Originally Posted by 2x4XLT
im highjacking!!!! does going 55mph really save gas? if you really think about it, when i go 65, the tach only goes up maybe 150rmps... yes it burns more gas then but i get there much faster thus, in theroy, useing less gas because im driving for less time....
sound reasonable or was i dropped on my head
Friction from the tires and drivetrain is fairly constant at any legal speed but aerodynamic drag increases as the square of velocity.
Aerodynamic drag = kV˛
Below 35 MPH, road friction predominates; above 35, aero drag is more important and increases at a much faster rate than the velocity. If all else stays the same and you double your speed, the drag goes up 4 times. So, even though you would get there in half the time, you have to overcome 4 times the aero drag.
According to Ford, driving at a constant 55 MPH instead of 65 MPH would probably save you about 15%.
The RPM argument does not hold up because how much fuel you use is a matter of RPM and
throttle position. The simple truth is that it takes more fuel per mile to travel as your road speed increases.