re: Tailgates
Sounds like you a good handle on it.
If you have a truck and don’t have a cover for your bed, installing one will give you a gain in mpg. If you don’t want to go to that expense you can drop your tailgate down for better airflow and get a net type barrier. There are also the fifth wheel tailgates and cargo gates that have openings for airflow.
This Fueleconomy site offers tips on saving mph and you can view what other people are getting for mpg of the same vehicle, unless you happen to be unlucky like me.
I guess I should share my mpg.