Originally Posted by ISurvivedNMU
I average 43 with the stick... but the winter has ulled me down quite a bit... 47-49 hand calculated is not out of the realm at 70 mph on a long trip... Speed does not effect my fuel milage that much ( not like the truck)
Please allow me to clarify:
Actually, an increase in speed has EXACTLY the same net affect on economy on both a Golf/Jetta/Beetle TDI and a F350 PSD or a 9900i
The calculation for air drag is 1/2(Cd) x (area) x V^2.
Since we do not change the Cd, or area, it can be said that the air drag is related to the SQUARE of the speed (velocity).
For a given vehicle at 55 MPH, V^2 = 3025. Now at 65 V^2 = 4225, a change of 30% GREATER drag, with a 15% increase in velocity.
Another way to think of this is that for every 10% increase in speed, you increase drag twice that, or about 20%.
Of course, this does NOT take into account economy differences due to engine RPM, and that can be HUGE. Which I think is what he ment.