esp on or off should not make any differance to the gas milage unless you are doing quick starts. if you have an auto running it in W instead of S makes a big differance.
my auto averages 27+ on a tank with 50% motorway at 80 to 100mph/town20/40mph

send me a bucketfull of gas at $2 a gallon, ours in the uk is ��4 or the equivalent of $6 a gallon