Hmm, I always thought the 3.5 V6 was a fairly thrifty engine. I know this is comparing apples and oranges but I owned a 95 Concorde with a 3.5 and have consistantly gotten 28-30 mpg from it on the highway at speeds between 70-80 MPH on cruise with the A/C running, I own a 94 LHS now and still get 25-27 out of it at the same speed range. I realise that there is a difference especially in HP ratings between my 3.5 and the 3.5's that are in the 300M, it seems weird that a 300M would get mileage that low on the road unless the gearing is different.