I see you only have this one post and it was back in May, so did you ever get the Mach1?
This may be too late but I have had my Mach since new, just turned 18k on it and I have filled it up everytime I have put gas in it and have checked every thank for the MPG.
I even made up a spread sheet that I enter each fill up on and it calculates the MPG and overall agerage.
My overall average is 19.5, a best of 28+ and worst of 15+. I have never had it on a trip that I got to run the Interstates for a full tank either.
I am pleased with this though concidering my '07 6er's overall average was only 20+ in the 25k miles that I owned it. It did get several trips too and only bettered the Mach's best a couple times.