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My 2 cents...
Take the weight of your vehicle and multiply it times gravity times sine of any slope you may drive on.
Example:
1200 kg car (~2600 lb) x 9.81 (gravity) x sin(3 degrees) = ~620 N
If you were traveling at 30 mph (~13 m/s) power consumption = 620 N * 13 m/s = 8000 watts = ~10 hp
Civic Cd ~0.36 & frontal area ~2 m^2...
Drag = .5 x density x velocity^2 x Cd * frontal area = .5 * 1.23 * 169 * 0.36 * 2 = 74 N
Power Consumption = 74 N * 13 m/s = 962 watts = ~ 1.5 hp
This calculation can be repeated with your vehicle specs to determine when you really need to worry about aerodynamic inefficiencies. From this example of a Honda Civic, 30 mph 3 degree hill climb results in 6x the power needed to overcome drag. At these speeds, I would say internal drags are your primary source of power consumption.
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