 95-pressent has the same case and ratios as the dodge. slightly taller 1st and 2nd. Sounds like you would be better with 4.56 and like a gearvender overdrive. it would let you split your gears and still give you overdrive. 2650 4th, 55mph, and 2100rpm overdrive.

4.3,
when you put it that way my plan does sound tall. My engine might indeed come up a little short. If that becomes the case, Mike, there's engine work or lower tires in the offing. I'll be changing the diff first. In fact, the job begins later today. I'll run the 4.10 for a while and see how she feels at the top. That OD would only come into play, if I had it, for empty/featherweight miles on highways. The reason I'm considering the Dodge SM instead of the GM one is the shorter low gears. I want the widest range I can get, without going as wide as my old granny 4-speed. There are some rough, unimproved roads between here and my favourite fishing.

-Mark

I have a 1-ton out back that has a GM 14-bolt rear under it with 3.70 gears (if memory serves). The guy I got it from said the 216 wouldn't pull it any longer once he changed the rear.

Granted 3.70 and 4.10 are different, but his experience does confirm it could be a problem. Please come back here and tell us how it goes after the swap!

By engine work do you mean on the 216 or maybe swapping for a 261? Few would ever notice that swap looking under the hood, but I'm sure you'd notice hauling a load or climbing a hill.

Buncha ways I might go there, Steve. I can't think how easy it would be to find a decent 261, but I have thought a bit about 250's. What I might do first, though, is swap out the tires for 225/75R16's.That would give me almost a 3" drop in tread height.
The rearend is first, though. I will keep the group posted on the result.

I don't know how easy it would be to find a 261 either, but I understand they were used in 60's Canadian Pontiacs and you might find one of those almost as easy as a 70's 250.

I've been to Surrey but never looked for wrecking yards there so have no idea what might be available.

But if you are going to deal with the conversion issues to go 250 why not consider a 292 and get a bit more torque and power?

I know, you're not there yet. I'll keep watching what you post!

You only get a lower first and second gear. GM sm420 is 7.06 first and 3.58 second, GM nv is 6.34 first and 3.34 second dodge and late gm is 5.61 first and 3.06 second. The 3.34 is nicer off the line. all four trannys have the same 1.7 3rd gear. If you still like the 3.06 2nd I would go with a mid 95-up gm nv. It uses the same 1.125 10 spine input as the sm420 and the imput is the same lenght. The dodge diesel,V10 has a larger and longer input shaft. You would have to get a custom clutch made for your small flywheel and make a 1inch adapter. A dodge v8,v6 nv has I think a 1.075 10 spine input. you could use a ford or dodge clutch with your pressure plate. still need the 1in spacer.

Yep . . I have trusted friends in the gear business. I'll have the 4.10 back within a week, and a test drive will tell me plenty. What concerns me most is what I'll find at the top, as I said. 55 is a sensible speed for an old truck, and I want to feel good there. I know that what I have in the SM, effectively, is a 3-speed road tranny with an emergency stump-puller boat ramp gear. I know, too, that there would be times I'd miss that low if I went to either of the NV's. I like goin' fishin', you see, and tire chains will be standard equipment. Even so, I think that it would be okay to slip a clutch judiciously on those rare occasions. But first the diff. Stand by . .

And... here it is. Picked her up a couple hours ago, and I've driven 20 or 30 miles in a bunch of different conditions. To review, what I've done is change the differential gear ratio from a far-too-short 5.14 to 4.10. Nothing else substantial except for whatever bearings were exposed in the process (all of 'em), and the front wheel bearings to boot.
Here are my early impressions: First gear is now useable for something other than rock climbing. I stopped on a 15% grade. 1st gear was (of course) an easy start almost without any throttle. At about 10mph I shifted to 2nd smoothly and easily without having raced the engine, and 2nd pulled away nicely. Ditto 3rd, and I levelled off at around 30 mph in the middle of third at a moderate throttle setting. Keep in mind that at 15%, the grade I ran was almost 3 times as steep as the steepest interstate. I could easily have gone fast enough to pick up 4th, but I'd have been exceeding the speed limit. So at the top I went to 4th and idled very nicely at around 35mph at the very bottom of 4th.
Then I took to a freeway. Evelyn followed me with her certified speedometer. It's German, you know. Therefore, of course, superior. Right. First I tried for my fastest comfortably sustainable cruise: 58 or 59 mph, she said. I could do that all day. But I also tried a "fuel miser" cruise, which turned out to be exactly 50mph. At that speed I could actually downshift, dump the throttle and scream up to 55 just like that! I didn't do that - no reason to - but I could have. I have the distinct feeling that the next time Epic is asked to haul a load that would void a new truck's warranty, she'll do it without a hitch, exactly like this, but at a slightly higher rate of fuel consumption.

There is no problem here, I'm telling you.
This is exactly what I was looking for. A truck that will still lift your house, but can now eat great distances without straining. I think I'll stay with this for a while. 2nd is still a flat ground starting gear. I don't need an overdrive, and probably wouldn't unless I really started to sock on the featherweight highway miles. Even then, this would do nicely, I think.

-Mark

Thanks for the information, Hotshoe36. Gives me more to think about. I've been thinking about another gear after driving my late model 1 ton SM465 for a while. It has a Dana 70 in it, so rear end ratios shouldn't be a problem.

Probably the reason I have been thinking about a 5 speed with OD fifth is that my last three cars have been a 1990 Honda Civic Si, a 1990 Toyota MR2 and a 1990 BMW 325is. (I really hadn't noticed before that the years were all the same.) After those three cars, I'm pretty used to having more than three choices for transmission ratio. I guess that five speeds are necessary in cars like these to make up for a lack of torque, which is not as much of a problem with a Chevy six cylinder.

I'm really going to end up with two slightly different problems. In the late model I'm not comfortable with the engine RPMs at highway speeds. It has a 350 4bbl in it and gets about 13 mpg when cruising at 50-55. I live in a hilly area and will have to shift down to 2nd fairly regularly. I can live with that, but the large ratio difference isn't what I am used to.

The 1951 4400, which I haven't driven yet, is probably going to have a really steep rear end ratio with transmission ratios similar to the SM 465. The difference is the engine.

I may put a slightly smaller engine in the late model because I don't do much heavy hauling. The 51 has been upgraded to what looks like a pressure lubed 235. I'm still considering other options, but I think it might end up with a 292 I6 in it. (Unless I can find a cheap Jaguar 4.2 liter engine/transmission somewhere.

I guess my first steps are getting a tachometer and getting the '51 into my back yard instead of theirs.