2007 Honda Civic EX, 5 speed manual @ 2000 rpm = about 45-46 mph.
When I try to go for maximum mpg (and I won't block/endanger other traffic), I try to keep the engine between 1750 and 2100 rpm. Nothing nicer than cruising around Mount Rainier going 40-45 mph.
Wasn't there a post on calculating the "ideal piston speed"? You entered bore and stroke and the formula produced a rpm range where the engine isn't lugging or working against the momentum of the piston motion. I immediately got an analogy of riding a bike - there is a range where your legs will feel most comfortable spinning. Slower, and 'yer thighs burn up, faster, and you start bouncing on the seat.