Gear ratio!!!!

It's all about finding a happy medium in keeping your rpm in a proper operating range and having a rear axle ratio that allows you to run @ the proper rpm and yet have startability AND the ability to pull hills without getting on the bottom side of the transmission the majority of the time.

 

several years ago when fuel got to be a big deal the large companies like JB and Schneider and some others started going with a 10 speed direct and 2.79 rears and low profile rubber to keep thier trucks in the 60 -62 mph range and that setup worked fairly well for them,biggest problem they had was the men behind the wheel drove the same speed in parking lots and truck stops as they did out on the highway.

 

thanks ralph

so on flat ground, overdrive is good, but uphill it is bad

is that pretty much the issue?

 

Parasitic loss of power is the problem. When you have a "Direct drive" gear, you have essentially made a straight shaft through a transmission, which means, there is a minimal loss in power turning other gears and what have you. When you're in an underdrive or an overdrive gear you're spending power turning other gears and losing power to get the load down the road. That loss in power results in reduced fuel mileage.

 

They still do...

 

I think 2.90s are too tall. I would have went with a 3.33 if the 3.55s were too low. And a 550 Cat with 4.11s...thats a heavyhaul setup. You could pull 300000lbs all day with those numbers.

 

We did alot of oil field work, the truck did a great job working around here in and out of the muddy and hill well sites...

They have moved into a more heavy haul company and do alot of rig moves and what not now a days, when I worked there we just had a couple step decks and hauled legal loads, he only had 2 trucks at the time...

 

Until you hit the mountains and had to start on a hill. Best be getting north of 5:1 rears if you want to run heavy in the mountains, unless you have a set of sticks and have a good compound deep-under. The truck I mentioned earlier having the 18-speed with no true direct-drive has 2-speed rears (3.90/ 5.27s) and is only matched in that fleet by an '90 Kenworth with 6x4 set of sticks pushing 4.56s.

The deepest geared truck I ever drove had a 13-direct with 6.80 rear gears.

 

HP and gearing are so important. 2.90s would be a gear a bullwagon would want. 600 HP and 2.90 gears, and you have a truck that will hit 130mph. A carhauler...? I dont know anything about carhauling, other than they have to work harder than flatbedders. I wouldnt care about being the fastest truck on the road, I would just want the truck to perform well. I wouldnt figure a carhauler to be as heavy as I am loaded, but the aerodynamics to be atrocious. Because of the aerodynamics, I wouldnt go any higher than a 3.36:1 ratio, even with 550HP. With a 13 overdrive (whats the point of an 18 speed? You dont needs the gears on the bottom to move 80k), at 70-75 mph, you would be running a comfortable crusing rpm on flat ground, and still have enough grunt to muscle up hills fully loaded.

 

Direct drive and 6.80 gears....military truck? 175 HP? at 50mph, you would be like a dog at the end of your leash. A 15 speed with deep reduction would get that 550 and 411s through the mountains even without the 2 speed.