Stick your hand out the window and try to push it forward at those speeds. Of course, if you’ve got a strong tailwind, the negative impact of speeding will be mitigated to some extent.
Not only does the extra weight hurt your fuel economy, it upsets the balance of the car.
Most modern cars have a display setting in the instrument panel that will show you your actual fuel economy in real time.
It’ll give goofy numbers like 1.2 mpg when you take off from a light, or 90 when you’re coasting down a hill.
These are small V-shaped devices that you can put in various places on your car, like in front of the wheels, that’ll cause the air to spin in a vortex, obviously.