The Stovebolt Site Forums The Shop Area Driveline 4spd vs 3spd questions

I'm at the point I want to order parts to rebuild a trans for my 42 1/2 ton. I'm just looking for opinions and real world experience to help me decide which to use.

I have the original 4spd crash box and, 46 and 47 top shift 3speeds with everything to do the swap.

I will be running a built 261 or 270 GMC , with 4:11 torque tube.

I can double clutch, but I'm leaning towards 3speed. I have never driven these particular trans, just 60's 3 and 4 speeds. I tend to run my stuff hard but try not to abuse it.

Does the 4speeds 3.? 2nd gear feel too low for normal driving?
Does the 3speed shift quicker?
Can the 3speed handle the torque?
Which would be more enjoyable in a daily driver?

Can't offer info on the 3 speed, yet, but from experience with a 4 speed and 4:11 rear end my '46 project will NOT be a repeat. Granny low is 1st, 2nd is roughly equal to 1st in a 3 speed. Top speed in 4th in my '37 is about 50 mph. Much over that and the engine and tranny sound like they are about blow out. For me, at that point the driving fun is gone.

My '40 has the 4spd non-syncro , I enjoy it :)I always use 2nd to start off , 3rd is just for slow corners , everything else is top

With my 245/75 16 tyres and 3.9 diff , 60mph is 2600rpm .

My friend had a hot 235 in his '37 , with the same 4 spd. It did just fine also.

I think , if you are happy with the more precise action needed to properly work the non-syncro 4spd , it will be ok , I also think you might want to be careful with how you treat that diff with the bigger engine you are using .

Thanks this is what I'm looking for. 99% of my driving I never need to get over 50mph. RPM and the sound of swapping pistons doesn't bother me anyway . I'm more interested in what its like getting to that speed.

I will take it easy on the diff and trans but I don't want to put something in that will break at idle. I have heard stories of 302's eating 3 speeds. I learned my lesson , not to shock a drive train too hard after destroying beyond repair a rear and muncie 4spd in my 65 GTO.

My 39 has the 3-speed trans. The 3rd gear has a 1:1 ratio. The 4th gear in the 4-speed also has a 1:1 ratio. At one time I thought of putting a 4-speed in the 39. After driving my friends 40 that had the 4-speed, I'll keep mine as a 3-speed. I purchased a ring and pinion gear set from a guy in California that had 25 sets made especially for the 39's. I travel down the freeways at a comfortable 55-65 mph. without the engine screaming at me. These old legs and knees need less clutching, not more.

Rick

Double clutching will give you a work out for sure. I usually don't use the clutch once I'm moving except to downshift. Does the 3spd shift quicker than the 4spd or is there still a 2 minute wait between gears?

The 4 speed isn't synchromesh like the 3 speed. So, that is another factor to consider. IMO, if the plan is to use this as a daily driver or anything much more than a grin and drive, I'd opt for the 3 speed. I really like my '37 with a 4 speed, but at my age and now being spoiled by more driver friendly modern trucks, the 3 speed seems more attractive.

While my truck has a 6.17 rear (with 7.50x20 rubber) so my experience might be a little different than that of a smaller truck with a higher rear ratio but smaller tires. You'll probably find that 1st in the 4 speed isn't useful except for creeping into tight spaces or maybe pacing a parade you've entered or if you're moving a heavy load in soft soil conditions. If you can handle shifting without the synchronizers the top gear is the same. The only thing you loose might be a little flexibility in some situations for maybe a little better ratio spread for unloaded cruising around town and slightly quicker shifts. I am also under the impression that the 4 speed box might be slightly stronger than the 3 speed from the same period.

With the 3-speed, push in the clutch, shift to higher gear, let out the clutch. Downshifting is the same procedure. Except for 1st gear, can't downshift from 2nd to 1st. 1st doesn't have a synchronizer. The 3-speed doesn't have a lever on the gear shift to go into reverse like the 4-speed. Some floorboards also have a different hole configuration between 3-speed and 4-speed transmissions.

Rick

Can always shift from second to first by double clutching, not hard to learn.

Ed