I recently upgraded to a 21 platinum back in June from an 18 sv. I averaged about 450-475 miles per tank with my 18 with non ethanol gas. Currently in my 21, I'm averaging about 400-425 miles per tank with the same gas and exact same driving habits. I know the platinum weighs slightly more than an sv, but doesn't seem like it would make THAT much difference in mpg. Any thoughts is appreciated