Stovebolt.com Forums The Shop Area The Engine Shop "Tall" axle vs. overdrive for cruising?

There is an important factor that has (as far as I have read) not been mentioned that affects the choice of whether to slow the engine down for gas mileage, wear, noise etc. while cruising to achieve the same engine RPM at 60 mph, etc. by using:
A. a low numerical axle (3.38:1, etc.) with a standard transmission (1:1 final drive ratio), or
B. a higher number axle (4.10:1, etc.) with an overdrive transmission
The engine will run the same with either.
However: the driveshaft speed always follows the axle ratio (not the final drive ratio), and will be turning much faster with an O/D + high numerical axle than with a standard transmission and low number axle.
This is not important for mild cruising, but definitely affects top speed especially with long driveshafts.
If you have a long wheelbase truck (= long driveshaft), or a substantial pinion angle misalignment (transmission swap, chassis lowering, etc.) this should be examined before making the choice.

This issue has been raised a few times here when the discussion is around long drive shafts though I don't recall seeing it in an OD versus rear ratio discussion.

In general the modifications talked about here reduce drive shaft RPM at any give road speed and therefore most of us are not in any real danger of turning the drive shaft too fast.

When someone suggests replacing a two piece shaft with a long single drive shaft I think it becomes a very important concern.

Standard practice for OEM drive shafts has long been to design them with a larger margin of safety. I don't recall the number, but I can see the engineering text with those numbers from where I am sitting...

A related discussion that I have not seen here is the parasitic power consumption of an OD versus a direct drive and the impact that has on performance and economy. My own feeling is that most of us would never be able to measure the difference.

So what statement did I make that you think is not correct?

I think you need to reread what I said.

unless your drive shaft is spinning above 5000 RPM, I would not worry about balance as much as you are. A standard drive shaft balance would be fine. I doubt if your running the shaft above 5000 RPM. with a 4.1 gear and 26" tires that would be close to 95mph. with a 3.38 gear and 26" tires that would be close to 114. Since these trucks are a brick - you will hit the aerodynamic limit before the drive shaft RPM limit unless the shaft is bent, out of phase, or improperly balanced.

If you have a 2-piece drive shaft, phasing is usually more of a concern than balance. As Steve mentioned, if your replacing a 2-piece with a long shaft, then you also need to worry about balance.

parasitic power consumption of an OD versus a direct drive

I agree - it's probably small, and also difficult to make a straight A:B comparo since to be valid the actual gear tooth counts, bearing sizes, U joint angles, etc. must match between the transmissions, and that's not going to happen.

Oldsub, I wasn't sure about your second line when talking about general modifications and slower driveline speeds. I just wanted to be sure anyone that reads this knows the difference between the two options being discussed and what each one does to the driveline speeds.


I will delete the post and shut up now!

Joe

Think of it this way. An overdrive trans does not change the speed the drive shaft at any given speed. But an axle gearing change does.

Most of the changes to rear gears lower the drive shaft speed if they change it at all and that is what I was saying.

I once bought a '50 Chevy 1-ton that had a 3.42 rear under it. The guy I bought it from said the old 216 would not provide enough torque to move the truck with any load at all. I never tested what he said but I believe it.

There is a reason for those original deep gears and I think some times people have unrealistic expectations about what they can accomplish with an overdrive or gear change.

So true, when I hear guys talking about gears and overdrive to gain fuel mileage, all I can think of is how litle torque the old motors make, and how are they ever going to get mileage working them that hard.

My truck didn't change much in fuel mileage going from 3.42 to 3.08, at highway speeds (65-70), its the wind that kills it, not the rpm.

Joe

its the wind that kills it, not the rpm

Yup - below about 35, wind resistance is very low compared to driveline loss, rolling friction, weight, etc.
Above 50, especially with a big boxy front, the power lost to the air goes up very quickly. Turning the engine faster (within limits) puts it closer to its peak power rating, and also adds mechanical advantage.

Just put a Gearvendors on my 3/4 ton and haven't checked the mileage yet. I still seems to go thru a tank of gas just as fast as before which is pretty darn fast. I'd need to lose about 700lbs, chop the roof, install a nosecone on the front in order to see any mpg gain. I'll post pictures.