Originally Posted by
Loren
Oh, for sure. The ENGINE is most efficient somewhere in the range of its torque peak. It's making the most rotational force with the amount of fuel that it's burning. That doesn't account for what gear your in, how fast you're driving, road conditions, aerodynamics, etc. There are absolutely other variables to be considered.
Note that I was talking about accelerating.
If you're CRUISING, you're going to get the best economy in the highest gear you can be in, regardless of RPM (assuming RPM isn't so low that the engine is lugging). This gives you the best gearing advantage and multiplies the efficiency of the engine.
But, hypermilers rarely cruise. Your best "record setting" MPG isn't going to come from just cruising down the road at a constant speed. It's going to come from accelerating up to a target speed, and coasting down to a lower target speed, working with the terrain where possible, and continuing to do that.
If you just want 45 mpg, then cruise and keep your speed under 65. If you want to see 60 or 70 mpg... it won't happen that way.
There are some hypermiling techniques that seem counterintuitive at first, but they work.
So, to use your example, if the target is 50 mph...
You put an experienced hypermiler in Car A, and tell him to get somewhere that's 50 miles away in one hour. He's not going to drive 50 mph. He's going to accelerate up to 60-62, and then neutral coast down to 42-45, and then accelerate back up to 60. Lather, rinse, repeat. And if he does it right (working with the terrain, effectively managing traffic and stops), he's going to get BETTER fuel economy than the driver in Car B who set the cruise control at 50.
He'll be accelerating in the torque peak range of the engine (even in 5th at those speeds), thus getting the most out of the fuel. And he'll be using gravity and momentum to carry the car between "pulses" of acceleration.
Car B will net some pretty good MPG in a Mirage at 50 mph constant. But, Car A will do better.
Find yourself a nice back road and try it!