Why is there such a difference between highway and around town miles per gallon?

Yes to starting and stopping. Gas is used to get up to speed; braking slows it down and repeat. Also on the highway you are in the top gear with fewer RPM’s less gas is used.

Also, when you’re stopped at a red light or waiting to turn left at a green one, your car is still consuming gas, while traveling 0 miles.

@Tropical_Willie is correct about starting and stopping. However, if you go faster than 55 or 60 your engine will burn more gas per mph. Keep it in the sweet spot.

Once an engine is at a steady power (speed), it takes less gas to maintain that speed. Your change in velocity is near zero. When you drive around town, your speed goes up and down and you have to stop and start. Every time you go from a stand still to a given speed, you are having to exert more force to change the velocity of your car and that means you have to burn more gasoline. Additionally, you aren’t burning all the gas…the engine isn’t as efficient as when it is already moving at a speed. If you do the fast acceleration from a light, it makes this even worse.
As for the most efficient speed, that varies from car to car. I have a car now that if I hold it around 60 it does really good on gas mileage. I once had a 1973 Buick Riviera which had a huge 8 cylinder engine. It got horrible gas mileage, but got its best mileage at around 85 mph. Don’t ask how I know this.

Every aspect of your car suffers from the wear of stop and go city driving compared to highway miles. All of it from wear on the pedals and tires, brakes, engine, transmission, you—EVERYTHING. Given the choice, I would much prefer to buy a used highway patrol sedan over a police cruiser or taxi with the same mileage.

In town you always accelerate away from stop signs which takes more gas, and you spend more time idling at stop lights which doesn’t help your MPG.
On highway your gas pedal pressure remains somewhat constant.

Think about pulling a laden wagon. You use a lot more energy to get better t rolling than you use continuing to pull it.

I remember the days of hauling kiddos around in wagons, I often wished for a downhill slope.