Tool for visualizing aero vs rolling resistance

[MetroMPG]

Yes, but it works only as long as the range of conditions examined is "small". That is, no large variations in speed, viscosity, or crazy angles of attack.

fortunately, using the equation to figure out drag for a road vehicle, there is no variation in viscosity or crazy angles of attack. so we don't have to worry about the writer's concern over those variables.

so we're left considering the writer's concern over the "large variation" of speed. and with no definition of what a "large variation" is, it's not a particularly useful point to have raised. (is 30-70 mph through air considered a "large variation"?)

i don't think the utility of the equation is seriously in doubt for the application we're looking at.