ntsqd was speaking to the speed vs wind resistance drag...not just to the "flat leading edges'...on any vehicle dependent on the coefficient of drag/friction for that vehicle the frictional drag of pushing through the air is going to increase much more rapidly than the speed increase..double your speed and get four times more frictional drag as ntsqd pointed out.
In short form, with or without the camper, slow down to lessen air drag and save fuel...
The wind deflector that many of us use to "streamline' the air flow over the squared front surface of the camper reduces the coefficient of friction and theoretically reduces the air drag. How significant is it? Tough to accurately quantify but several forum folks have reported measurable improvements in mpg after installing wind deflectors.
You [Turn-One] reported losing 4mpg in a diesel [with superior torque] this seems a bit much, but your speed was not mentioned...and most of us never truly get an accurate mpg determination until after running numerous tanks of fuel through the vehicle...way too many subtle but significant variables to have a sample size of "1" and call it accurate. Many runs are required to smooth out the variables and really find out the amount of any mpg fluctuations.
Just my take..and in any event...like was posted, add weight/wind resistance and expect to pay for it at the pump. it is all good.
Phil
