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Under 5K and 'spirited' don't really mix well. Getting good mileage in the Si really means keeping your foot out of the gas, keeping your RPMs down, and going rather slow on the interstate (67ish or under). Really, its a zen exercise and likely to cause an stroke if you are impatient

(or sick, like doing 47 in 6th gear on a flat several miles of road just to see how 'high' you can get your MPG reading to go for a sustained period...or coasting downhill to watch the MPG go over 300mpg, just for fun)
Most are calculating mileage based on "trip miles/fill gallons".