must be pretty flat grounds and what 55mph or so for how many miles? the best i can get with my 2006 LS manual is about 40mpg give or take 2mpg's with cai, cat, cat back and jet maf sensor. oh and 185/70r15 Michelin pilot sport a/s 3's. thats driving from anchorage, alaska to homer, alaska thru a couple mountain passes.