finding right cruising speed for economical gas mileage
We just purchased a used 2008 Winnebago View with 5400 miles. Does anyone know a good speed to travel at to get the most out of the mileage? It is a V-6 diesel motor.
Forum Discussion
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.
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.
