Need opinions on trailer towing with 3.08 vs. 3.42 gears.

[Larry E.]

Re: Need opinions on trailer towing with 3.08 vs. 3.42 gears.

Dave: Excellant in explaining the above. The only thing I do no understand completely is that with my setup the transmission will always be in a lower gear then with the 3.73 setup. eg.: At a given speed I might be in 3 gear (transmission) and the other setup would be in 4 gear. And to confuse matters with these newer 6 speeds the last two gears are "overdrive" gears. Would not my setup stay in 4 gear when the setup is trying to get in overdrive? Also what I don't understand is when the top driving speed is achieved (Say 55 MPH) little torque is required. Is there that much difference in wear/tear for one gear?? Larry

[Larry E.]

Re: Need opinions on trailer towing with 3.08 vs. 3.42 gears.

[QUOTE=Dave Perry (19643);708073]We need to remember that what we call "reduction" is really torque multiplication. Torque is the work required at the rear wheels. so what we want to do is look at the various combinations that torque produced by the engine is multiplied to the level required at the rear wheels.

I'll try it one more way:

We need 1,000 ft lbs of torque at the rear wheels to pull the hill the rig is on.

Dave: Thanks again for the response. I understand the physics of this a little better; but not completely. Not to beleaguer this subject but where would
the trade off point be? All I'm hearing is the lower gear is better. Why not put a 4:56 in?? Let's not even bring the engine in on this; for this topic we will assume
the engine can handle it. By running a lower gear I assume the transmission speed would have to be increased for the same result. Does transmission speed
effect wear/tear on the transmission??? Larry

[Duke W.]

Re: Need opinions on trailer towing with 3.08 vs. 3.42 gears.

You guys are over-analyzing this. Compute the RPM in the top two or three gears at your planned tow speed. Look up the tire size at tirerack.com to get the revs/mile. With a 1:1 trans ratio, revs @ 60 is simply tire revs/mile times axle ratio. For other gear ratios you linearly proportion... same for other speeds. Since you won't have to deal with hills you might be able to pull 6th on the flat.

Figure the engine has 80 percent of peak torque at 1500, 90 percent at 2000 and 95 percent at 2500.

Don't these 6-speed autos allow you to shift manually or at least lock it in a gear? If 6th won't pull on the flat lock it in 5th, and whatever lower gear is required for hills. You don't want the transmission to "hunt". That will wear clutches and generate heat. If it's locked in a gear there is no clutch wear and associated heat generation.

Sure, the transmission is going to generate more heat when towing because you're transfering a lot more power through the planetary gear sets than with no trailer, but I assume the tow package includes a decent transmission cooler.

The most efficient engilne operating point is at to somewhat below the torque peak at near WOT. For climbing, use a gear that will maintain speed at the torque peak +/- 500 revs. Don't push the throttle to the floor. That will put the engine into open loop and richen the A/F ratio, but it really won't give you that much more power and fuel consumption could go up 10 to 20 percent. Keep it just off the floor. At medium revs if the throttle is 3/4 open manifold vacuum will be near zero and more throttle won't allow materially more air flow.

A few months ago I started a post that included an XLS "gear chart". That tool would be very useful in this situation. You input the data, it does all the math and computes revs @ 60. You will find it very quickly by searching for posts started by me. There aren't that many.

You might be able to find a GM published torque/power curve on the Web, too.

My bet is that the 3.08 axle will be just fine for your tow application, but I'd opt for the 3.42 if I had to regularly tow over 7000+ foot Donner Pass because at that altitude the engine can only make about 65 percent of torque/power as it does at any RPM at sea level. As you approach the Eisenhower Tunnel at the top of 10,000+ Loveland pass you're down to barely over 50 percent.

One of the toughest tows in the country may be southbound I-5 over Tejon Pass. You're faced with several miles of six percent grade climbing from the San Joaquin valley floor at about 500 feet ASL to the top of the pass something over 4100 feet. Loaded big rigs are lucky to maintain 25 MPH.

I have a friend who was enamored with the original Hummer - the one with the 150 or so HP naturally aspirated GM V-8 diesel. He thought it would be good for towing. I told him he was insane. He actually towed a GTO in a trailer over Loveland Pass and was a mobile road block to all the big rigs. After that he got rid of that PoS and got a real tow vehicle, but it was a Ford.

Towing requires horsepower, which you obtain from the product of torque and RPM. The required power is total drag times speed, and the power requirement increases with the cube of speed. My guess is that a typical pickup with 7000 pound loaded trailer load requires something on the order of 70-90 HP at 60 MPH on the flat at near sea level. For climbing you want at least 200, preferrably 250 near the torque peak to tow up a mountain grade, but that can be cut as much as in half at extreme altitude.

Duke

[Dick W.]

Re: Need opinions on trailer towing with 3.08 vs. 3.42 gears.

I am seriously considering a new 1/2 ton crew cab with a 5.3 and the tall gears. For the amount of towing that I, and most here do, the original set up he is considering is adequate. With the smaller engine you learn that you are not going to tow at 80 mph, set 0-60 records, etc. Learn to take it easy, no jack rabbit starts, feather the throttle and drop a gear when pulling hard.

You can tow with anything, I saw a Toyota Land Cruiser tow a LS/6 from St Louis to Reno and back. You just don't tow in style that you would with a bigger rig.

It is going to be hard for me to go from about 400+ hp 500+ ft lb of torque diesel, but I have learned to leave a little earlier and not get in a hurry