Does freeway driving mileage go down with pbuttengers in the car

Mike Tantillo

Pressure goes as the square of the velocity, power required as the cube of velocity as a first approximation.

Doubling the speed would result in four times as much wind resistance or drag. Or whatever name you want to give to pushing air out of the way. Typical air resistance drag for a pbuttenger automobile is in the range of 50 to 85 pounds at 50 MPH, depending on area (18 to 23 sq.ft.) and drag coefficient (0.27 to 0.55). Total drag is in the range of 90 to 200 pounds. (15 to 20 horsepower 11 to 15 kW).

BTW, air resistance is the ultimate "speed limit". Air resistance determines how much the engine can breathe. Air resistance is the limiting force at speed.

Significant forces at steady speed are tire deformation and air resistance. Unless the tires are underinflated to begin with or the car is loaded beyond nameplate data, or the car has the wrong tires installed.

Cold tires may have more effect on drag than the weight carried. Tire resistance is about 25% greater during the first 10 minutes of movement. It may take as long as 20 minutes to reach equilibrium temperture.

The technical term "aero horsepower" is used to denote power required to overcome air drag at 50 MPH on a level highway.

A reduction of 1 aero horsepower is the fuel-efficiency equivalent of taking 300 pounds out of the car. But fuel reduction is more a factor of acceleration. 300 pounds increase in weight increases fuel consumption (CAFE cycle) by about 1% (0.3 MPG at highway speed, 0.8 MPG city driving).

Aero Horsepower of a mid size car is about 7 to 8.

For the population in general, "underinflated" tires seems to be the norm. Underinflated tires is related to ride comfort. People like a softer ride and the roads aren't all that smooth. Except in North Carolina. For the motoring public in general carrying unneeded weight in the trunk seems to be the norm.