The Stovebolt Site Forums The Parking Lot '60-'66 Trucks 64 vs 73 questions

I have a question. My truck is a 64 it has disc brakes and a 350 motor that I was told was a 1973. ( not sure if that is correct)

It has a two speed automatic transmission. Right now it has take off power up the wazoo but revs very high at higher speed.

I'm trying to tone it down. Right now it has a summit double pumper carb 600 CFM.
With high rise manifold
A speed demon would love the way it smokes the tires but I am looking to convert it to a cruiser

There are alot of factors the rear end ratio, the transmission and that carb.
I'm sure I'm being vague on the set up but are there any suggestions or input
That may help me come up with a plan. I'm thinking a different transmission and carb. I'll check rear end gear ratio in a day or two.

Help

If it’s screaming at higher speeds one of two problems come to mind. The rear end is geared for pulling instead of cruising or the transmission is not shifting into final drive. A ‘73 transmission should be a 350 or maybe a 400 not a 2 speed powerglide.

I agree that the two speed is not my cup of tea and am in process of finding a different transmission and may get new carb to replace the double pumper.


Like I said I'll check the rear end this weekend

The powerglide is a bullet proof tranny well sought after I had one in a old Nova that was sitting abandoned and the transmission shop wanted it as they basically had it sold before I got it there .I think the final gear is the same unless it is a overdrive unit..I also had one in my 1970 gmc truck worked good.

Figure out one thing at a time. Start by finding out what the rear axle gear is. Pretty easy to do. Watch this video.
If you have a power glide that works properly, then you might not be any happier with a TH350 automatic. Both have the same 1:1 final drive ratio.
A good cruising rear axle gear is a 3.08 or 3.42. If you decide on an overdrive transmission, then a common rear axle gear of 3.73 works well.

But I’d definitely start by figuring out what your rear axle gear is.

Looks like I have a 3.55 ratio rear end

If you install a tranny with OD, I’d recommend a 0.80 or numerically higher gear. A 3.55 gear is a nice middle ground axle gear.

Lug, you kinda lost me here.
You think it would be good to keep the rear end and a different transmission?

Sorry but I know nothing about gear ratios and little about transmissions

I found a 700R4 with OD would that be a good match for that rear end?

The other unknown is the tire size.

If you have a tach and a GPS, you can drive at (say) 50 mph and get the engine RPM. This will provide a benchmark for any changes you plan to make such as tires, gears, transmission.

P275/60R15

Pretty sure what I say below is accurate.
All factory 3 speeds and 4 speeds have a final gear ratio of 1:1.
The exception would be a factory 3 speed with the solenoid style OD gear, and I’m not sure those were available in these trucks from the factory.
All TH350 and Powerglide transmissions have a 1:1 final gear.
The 700R4 transmission has an OD gear.
Most of the 1960-72 C10 tucks had a 3.90, 3.73 or a 3.42 rear axle. I’m sure some were 4.10 too.
An engine at any given RPM will turn the wheels faster or slower based on the transmission gear X the rear axle gear.

So if your engine has higher RPMs than you desire at say 65mph, and you have a transmission with a 1:1 final drive gear, then you can bring the RPMs lower by either changing to a different rear axle gear (numerically lower like 3.08) or you can change the transmission to add an OD gear. The trick is to find a happy place where the truck has power to get going in 1st gear and yet not lug the engine in the final gear.

Example: Some folks with a 3.73 rear axle change to a 3.08 axle. They say it drives really well on the freeway with the factory 3 on the tree and a transmission final gear of 1:1. The RPM calculator I use tells me that combination will get the RPMs at 2320 while cruising at 65MPH (Assuming a 29” tall rear tire.). However, they have effectively reduced their power in 1st gear for things like pulling trailers and heavy loads. They have effectively “lost power” in all gears. That’s ok depending on how you use the truck.

Some folks install an OD transmission to reduce the highway RPMs and leave the rear axle as is. An OD gear can be a good solution, but not always. The engine must have enough power to move the vehicle when in overdrive gear or the driver will be constantly downshifting at the slightest upgrade.

Modern drivetrains usually target RPMs in the 2200 range on the highway. I owned a Miata that had a small short stroke engine that had RPMs above 3000 on the highway. That’s because that little Mazda engine didn’t make much power in the lower RPM range being that it was a short stroke engine.

So if you keep the transmission as is, you can reduce your highway RPMs with a numerically lower rear axle gear, but at the cost of losing some power in all gears.
If you install a transmission with OD, a 3.55 gear X 0.86 overdrive will get you 2299 RPMs at 65MPH with a 29” tall rear tire.

Finally, simply changing to a taller rear tire will functionally reduce the RPMs too. It all depends on what stance and look you want.

A 275/60R15 tire has a 28” diameter.
So a 0.86 OD gear with a 28” tall tire and 3.55 rear axle will give you RPMs of 2381 at 65MPH.

A 700R4 has a 0.70 OD gear according to the internet. If your 350 SBC is as strong as it sounds, your engine should have plenty of power to use it effectively. A 0.70 OD gear X 3.55 rear axle with a 28” tall tire at 65 MPH will have RPMs at 1938.

700R4 and 350ci will work very well together. I’ve had that combo in 3 hotrods over the past 10 years and have been very satisfied. I do not know what the rear end ratios were/are.