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Old 09-17-2017, 10:27 AM   #1
Bus Nut
Join Date: Jan 2016
Posts: 253
Best fuel economy...

I have been reading about this topic fromn a variety of sources and wanted to check my understanding. A bus gets the best gas mileage when the transmission and differential are geared to the engines most efficient rpm range; increasing the torque of an engine raises the most efficient rpm range; increasing horsepower increases top speed. (All of these statements should have the phrase "within reason" attached to them. Am i right?

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Old 09-17-2017, 12:49 PM   #2
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I drive a big truck for a living and while there's some truth to all the above factors, it's a lot more complicated than that.

It's true that keeping an engine within its optimum power band will help with economy, and adjusting gear ratios will help with that, it can only do so much. You have to factor in things like mechanical drag, brake drag (honestly, you shouldn't have any), tire rolling resistance, and perhaps the biggest factor, wind drag.

For our purposes, we can reduce mechanical drag by making sure we have the right oil viscosity, transmission fluid, grease in the driveshaft, the front & rear wheel bearing lubricant, and everything in order with the rear axle. Brake drag shouldn't even be a factor (I will assume for this discussion that the brakes are not dragging). Tire rolling resistance varies by tire brands and models, tire life and age, and the one thing we have the most control over, tire inflation (tire pressure).

Wind resistance deserves a paragraph of its own. Much has been said and written about wind resistance. Consider we are driving large brick-like objects that do not have perfectly designed aerodynamic bodies, with design priorities given to safety and functionality. Add to that all the exterior stuff some of us add to them - awnings, roof racks, decks, solar panels, and whatever else I'm forgetting. Turbulence underneath and behind are factors too (perhaps minimal, for our purposes here). But the one thing that matters more than any of the above mentioned factors is speed. The faster one goes, the more air that must be pushed out of the way in a given time, and that pushing must be done more quickly than at a slower pace. At 45 MPH, in 1 minute, you will need to push 3/4 of a mile of "air" out of your way, at a somewhat leisurely pace. At 60 MPH, that becomes 1 mile of air to move out of the way, at a moderate pace. And at 75 MPH, you'll move 1 1/4 mile of air out of the way, at a really quick pace. You'll find that fuel consumption starts going up above about 60 MPH, with gearing and RPM ratios being otherwise comparable.

Case in point, I drove a Freightliner Century over the road a number of years ago. At an average speed of 70, it averaged about 6 MPG. At that time, Illinois, Indiana, and Ohio had split speed limits and I'd set the cruise at 62 in those states. I could reset the average fuel economy tracker in the truck, and it reported getting 8+ MPG when set at 62. It is for this reason that many fleets set their trucks to about 65 (some claim safety, which is debatable, but fuel economy is measurable and hard to dispute).
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Old 09-17-2017, 07:17 PM   #3
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Mechanical resistance rises in proportion to your speed.

Wind resistance, on the other hand, goes up by the square of the speed. In simple terms, to double your speed you need to quadruple your power.
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Old 09-17-2017, 07:53 PM   #4
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Originally Posted by roach711 View Post
Wind resistance, on the other hand, goes up by the square of the speed. In simple terms, to double your speed you need to quadruple your power.

Not when you draft an 18 wheeler. Easiest free MPG's.
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