That statement has nothing to do with the speed limit, though. It may be trivially true, but adding the speed limit ties an irrelevant variable to it. Speed limit is a semi-arbitrary number that has nothing to do with how much fuel a car uses and how efficiently it uses it.
Not necessarily, because of gear changes. Not all cars can use 5th gear in a 50km/h zone, so they'll drive in 4th gear. It seems likely that doing 60km/h in 5th gear is more efficient than 50km/h in 4th gear.
Funny, I had always heard 55 MPH was the "sweet spot".
Drag Coefficient will be dramatically different on all car models.
Driving slower can have as dramatic of an impact on MPG as driving too fast. Your engine has to work quite hard to keep the car moving in both cases. Once up to a cruising speed, your engine will have to work less hard (and burn less gas) to maintain momentum. This is why your foot is not on the accelerator pedal harder while cruising on the highway when compared to driving on the streets. This is also part of the reason overdrive gears work (<1:1 ratio).
From a dead-stop, you dump the most unburned hydrocarbons (as a bunch of fuel is dumped into the chamber suddenly). So, constant highway driving will be more efficient than a slower city-street driver.
>How does a number on a sign affect how much fuel I'm burning?
I never claimed that it would. I said "Driving at the speed limit will definitely save you more fuel than speeding.", which is definitely true. If you speed you are going faster and using more fuel.