The Stovebolt Site Forums The Shop Area Driveline Balancing trans & rear end gearing, what's best?

I've read through a bunch of threads about increasing highway speed and fuel milage and the importance of trans and rear end gearing. I have a two part question.

1. My first question is does it make a difference where the gearing improvements are made? By that I mean do you get better results from a .50 overdrive trans and 3.90 gears vs. a regular 3 or 4 speed with 3.08s? Is a balance of say at .73 OD trans and 3.55 gears better?

2. I think I read that you can go with some higher rear end gears without sacrificing much at take off. Is this true? Is there a tipping point where a goal of easy crusing keeps you from getting the thing rolling in the first place?

Question 2 ties back into question 1. My overll goal for my 58 Suburban is to build a pretty strong truck that will cruise nicely on the highway but remain capable of towing and carrying a load when necessary.

I'm thinking a fairly tall (~.5) OD 5/6 speed trans (T5 or T56) mated to a set of 3.55 gears. That should give me some descent launching/towing power while also providing some descent cruising?

A lot depends on what engine you are useing & diameter of the rear tires.

Here's a gear calculator to help you figure speeds and RPM.
http://www.onlineconversion.com/bigger_tires.htm

Important to have for towing are gears low enough you can get the load moving.
As well as your direct drive gear, axle gears, and tire size such that you get a comfortable speed while towing with the appropriate engine RPM (higher than cruising rpm when empty but not wound out to tight)
And if you don't have enough to think about already consider a transmission with a close 4th and 5th gear such that they are both useable. Not much more frustrating when towing than to be stuck in a low gear because the next higher is so far away you can't get to it.

HP also plays a factor, the right gears for a 235 with it's somewhat limited HP will be slow for an engine with significantly more HP.

Grigg

One other thing to figure in is the actual cost of parts. Using an sm420 that can be rebuilt for $200 and will last a million miles is one choice. An overdrive transmission may cost you $2000 or more to purchase, install and setup.

How long will it take for you to burn through $1800 in gas?

Certainly less than it would have 10 years ago, but it's something to keep in mind.

Jeff

Grigg:

Excellent advice. That is one reason I thought a 6 speed might be the better choice. You've 6th for empty highway cruising and 4th or 5th for towing use. I'll have to look at the relationship between the higher gears.

Rustednuts: I hear you. Economics are always a factor. I'm facing the same issues with engine choice. I may end up with "good enough" in the short term, while I save my nickles for "best choice" down the road.

Anyone have any thoughts on the rear end gears vs. transmission gears issue? Maybe if the rear is in good shape and doesn't have some crazy gear set, I'll leave it and fine tune overall road speed/RPM using the trans and tire size.

Larry

How fast do you want to drive?

Having a 59 gmc, with an OD 5 speed, I can offer my thoughts

3.73:1 rear gears with a 0.73 OD and 28 inch tires was a pretty good combination on my 59. I recently changed to 31 inch tires with the 3.73 gears, and I feel like I have lost some of the grunt the truck had. I will likely drop the gears down to 4.11:1 in the near future. I have a 9 inch rear in my truck, about 325 hp from a 447 inch v8 and a 5 speed NV4500.

Currently my truck will do 70 MPH at 2050 rpm. Changing to 4.11 gears will add 220 rpm or so to put me just under 2300rpm at 70mph. The current gearing will not pull the OD on an incline at 60mph.

I would reccomend gears in the 3.73-4.11 range and tires in the 28-31 inch diameter range with a 27 to 30%od (0.73:1 to 0.70:1). I would stay on the lower side of the gear range with a 6cylinder motor.

My 55.1 3100 has a 3.90 axle with 28" ties and a BW overdrive. That puts my 60 mph cruise rpm at about 2000. This is just below the torque peak for my 235. The setup is pleasant to drive.

regards,
Leon

Swingdaddy/Leon;

Great advice. Thanks. I'll need to find the torque peak for my 250 and taylor the overall gearing from there. 2000-2300 RPM at 60-70 sounds like it would be right in the sweet spot.