re: city driving has tremendous effects on my mileage.
Whether I just drive 'to' the city with a few errands, little rush hour, no stop & go, etc, I can expect my over all average of 22.4 (over 5000 miles all combined) to drop to sub 18. If I'm doing more driving IN the city under heavier traffic, my absolute worst has been around 16.3 IIRC (posted elsewhere). But that was for the whole fill-up to fill-up, and included all day city stuff. Bleh.
re: HP demands related to MPH.
There's a wonderful youtube clip on some of the engineering of the 1001 HP-Bugatti Veyron which reports they only need about (IF I have these numbers right)150hp to get up to 100 mph. Then to get to 200 mph then need another 250 hp. Then to get to ~300 mph they need another 700 hp, despite the slicked aerodynamics they can slick up their little road burner. Not only is there "resistance" via air-friction, there is "drag" and "lift" factors which also consume HP and fuel.
Here's one clip: