Let's first answer what would be the cost of gasoline for each route.
The car needs 18 gallons of gasoline for every 296 miles. So the cars efficiency is

The southern route has 3102 miles long with an average cost of 2.20 dollars. Then the cost for the southern route is

The cost for the southern route is 310.20 dlls.
The northern route is 2918 mi and the average cost is 2.28 dlls. Then the cost for this route is

he cost for the southern route is 302.41 dlls.