finding right cruising speed for economical gas mileage

For most motorhomes, no-wind conditions, the "best mpg" speed is going to be too slow for safe highway travel in North America. It will be in the 30-40 mph range for most class Cs. EPA throws around numbers like 55 mph, but that is for vehicles with much lower aerodynamic drag, and parasitic loads (primarily A/C) that are much closer to what it takes to actually move the vehicle.

In a tailwind, best mpg speed might be even slower, so that the tailwind does more of the work for a longer period of time. For the same reason, it is usually better to speed up into a headwind. These ideas seem counter intuitive, but are well established in aircraft cruise practice for the past 80 years.

The engine being more efficient at "peak torque RPM" applies to full-throttle conditions. At less than full throttle, peak efficiency for the lower torque output level is at a reduced RPM. For most IC engines it is the minimum RPM at which the engine can produce the power needed.

The engine manufacturer has likely run the suite of specific fuel consumption tests over the full range of power outputs at selected RPMs, and has mapped them into the engine and transmission control programs. This is why so many modern automatic transmissions now have five, six or more gears (or CVT), to let the engine find that optimal MPG condition at expected city and highway cruise speeds, over a range of loads, vehicle and parasitic.

So practically, for highway driving in this country with a RV, best MPG will be at the slowest speed you are willing to travel, given your time constraints, patience, and tolerance for holding up traffic.

KD4UPL wrote:
Around here the minimum speed on interstates is 45 but the limit is 70. Are you saying everybody is happy if you poke along at 50 in a 70? I'm pretty sure they're not.

OP, slower is the answer to a point. You need to be at the engine's peak torque RPM while your transmission is in it's top gear. That should theoretically be your best MPG.

Frankly, I don't care if they're happy or not. There's a passing lane, so they can go around. On a two lane road, I keep up with traffic.

Every motor home will have it's own best fuel economy via MPH sweet spot pending aerodynamics, weight etc, but it won't vary by much. That is why people make blanket statements like 55 MPH. It's going to be right around 55 MPH.

With a Scanguage-II the TPS (Throttle Position Sensor) display number indicates how far down you are pressing the gas peddle. Between that number and real-time fuel economy, you can get a good idea of the optimal speed for best fuel economy.

gbopp wrote:
Welcome to the forum. Slower is usually more economical.
In our Class A gas Southwind I usually travel about 58-62 mph. That gives me the best mileage and isn't unreasonably slow.

I don't know about a diesel but, I would think the same is true.

Mine is a gasser, but I have found this to be accurate. Faster uses more fuel. As a matter of fact, 16% faster (72mph) gives 25% reduction in mileage (8mpg down to 6mpg). Slower (55mph) causes it to downshift more often on small grades, causing it to use more fuel. I usually set it on 62.

Time for a physics lesson. The force of friction is proportional to the square of the speed. It means doubling speed uses four times as much fuel. Going ten percent faster uses 20% more fuel per mile. You can see it on a graph here: https://www.fueleconomy.gov/feg/driveHabits.jsp
This doesn't depend on the vehicle size or shape because it is comparing a vehicle to itself at a different speed.

Safety also increases non-linearly with decreasing speed because the energy to be dissipated in a crash is proportional to the speed squared - down to the speed of traffic. There is something magic about all the vehicles going the same speed: no passing means MUCH safer.

I mostly drive the MH on two lane roads. I like to go just a little below the speed limit but will speed up to go with the flow, then get off the road when opportunity presents and let all those folks in a hurry go on by.

Seems that most everybody moans about gas prices but they jam from stop light to stop light as if they don't care in their huge pickups and Suburbans and SUV's. I try to drive a MH at around 55 to 60 mph and enjoy the ride. If local yokels want to ride my rear bumper, I'll pull over and let them pass when it's safe. When I am on an unfamiliar twisting down grade, I obey the speed limit signs and ignore the yokels. They are not driving a top-heavy motorhome.

KD4UPL wrote:
Around here the minimum speed on interstates is 45 but the limit is 70. Are you saying everybody is happy if you poke along at 50 in a 70? I'm pretty sure they're not.

OP, slower is the answer to a point. You need to be at the engine's peak torque RPM while your transmission is in it's top gear. That should theoretically be your best MPG.

This is basically what I do. My transmission shifts into 4 gear at 45 mpg on a flat road, if I travel at 50 mpg it will not downshift on slight hills. Also by not using cruise control and accelerating slightly I can keep it in 4th gear without downshifting when approaching a hill. I also still use a vacuum gauge to drive(I'm old) and know when it will down shift based on vacuum readings so I take my foot off the pedal and reduce speed before it downshifts keeping it in 4th gear. If you have a 5 or 6 speed your speed will be higher.

Around here the minimum speed on interstates is 45 but the limit is 70. Are you saying everybody is happy if you poke along at 50 in a 70? I'm pretty sure they're not.

OP, slower is the answer to a point. You need to be at the engine's peak torque RPM while your transmission is in it's top gear. That should theoretically be your best MPG.

The only way a MH will get good gas mileage is if you drive it over a cliff, but the sudden stop at the bottom is uncomfortable. I've found that driving about 5 MPH over the minimum speed limit on expressways results in less downshifting out of overdrive and allows everybody to pass me. Then everybody is happy. Based on the number of hours I plan to drive, I know about where I'll be at the end of the day so that I can call for a reservation if its a popular area. It helps if you don't have a time limit on your trip, as in "retired".

Your mileage will ALWAYS be best when you travel slower and more steady. slower speeds mean less wind resistance which equals better economy and steady speeds means using less fuel to achieve the speed you've chosen to travel. Hit the interstate at a constant 35 MPH and never let up and you'll do best regardless of what you're driving. OF course it's hard to imagine you'd enjoy traveling that way even if you weren't arrested for obstructing traffic. My advice is to travel at a speed where you enjoy the trip the most whether that's 80 MPH and 6 MPG or 50 MPH and 12 MPG. The trip to where you're going SHOULD be as enjoyable as being where you're going.
Good luck / Skip