I was wondering how speed and engine running time affects gasoline mileage. For instance, driving at 60 mph to go from point A to point B and driving at 25 mph to go the same distance. Would the driving time that it takes to cover the distance at 25 mph burn more gas than going the distance at 60 mph? Usually the faster you drive the more gas is consumed. John
Yeah, but at 25 mph yer still in second gear ie if you were in a higher gear you would travel a longer distance with the same amount of fuel.
On the surface, you'd think "faster burns more fuel" because wind resistance goes up as the square (or is it the cube? No, I think that's energy used) of speed.
But its not that simple. Below 60-80 MPH for most cars, you're still on a fairly flat part of the wind resistance curve so even though the energy used due to wind resistance is changing as the cube of speed, its still small compared to rolling resistance and the internal losses in the drivetrain. What dominates mileage in that case is whether the engine and transmission are running at their optimum points. For example, if you're driving so slowly that the transmission can't shift into overdrive and/or lock its torque convertor, you're wasting more fuel than you would be by driving a bit faster to get in top gear and lock the convertor.
Most cars are designed to be efficient in the range of common legal speeds. IOW, in the US where most speed limits are between 40 and 85 mph (and 85 is only posted on a few rural interstates) the mileage is terrible below 20-30 mph, starts climbing as you go up to between 50 and
70 mph, and then falls off very quickly as you go faster than that.
It is a very complex problem and the optimum speed for each brand and model of car is different. Here are some of the issues.
Usually the efficiency of an IC engine is maximized at low rpm but maximum throttle opening. However, car engines have a problem with full throttle and low rpm because of drivability (the mixture does tend to lean out under full throttle. So most engines have max efficiency at about 2/3 to 3/4 throttle, again at low rpm.
Aerodynamic drag is the main loss of energy at high speed, but rolling friction is at low speed. The exact crossover speed depends on the streamlining of the car, the tires, bearing design, and vehicle weight.
Back in the days of the first oil embargo (mid-70s) there were a number of articles about optimum speeds for various cars. I seem to recollect most were in the low 50s.
A low numerical ratio (high gearing) maximizes mpg. In fact, the first "Mustang MPG" model was merely one with a lower numerical rear axle ratio.
If you have a stick shift you can maximize fuel economy by short shifting, picking very low rpm shift points. Most FI cars can lug quite well without shaking, which is hard on car.
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