Swapping Detroit for CAT or Cummins

There's a lot of great information already on this site about gaining mpg in a true and honest way. Also great info from very reputable members on how to increase hp on about any engine make and model.

But one thing that is seldom discussed and I believe is in large part the reason for so much brand bashing, it gearing and trans spec. If you are holding your coffee cup in a company rig, you have little to no say in spec. But if you are running your very own investment, then some time in study regarding your truck's spec can return your investment time and again.

You'll never realize the true power or mpg of the engine you now have if you are always compensating for some component that is mismatched for your average load and driving conditions. Running in OD or any gear other than direct when at your nominal speed, will cost you. When in direct, your r&p ratio need to put your engine right on it's toque peak.
If you're grossing over 100k and running in the hills, then buying and running a truck that was spec'd for 80k on the open flats aint gunna work. You'll be mad at every aspect of that truck.

 

The problem with specing a truck to run in direct is in order to cruise at an acceptable speed you have to have a ridiculously fast gear ratio, which them means you can't pull a hill. Besides that, I'm not even sure you can get a 2.70 or faster gear in a 44k or 48k rated rear end, I've never tried to spec big rear ends in anything faster than a 3.55

 

Great point. But it does depend on your specific needs. There are now used otr trucks showing up in truck paper advertised with 2.70:1 rears. Likely not heavy 48k, but 40k. With engine tech changes, component changes will become available. I have not looked for ratio availability in heavy axles. OD in the transmission is not all that bad, but when one is really trying to go for that last decimal of efficiency, direct is on average, the most efficient. It's good to have overdrive, and maybe even a 1:0.7 when needed or can be used in a monetarily beneficial way. In my opinion anyway.

 

2.70s with ten speed directs are common with typical big fleets. Melton and maverick used to have a bunch of them, maybe they still do. And they were pigs in the hills. As big entry level carriers i understand their need to squeeze every last cent out of their fuel, but for those of us running our own equipment I'm happy to trade a very minimal decrease in fuel economy to not have to throw the 4 ways on at every little grade.

Id make sure you looked into the rears on a truck before buying, most of the used trucks on the market are from large carriers, and very few of them order 40k rears. 38k rears is the most common. If all you do is dry van and reefer freight that's perfectly fine, but if you ever want to branch out into other markets 38k is too small. Many states allow you to permit 92k on 5 axles, that's going to be at the very top, maybe even over, what 38k tandems are designed to hold up to.

Now if you got a long term contract pulling light freight by all means, go with little rears and a direct speed transmission. Just be aware, speccing a truck for a specific gig is not always the right move. Ive been down that road and the contract disappeared in under a year and then I was taking a truck specced for lightweight loads and running the open market for it. All for a penny or two a mile in fuel cost.

 

Why can't you pull a hill with something like a 2.70 gear ratio? The only difference is you start at the bottom of the hill in a lower gear but at the same speed. You're putting just as much or more power to the ground so you're going to down shift the same number of times to get up the hill and you'll be going the same speed at the top regardless of the ratio.

 

It takes more energy to turn a small gear than it does a big gear. Basic physics. Ever ridden a 10 speed bicycle?

 

It's a loosing battle I have fought many times, just because the numbers are equal they think it's the same. With no thought of where the rubber meets the road.

 

It takes more torque to turn the smaller gear but less rpm so the power required stays the same. Parasitic loss aside, you can't change the power to the ground by making the transmission turn faster and the rear gears turn slower. Rather than getting your information from the driver at the next fuel pump or some salesman who doesn't understand, you should try picking up a book once in a while

 

It's always a losing battle when you're wrong and don't understand the basics of what you're talking about

 

And your engineering degree is from where?