Less RPM, More MPG: Fuel Efficiency for Modern Eco Friendly Cars
Mpg, rpm their relationship and how they can be improved.
Cars Then and Now
The second car I remember my father driving was a Chrysler, like the one pictured above. I remember it had a strange method of shifting gears, way superior to his previous car (a stick-shift DeSoto). But it wasn’t quite automatic transmission. Instead, an indicator of sorts (I forget if it was a beeping sound or a blinking light) would inform my father that it was time to shift gears, at which time he would press a button to achieve that effect. I’m not sure how it worked, but I assume that the car would ask the driver to shift up if the motor was spinning too fast, or conversely, to shift down if it were spinning too slow. If this is so, then we can learn 2 things:
- Cars have known about their RPM for (at least) 50 years now;
- Tail-fins didn’t fly then and they don’t fly now.
MPG, RPM, and how they relate
MPG refers to miles-per-gallon, of course, an indicator of how fuel-efficient a car is, while RPM refers to revolutions-per-minute.
But what exactly is their relationship? The car’s engine uses one volume of gas-air mixture on every revolution, e.g. 1.6 liters for a typical compact car. The higher the RPM, the more mixture-per-minute is used. So, all things being equal, higher RPM should spell proportionate higher fuel usage, e.g. a car running at 4000 RPM should use double the gasoline than when running at 2000 RPM. But that’s only half the story. When we hit the gas pedal we are also opening the throttle, injecting a richer mixture of gasoline into the cylinders, resulting in perhaps doubling again the gasoline usage.
How it can be improved
So keeping your RPM low is doubly-good advice. Most cars these days have RPM dials, and it is easy to see how it jumps when you accelerate from a stopped position. The faster you accelerate, the higher it goes. I try to keep mine below 2500 and that’s what I’ve been telling other drivers in my family. But then I realized that we’re back with the old semi-automatic Chrysler: Why does the car tell us what to do? Why should the driver even have to worry about these things? Just as we mastered the technology to shift gears automatically, it shouldn’t be too difficult to have the car itself limit the RPM. Car manufacturers can set the max RPM to, say, 3000 revolutions-per-minute, and then no matter how hard the driver presses on the pedal it won’t go past that number. Besides saving gasoline and cutting pollution, it can also make for a safer driving experience.
by Oran Bodner