If you don't ever drive over 40, forget aerodynamics. Under 40 MPH, a Scion is just fine.
At 40 the road load equation says that aero drag passes rolling resistance and from there the aero drag HP goes up with the cube of the speed increase. To double your speed, you have to increase HP (and fuel burn) eight times. So trucks that drive the open road really need to look at aerodynamics if they want better MPG
The aerodynamic component of the road load equation is:
HP = k x Cd x A x MPH^3
k is a constant that converts units
Cd is coefficient of drag
A is frontal area
MPH is road speed times itself times itself
Cd for pickups starts out being about 0.45 to 0.48. A Corvette is the slickest common seen car on the road and they run about 0.30. So if the Cd of a truck could be reduced to say 0.40 (a good reduction) that would reduce drag by 12%.
Area is easy to compute. Width times height, but with one kicker. Double the area between the bottom of the front bumper and grade. For instance, my truck sits 72" tall and is 80" wide. Raw area is 5760 square inches or 40 square feet. My front spoiler sits 7.5 " above grade so I must add in the area under the truck again. I have to add (80x7.5=) 600 square inches so my true frontal area becomes 6360 square inches or 44.1 square feet. You can see how a lift rapidly increases frontal area and therefore drag. Use your tape measure, but SRW trucks are usually just under 80 inches wide. Over 80" wide and they need side marker lights. Duallies measure at the "hips" and is usually roughly 90". Height is the highest point on the cab or cap (if the cap extends above the top of the cab). for a F350 SD
, my 72" height represents the fact it has been slammed 4" in front and 6" in back. Measure under-vehicle area from the bottom of the bumper (not the pumpkin) to the road. A unmodified 4x4
SRW will stand about 80" high and 80" wide and have an under-vehicle height of about 14". So the stock area is (80 x 80) + (80 x 14) = 7520 square inches or 52.2 square feet.
As you can see the equation is very sensitive to road speed. There is a Dodge Cummins guy who get excellent MPG
simply because he never drives over 40 MPH except down hill. The same truck at 60 MPH needs 3.4 times the HP for aero drag, so no wonder he gets sparling MPG
. I can't drive that slow. People in Indiana would shoot you. He would be beheaded with a dull knife in a hammer-down place like SE Michigan.
Note that all pickup sized diesels have roughly the same engine efficiency so more road HP equates to proportionally greater fuel burn.