Just did a round trip from Orlando to Jax and back. I was in search of 40 mpg but didn't even come close

i only got 34.5 mpg. I cruised at 75 mpg the whole way, with the windows up AC off on the way there but AC on on the way back. Had one passenger in the car.
I don't understand I went to Naples and back and got 35 mpg with four people in the car, luggage and A/C on the whole time and was cruisin at 80 mph???
I guess to get 40 mpg you got to be cruisin at 50 mph AC off and windows up with no one else in the car