The Stovebolt Site Forums The Shop Area Driveline 3.08 or 3.90 to replace 4.11?

I know this has been addressed many times, but I'm having trouble sorting through all the info and coming out any smarter. I've collected some of the major parts for my project but am missing one part; the rear end. This is for my '53 GMC 1/2 ton. Plans are to run a stock '59 GMC 270 and a '87 T-5 with a 0.72 fifth gear. I'm planning to swap out the rear center section for an open center section from a "55.2-62 Chevy 1/2 ton. Tires will be about 30" tall. I have a line on a couple 3.90 rear ends but it has been suggesteed that I would be much happier with a 3.08 This truck is my (mostly) daily driver and I drive it forty+ miles a day and would like to get decent gas mileage and take the old truck on long trips on occasion. This is not a low rider or a show truck; it will be used mostly for transportation but also to haul stuff when I need to. So . . . 3.90 or 3.08? Or is there another option that will allow me to keep my original axle?

3.90 it is, when the 3 speed OD became optional they came with the 4.11 and did pretty good on the highway while still being able to haul a load, you'd never use the OD with a 3.08 and likely be frustrated with having to slip the clutch to get rolling not sure with it being a GMC, but the only other possibility would be the 3.55 gear set Patricks offers, which some folks use instead of going OD

Bill

Twotone, there are some 3.38 gear ratio's out there but hard to find. Someone here mentioned looking for a GMC/Chevy that is an automatic for the 3.38
I got lucky. Just a week ago today a fellow bolter gave me the rearend out of his 59 Fleetside. When I got home and pulled the over and counted the teeth I was in shock....a 3.38 which will replace the 3.90 I had in my 52panel.
(I put the 3.90 in to replace the old original torque tube 4.11)
Don't give up yet.

Using an overdrive transmission, the 3.90 will be perfect. The higher gears (3.38 being the only option with the HO33 rear) will leave you with a pretty much unusable 5th gear. If you weren't using the overdrive trans, then the 3.38 would be ok.

The 270 gmc could pull a 3.38 over drive with 26 inch tires. it won't be an ideal setup with 30 inch tires.

Just for giggles here are a few rpm/speed calcs with the 3.9 rear and .72 od and 30 inch tires.

55 mph @ 1730 rpm
60 mph @ 1887 rpm
65 mph @ 2044 rpm
70 mph @ 2201 rpm
100 mph @3145 rpm
108 mph @ 3400 rpm, the maximum reccomended rpm of a 270.

You really can't do much better than that! Just a note, tires can vary by 1 inch or more of their advertised diameter, usually on the shorter side. 1 inch shorter tires will add about 75 rpm @ 70 mph. This setup should be both agressive off the line and comfortable at cruising speed.

I have had overdrives for over 50 years...Borg Warner, but the top 2 is about the same split. 3.90 is ideal. I used some 4.11 in my Y-Blocks which gives you a little more power in overdrive, but I don't think GM made a 4.11 open rear. You must remember these old engine are happier at higher rpms than current production engines, even if they don't sound like it.

Thanks guys! Thats the info I'm looking for. And thanks Bigbadswingdaddy for running those numbers for me.

Hey Alvin

Where do you look up the "tooth count" to establish the differential ratio?

Dave

You can count the number of teeth on the ring gear and pinion gear, then divide the smaller number into the bigger number and you will have your ratio.

Just for reference, I have 3.38 gears with a 3 speed OD trans (1968 1st gear synchro Saginaw) in my 1957 Chevy NAPCO truck. It works just fine for me. However, I do have a warmed over 283. I was concerned with the same issues as mentioned in this thread relative to bogging down. But the 283 seems to help mitigate that issue. I got my gears from Patrick's. He gave a discount because I bought two sets (front and rear) and was most helpful.

Note:
I had 3.90 gears, but the front differential had some chipped teeth, so I had to replace them anyway and opted for the 3.38s. I'm working on rebuilding a 348 engine, so if long-term the 283 doesn't cut it, I'll go with the bigger engine (I need something to keep busy).
don

Each engine family has a different torque range. The 3.38s with an OD may be OK for a 283's torque range but it aint gonna work with a 248 six and I'd bet not for a 235 either. With 3.90s my 248 rpm is well below the power band at highway cruising speed. If it had any higher I couldn't use OD.

Ok, so, what is the power band of a 1958/59 270? It's hilly here in southern Iowa and I don't want to be down shifting every time I come to a hill. darn Cletis, you got me scared.

My 248 seems to kick into the power band at about 2900. (I don't know how accurate my tach is.) I imagine the 270 is probably close to that. Unless I'm running 65-70 I have to kick it in passing gear for some of the hills around here. The 270 and being in a pickup instead of a Suburban may do better.

Don't let it worry you. Go with the overdrive. It's the only way to fly.

What tires are you running? The gears are only part of the equation.