If you manually push the insight, you can get infinate MPG,If you coast down hill, you can also get infinate MPG. Pulse and Glide with a relitively slow average speed can peg the MPG display at 150MPG.(Thank you Wayne)
Speed and terrain also have huge effects on the final MPG number.
In my book the only way to compare MPG between cars and drivers, is on the same route at the same average speed. The mountain drivers that drive with the flow will never approach the flatland drivers that also drive with the flow. Taking back roads where the flow is 35-45MPH will always do better than highways at 65-75MPH.
Then there is the highway drivers that go the minimum allowed MPG for the highway. they will usually do much better than at higher speeds.
Adding a certain modification and some booster batteries can improve all situations, and of course a pure electric mode that an Ewheel type modification makes possible, is like pushing with the motor off, infinate MPG.
So don't get too hung up on the numbers, unless the speed and terrain is considered.
I tend to think in terms of the road average MPG, which considers the other drivers, and the total gallons of fuel used by all vehicles. A truck that has to slow down, on the way up a hill, because I am driving much slower than he, and the passing lane is full so he cannot pass, will waste more gas when resuming his speed in that single event, than I will save by driving slower than the flow for many many miles. When everyone on a road is cruising at a constant speed, the MPG for that road will be maximized for that speed. If we could lower the speed limits like they did in the past, that would save a huge abount of gas.
I am not suggesting that hypermiling is a bad thing, it is only that it should only be used when the overall effect on the MPG of the cars around you will not be adversly effected. The big picture.