I once did a trip which started in San Francisco-Los Angelas-Las Vegas-Grand Canyon-Dallas-New Orleans-Tallahasse-Atlanta-Chicago-Denver-San Francisco.

The Chicago-San Franciscio portion I pretty well drove non-stop and had 2-3 hours to spare before my flight.
I think the trip totaled about 7000 miles in 2 weeks.
I paid around $160 for the 2 week car rental.
The lowest price I paid for gas was in Georgia for around 80 cents a gallon .
The rental had about 4100 miles on it when I started and ended with around 11200. Well worth the cost of the rental.
Wonder if the rental company made any money renting to me.