I'm going to ignore the "better gas mileage at 80-mph then 55-mph" claim. I've got the technical background to demonstrate that such is only the case in very limited cases with vehicles that are incredibly over-powered such that their engine reaches its peak efficiency zone well higher up the speed scale then most vehicles with reasonable power capabilities, but then I'd have to do about an hour or two or researching appropriate diagrams and source material and doing match and write a big long monster post. I don't do that unless I'm really passionate about the issue.

And saving gas is not why I'm passionate about going reasonable highway speeds like 55-mph rather then being a dangerous speed demon motorist.

It's not about saving gas ~ It's About Saving Lives

An large, heavy, fast moving automobile vehicle IS a kinetic energy weapon. That was what I was taught when I took drivers ed. The old guy who taught the class I took drilled that fact into our head more then any other, we were all country boys and girls and we knew well the lethal power of those other kinetic energy weapons known as guns and bullets, and the drivers ed teacher we had demonstrated very clearly to use and repeatably drilled into our adolescent thick skulls that we were "Driving a Bullet" and that an automobile at speed packed more lethal energy then even the most powerful rifle guns that it is possible for a human to fire from the shoulder and aren't so big they have to be mounted (like a battleship gun) and that people being stupid with automobiles killed and maimed many, many, many times more other people then people being stupid or doing bad things with guns and we had to treat an automobile when we were driving it with as much if not more respect then we had already learned to treat a loaded gun with. And, yes, he had the facts and figures to back every one of those facts up and he used them.

Kinetic Energy = (1/2) x (Mass) x (Velocity)^2

That kinetic energy is the lethal "knock down power" that an speeding automobile or a speeding bullet uses to kill, maim, and destroy. The more kinetic energy the more "knock down power" and lethal potential, and for cars also how long the stopping distance is. Twice the kinetic energy means twice the distance to stop.

The important thing is that kinetic energy raises not linearly with increased speed but rather to the square. Which means if you double the speed you increase the kinetic energy to four times, if you triple the speed you increase the kinetic energy to nine times, and if you quadruple the speed you increase the kinetic energy to sixteen times.

(2)^2 = 4
(3)^2 = 9
(4)^2 = 16

Now lets look at what happens with kinetic energy and stopping distance when you increase speed above 55-mph

----- Increase to 60-mph = (60/55)^2 = 1.19 = A 19% increase in both potentially lethal energy and stopping distance
----- Increase to 65-mph = (65/55)^2 = 1.40 = A 40% increase in both potentially lethal energy and stopping distance
----- Increase to 70-mph = (70/55)^2 = 1.62 = A 62% increase in both potentially lethal energy and stopping distance
----- Increase to 75-mph = (75/55)^2 = 1.86 = A 86% increase in both potentially lethal energy and stopping distance
----- Increase to 80-mph = (80/55)^2 = 2.12 = A 112% increase in both potentially lethal energy and stopping distance

Long story short, if you drive 80-mph instead of 55-mph you have more then doubled the lethal energy of the dangerous machine you are driving around other innocent people and have also more then doubled your stopping distance. Could that not alone, in and of itself, potentially qualify as criminal negligence if your doing it around other innocent people? That's a serious question you need to ask yourself.

Also, as to gas mileage, that greater kinetic energy of greater speed needs to come from somewhere and with a motor vehicle it comes from the motor. The only way gas mileage goes up as speed goes up is if the motor is sufficiently more efficient at higher speeds (higher RPMs for the motor) to more then make up the distance.

It also explains why you have to pedal a bicycle so much harder to go only a little bit faster, for a bicycle lets use 15-mph as our base point:

----- Increase to 20-mph = (20/15)^2 = 1.78 = A 78% increase in pedal power required to increase to this speed
----- Increase to 22-mph = (22/15)^2 = 2.15 = A 115% increase in pedal power required to increase to this speed
----- Increase to 25-mph = (25/15)^2 = 2.78 = A 178% increase in pedal power required to increase to this speed

And, yes indeed, those increases also mean an increase in potentially lethal energy and stopping distance for bicycles as well just like for automobiles, thankfully bicycles aren't near as heavy and don't go near as fast as cars, but its still something to keep in mind especially when it comes to pedestrians and other cyclists where there is certainly the potential to do potential lethal injury even at the much lower weight and speed of a bicycle.

And yes the same question about the potential negligence of higher speeds also applies to bicycles at least when they are being operated in close proximity to other "soft targets" which are open and vulnerable namely other cyclists and pedestrians. Really doesn't apply when your operating a cycle at increased speed and thus increased kinetic energy around "hard targets" where you don't have much chance of doing anything beyond scratching paint.