tester seems to give a very scientific answer for u.........hope it's good enough hehe...........

btw i would like to add some ........... 4x4 pickup is usually quite heavy (1.6 tonne >) and uses a large diameter wheel, with lots of tyre surface to roll around ......... the 4wd mechanism with center and some differential also give out quite a lot of loses due to viscosity effects of the lubricant inside....... constant movement would give u very high efficiency but not in the mode of stop and go.........so all of these sums up......u make the figure urself.....

as for sedan like subaru (all models are AWD supposelly), the drive train need to push a lot of extra mechanism too........even the drive train itself would simply add around 100kg already....... imagine thats..... if it come with differential........more is lose but scarifice for better control is out weighted for that little increase in FC rite.......

so lets take a very simple car subrau impreza 1.6 TS come with AWD produce about 100hp and 14.5kgm or torque.......it does not make a lot of power, (this would be explain better with a torque curve)......... so usually in order to make up AWD cars has lower final drive ratio in the final drive even all the gear ratios are simliar with 2wd brethrans........more torque is need to drive the AWD drive train for daily usage rite..... so the engine needed to be working harder in order to move the car, which ends up FC ++ situation.......

anyway about the tyre surface thing, drag is about the same for 4wd or 2wd car as long as it has 4 wheels, but weight does kill on FC here........... tyre are meant to provide low rolling resistance while maintaining sufficient side and some forward grip for control and speed..........

but accelerating make a different story > 4wd accelerate better and does need more torque to get all of the 4 wheels tyre surface to break surface grip to move......... 2wd car just need to move 2 while the other 2 merely mean for supporting.......rolling resistance is rather low..........