At highway speeds your vehicle is constantly rolling down the road without having to rev the engine more to get going again.
When you're idling your fuel injectors are still spraying near the same amount of fuel into the engines cylinders no matter how slow you're going.
And when you drive in town and slow down and then have to get started again you increase the fuel intake into the cylinders to get going again.
So all that stop and go driving uses up more fuel to get less distance but when your vehicle is already at high throttle going down the highway the same amount of fuel is being used to keep the vehicle going at the same speed.
It takes less energy to keep the vehicle going at a constant speed than it does to get the vehicle going again.
Actually when you drive slower you use more gasoline because the fuel is still flowing through the fuel injectors even when you're not pressing on the gas pedal too much and going slower.
But it takes just a bit more fuel to go a bit faster and it actually gets you further on the same amount of fuel so your fuel mileage will improve if you keep at around 55 MPH to 65 MPH or whatever the speed limit it.
Speeding however uses more fuel and puts more strain on your engine and transmission but going the speed limit helps you get the best fuel mileage out of your vehicle.
Going slower causes you to take longer to reach your destination and uses up more fuel so go the speed limit when possible to get the best fuel mileage.
Also when you're just driving around town your vehicles uses more fuel to go shorter distances because your fuel injectors are injecting the same amount of fuel even when you're just cruising around town.
So driving on the highway at higher speeds get you more miles per gallon of gasoline.
That's why there's a city mpg and a highway mpg and the city mpg is usually lower than the highway mpg.