Fuel economy at 65 mph vs 60 mph

The reason objects have a terminal velocity when falling through the air is because they reach a speed where drag increases to a level where they can no longer accelerate. That's why parachutes work. Going faster requires more power and more fuel.

The 4100 Lb Bugatti Veyron requires 1000 hp to reach its top speed of over 250 mph and burns 1.4 gal per minute in order to stay there.

SabreCanuck,

Rolling resistance is a constant value at any speed a vehicle travels but air resistance squares itself in value. That's why aerodynamics (resistance to air flow) make so much difference in fuel used/efficiency. The unloaded truck bed cab back is a vertical wall which creates a suction just like the flat rear of an RV which pulls backward on the unit.

The wood piled up on the truck diminishes/reduces the suction effect by filling the void and having rounded surfaces for better aerodynamics. Another factor is which direction traveled when going back a forth to the lake due to the prevailing winds. Due to the Earth spinning and the ocean's currents influence. The prevailing winds in North America blow from west to east generally. Other factors can have some influences also. Gas engines run at high RPMs than diesels to achieve required torque needed to move the load to the speed desired. Remember that diesel fuel has 11% to 12% more BTUs per measure than gasoline and it's the heat expansion explosion force produced from the compressed air/fuel mixture in the cylinder that drives the piston downward and turns the crankshaft which turns the drive wheels. ANY resistance beyond the drive line lowers the fuel's efficiency values.

filrupmark wrote:
We got 11.5 yesterday running 63 mph. "Perfect Conditions "

Coming home on same trip I ran 70 mph with some headwind and our mileage dropped to 9.4 mpg. We are going back to the old 63mph we have always run . Glad you brought the subject up. We did our own test and now we know. Thanks

SabreCanuck wrote:

laknox wrote:

johndeerefarmer wrote:
What fuel economy increase have you observed from pulling your fiver at 60 mph vs 65 mph?

My personal experience is about 1.5 mpg increase by dropping to 60 from 65. See the rig in my sig. I've simply not towed far enough at any given time to see if I have a "sweet spot" somewhere in between, as many people say. A lot of D'max owners seem to do well at about 62 mph, for some reason. That being said, the best mileage I've ever seen in my truck, not towing, was on a trip to CO 3 years ago, with an empty 14 cu ft chest freezer up against the back of my cab and driving 75 mph the whole way. Got right at 23 mpg and it =still= has me scratching my head on how it changed the aero profile. :-)

Lyle

Funny you should say that... My dad used to haul firewood out to the lake about a 2 hour drive. Loaded and heavy on the way out and empty on the way home. He ALWAYS got better mileage loaded than empty. Back of the truck was piled to the roof with firewood.

Aerodynamics play a much larger role in fuel efficiency than weight.

Yep. Were I driving long distances not towing, I'd certainly look into getting some sort of cap/shell/cover for the bed.

Lyle

Stefonius wrote:

ACZL wrote:
You're pushing the same amount of air out of the way at 60 mph as you are at 70 mph.

Not true from a physics standpoint. When you double your speed, you encounter four times the wind resistance, and you need eight times the horsepower to maintain that speed.

Could have sworn it was speed squared and drag cubed. It =was= 40 years ago when I had physics... :-/

Lyle

ACZL wrote:
You're pushing the same amount of air out of the way at 60 mph as you are at 70 mph. Technically, like others have said, going slower will net a bit better mpg's, but overall with today's engines (specifically diesels), they are designed to be most efficient around 65-70 mph.

Drag cubes as speed squares. Can't get out of that exponential difference.

Lyle

laknox wrote:

johndeerefarmer wrote:
What fuel economy increase have you observed from pulling your fiver at 60 mph vs 65 mph?

My personal experience is about 1.5 mpg increase by dropping to 60 from 65. See the rig in my sig. I've simply not towed far enough at any given time to see if I have a "sweet spot" somewhere in between, as many people say. A lot of D'max owners seem to do well at about 62 mph, for some reason. That being said, the best mileage I've ever seen in my truck, not towing, was on a trip to CO 3 years ago, with an empty 14 cu ft chest freezer up against the back of my cab and driving 75 mph the whole way. Got right at 23 mpg and it =still= has me scratching my head on how it changed the aero profile. :-)

Lyle

Funny you should say that... My dad used to haul firewood out to the lake about a 2 hour drive. Loaded and heavy on the way out and empty on the way home. He ALWAYS got better mileage loaded than empty. Back of the truck was piled to the roof with firewood.

Aerodynamics play a much larger role in fuel efficiency than weight.

mtofell1 wrote:
(snipped) Some vehicles may seem to be at a better RPM range and "feel" better but it's still taking more fuel to move that load the faster you go.

Not always. Some engines flat will be more economical at a higher speed than at a lower speed. Just the nature of the beast. My dad showed me how my old gasser, with GearVendor OD, would get a lot better mileage by running it at a higher RPM than I would by running 1 higher gear and lower RPM. Engine just ran "freer" at the higher RPM where I was somewhat lugging it, causing it to run fatter.

Lyle

johndeerefarmer wrote:
What fuel economy increase have you observed from pulling your fiver at 60 mph vs 65 mph?

My personal experience is about 1.5 mpg increase by dropping to 60 from 65. See the rig in my sig. I've simply not towed far enough at any given time to see if I have a "sweet spot" somewhere in between, as many people say. A lot of D'max owners seem to do well at about 62 mph, for some reason. That being said, the best mileage I've ever seen in my truck, not towing, was on a trip to CO 3 years ago, with an empty 14 cu ft chest freezer up against the back of my cab and driving 75 mph the whole way. Got right at 23 mpg and it =still= has me scratching my head on how it changed the aero profile. :-)

Lyle

I do it when the prevailing traffic is moving at a speed in which I am presenting a hazard by moving too slowly.