Remember your own words, " Driving slower in fact does produce better gas mileage."?
But seriously, I was only making a point that the MPG of any auto is a curve and not linear. And I wouldn't even grant you that 60 mph produces better mileage than 75 mph. It really depends on the car. Many cars made before the late 80's were designed to be efficient based on the national speed limit of 55 mph. Those cars still on the road after speed limits went up in the mid 90's found themselves on the backside of the curve peak when running 75 mph. Newer cars are better designed to be operated efficiently at 75 mph with a higher curve peak.
The fuel mileage killer is not as much the difference in 10 mph. It's sitting still with the engine running (which is always 0 MPG) and acceleration. Especially from a stop where the lower gears produce less rotational output while the engine is consuming a higher quantity of fuel.
Again, every car is different. Just because someone is going faster than you, doesn't mean they are consuming more fuel or getting less mileage that the car is capable of.
Some of them may indeed be idiots, but not because they are passing you.