Can you give me some details of your driving style and speed? I actually can't believe a Hemi can get 21mpg, except for maybe downhill with a tailwind and in neutral! Mileage was a main reason for buying the ED but I have only achieved a best of 23mpg *with* the GDE tune. I drive 90% hwy at 75-80mph. Tires are 34" tall vs stock ~33", other than that it is stock afaik. If I could get a Hemi to crack 20mpg (even 18-19 would work) then I can offset the mileage difference with fuel cost difference. I don't tow often enough to matter. I just can't believe it will do it...I’ve been driving a Hemi all summer because of a similar glow plug issue. My hand calculated mileage has averaged 21 mpg, my ED was averaging 24mpg, based on fuel prices in my area the price per mile is the same. Although I have found the Hemi has a throttle response problem too...if you mash it you might go sideways!