Driver for mileage

my daughters 2005 cavalier, 30 plus average...highway 33-35
My other daughter drives a 2001 neon ...28 city 30-32 hiway
I have a 2000 buick lesabre ,v6 ,loaded , real comfy ride. it gets 28 average have nt taken any trips yet. but i ll bet it sees 30 +..and american cars to boot..
My hemi ram 2003 4x4,w/ 3.91s...as much as 20 hiway, as little as 10 climbing the hills here.

I have noticed the area of the country that i am driving in changes milage quite a lot.... sea level and no hills make a big improvement ...
on one trip from key largo fla . to willmington north carolina ..2002 ram 4x4 w/ 5.9 and 3.55s ...i got over 20 mpg.....same truck up here in NY , best i ever got was 16 hiway...