Rear gear Ratio Opinions/Suggestions

[MikeB]

Originally Posted by Palf70Step

350 yes, Tach yes, 3 speed manual, so at highway it would be same as T350. Rpms are about 2300 at 70MPH, 60 is a hair over 2K. 265/70x16 tires or about 30.5 in high

A TH350 would have a little slip at highway speeds, depending on load. My old tach wouldn't fit after I shortened my column, but I remember there was a 200-300 PRM difference between a slight downslope (light throttle) and an upslope. So with 29" tires, let's say if you see 2100 on level ground to downslope, that would be 2300-2400 so on an upslope. Of course the amount of "slip" depends on the converter.

[BRL]

Originally Posted by pjmoreland

I had a '67 C10 that was ordered originally with 4:10 gears. I put a 350/350 combo in it, and it was miserable on the highway. I swapped in 3.08 gears, which made it great for highway cruising. I drove that truck between California, Colorado and Oregon many times over the years I owned it. A set of 3.08 gears would drop you from 3300 down to 2700. The down side, of course, is that you will end up with a very sluggish first gear.

That's exactly what I had and exactly what I did and I couldn't be happier!
As far as a sluggish 1st gear goes I have what I consider to be a rather healthy low end torque engine and I don't drag race the truck.
This pickup has plenty of pick-up!
God I'm witty.

[MikeB]

Originally Posted by pjmoreland

A set of 3.08 gears would drop you from 3300 down to 2700. The down side, of course, is that you will end up with a very sluggish first gear.

My truck has a 3.08 posi behind a mild 350, and will spin the tires for 50 ft if I stomp on it in first gear. PJ, you must be one of them there hotrodders with a big cam and 850 double pumper.

[slikside]

Originally Posted by Palf70Step

350 yes, Tach yes, 3 speed manual, so at highway it would be sam as T350. Rpms are about 2300 at 70MPH, 60 is a hair over 2K. 265/70x16 tires or about 30.5 in high

Yeah, mine came out similar on Geezer’s calculator. I have 30” tires.

[pjmoreland]

Originally Posted by MikeB

My truck has a 3.08 posi behind a mild 350, and will spin the tires for 50 ft if I stomp on it in first gear. PJ, you must be one of them there hotrodders with a big cam and 850 double pumper.

I did that swap 27 years ago, and I'll admit the gas pedal spent a decent amount of time mashed to the floor back then. I had a 70s smogger 350 with a stock cam, so it was nothing too crazy. With the 4:10 gears it would squeal the rear tires effortlessly. Not so much with the 3.08 gears.

A few years later I swapped in a 300HP crate 350 to liven things up. The younger brother of an ex-girlfriend asked to borrow my truck for a high school day at the local drag strip. My truck did quite poorly that day, getting 17 second quarter mile. I ended up installing 3.73 gears after that, but once again it was miserable on the highway.

[jocko]

custom10nut, if you share your tire diameter we can tell you exactly what your current rpm is at various speeds and what it would be with different trans and/or rear gearing at various speeds. Engine size and observed tach/mph readings aren't relevant.

[custom10nut]

Originally Posted by HO455

You haven't mentioned what size rear tires your running. For my GTO with a T400 3.55's with a 26" tall tire was perfect for me. But I wouldn't have been happy with 28" tires.

In my Burban I run 3.73's with a Gearvendor (Which I got off Craigslist for $600. It was the 3rd one I've bought used.) on 28" tall tires.

275/60/15 BF Goodrich T/A Radials.
I think they’re 29 or 30 inches tall
Is realist here.
I never going to chase the dream of 18-20mpg (right now I get about 15 Hwy)
My concern is the constant running at 32-3300 rpm at 70-75.
Oh the truck will easily do it, but I don’t want to abuse the engine.

[Already Gone]

Originally Posted by custom10nut

275/60/15 BF Goodrich T/A Radials.
I think they’re 29 or 30 inches tall
Is realist here.
I never going to chase the dream of 18-20mpg (right now I get about 15 Hwy)
My concern is the constant running at 32-3300 rpm at 70-75.
Oh the truck will easily do it, but I don’t want to abuse the engine.

Your 275/60 15's are 28 inches tall.

[jocko]

the actual RPM your truck is experiencing in the upper left red box - along with various trans and gearing combos that have been mentioned in the thread. Obviously you won't want to do BOTH an O/D and 3.08 gearing - they're just listed for reference only. It shows why 3.73/700R4 is such a popular option (and why GM built zillions of modern 1/2 ton trucks this way) - it improves 1st gear acceleration and also reduces your cruising rpm more than a 3.08 does. But, several options can get you closer to what you're trying to achieve. Hope this helps!

[MikeB]

Nice charts! Interesting to see the RPM increases from 55 to 65 and 65 to 75. More than I would have guessed off the top of my head.

Looks like the TH350 numbers assume no converter slip at all, which rarely happens even on a level highway.