I used to use that site, but there are a lot more people using Fuelly and the website is gives you much more information about your fuel spending, etc.
Speed is a big factor in determining MPG.. Following is coming from Mazda 3's test drive experience.
"It turns out that the SKYACTIV Mazda3 is especially sensitive to driving style. Driving with a very light foot, shifting short of 2,000 rpm, and paying extreme attention to impending red lights (so as to get off the gas as soon as possible), I managed a high of 41.0 mpg on my standard drive from home to the kids’ school (with a trip computer reset after the engine had warmed up). Now this is more like it!. Speed is a big factor. First, air resistance rises at the cube of velocity. Second, engine speed increases in lock step with vehicle speed, and at a certain rpm efficiency begins to fall off dramatically. Driving at a steady 78 mpg, the trip computer reported 30.6 (again close to my nearly decade-old Protege5). Drop ten mph, and the number ticked up to 31.3. Drop down to 62, and it took a larger jump, to 33.6. Perhaps at the double nickel the promised 37 would materialize—but I just can’t drive 55! The implication is clear – the EPA highway figure is only going to happen at speeds much lower than most of us drive."
Source: Mazda3 SKYACTIV: The Truth Behind The EPA Fuel Economy Numbers | The Truth About Cars
But again, I guess this is going to be the case with every car.. Not just Mazdas with Skyactiv Technology.
Why would you put 91 octane in the car? It should be designed to run on 87. I don't think the higher octane will do anything for you in a stock car.First fill up I calculated 26.7 mpg which was .2 lower than the onboard info. Dealer filled tank up with who knows what grade gas. I filled up with 270 miles on the teip, most of which where in town driving. I filled up with 91 octane. I will see if I get any better or not.