Best fuel economy speed

Airstreamer67 wrote:
Mr. Mark's data follows the physics involved: the faster you go, the more fuel you use. After about 45 mph, it's the wind resistance and related aerodynamics of the vehicle that determine the fuel economy differentials.

Right. But this is still very interesting. At any reasonable speed highway speed, mileage decreases as a function of speed. Lots of data are available for cars (e.g. here: https://www.fueleconomy.gov/feg/driveHabits.jsp). Since a major part of the equation is wind resistance (drag) (Drag = 1/2 * density * area * drag coefficient * speed-squared), one would think the mileage-versus-speed effect would be even greater for big boxy vehicles like a motorhome. Based on the posts above (Mr.Mark shows about .5 mpg decrease for each 5 mph above 55; tyoungs reports Cat and Cummins as .1 mpg loss for every mph, both of which agree), though, the speed penalty works out to about 1.4% decrease per mph at the low end to 1.7% per mph at the higher end. If my math is right, that's only about double the speed penalty reported for cars (which is 0.6% to 1% over the same range). I would have thought it would be more than that, but the data indicate otherwise. Very interesting.

Skip

Mr. Mark's data follows the physics involved: the faster you go, the more fuel you use. After about 45 mph, it's the wind resistance and related aerodynamics of the vehicle that determine the fuel economy differentials.

Wolfe is right on - generally your most economical "travel speed" is the lowest speed where you can easily maintain 6th gear (I'm assuming a DP with a 6 spd Allison). In my current coach that is about 60-62 MPH. Rule of thumb from Cat and Cummins fuel economy information is that you lose about .1 MPG for every 1 MPH over 55. So the difference between 55 and 65 is about 1 MPG - this is mainly based on aero effects because aero drag goes up with the square of the velocity.

Garbage trucks get 1-2 mpg. It's all about driving long and steady at a constant speed in the highest gear possible.

MM.

Speed and low gears kill mpg, so the best is the lowest speed the vehicle will go on highest gear without downshifting when somebody farts in reverse.
On hills that takes some technique. When you see longer grade, often speeding up at the bottom to avoid downshifting 1/2 the grade will save money.
Than on some configurations not.

For me it mainly depends on headwinds. If I am bucking a stiff wind I probably won't go over 55-60. If there is not a strong headwind I like to cruise at 65-70 because like a another poster stated that I can cruise over the hills in 6th gear in economy mode and usually never has to shift down into 5th. Now this is a DP.;)

I feel most comfortable at 60 to 65 MPH.
I have no idea what the "fuel economy sweet spot" is.
I'm driving a small house on wheels, towing a half ton pickup.
IMO, "fuel economy" and "motor home" are mutually exclusive terms.
They simply do not belong in the same sentence.

I think most vehicles are designed to have the "Sweet Spot" mpg wise between 50 and 70 MPH, Motor homes tend to be in the lower ragne. Mine does best at 55-60 both when towing and when bobtail. 38' Class A. bobtail I've hit 11MPG just driving under normal conditions, Usually get 8-9 Towing.

Under optimal conditions (100mph dead astern tail wind) 13.8 towing. But alas that does not often happen and I really do not like those conditions.

Like others we don't have the time available to us to drive for peak mpg's. We just returned home from about 3000 miles and I can't imagine how much more windshield time it would be slowing down to 55. I prefer 65-70 and at that we are about 7 mpg.

Over a typical 5000 mile trip I seem to avg 7.5 and nothing I do seems to affect that avg more than +/- 0.2 mpg. Sometimes I drive 55 sometimes 65 depending on the speed limit and/or conditions and it's always the same.

Granted some legs I might see 8.5 and some 6.5 but the avg is still 7.5 and has been since the engine was born 95,000 miles ago.

Info from my Silverleaf so it doesn't account for genny or aqua hot running.

Thinking about how to improve the economy makes for good entertainment on lonely stretches of road but that's about it.