Yes, you are impressed. And that impression tied in with the unmatched smooth and silent driving experience remains in your mind; tempting and seducing you... at least until a few weeks later at the pump!
OK. Perhaps not all of us purchase a fuel efficient hybrid based solely on the fuel economy numbers we achieve in a first test drive. But, wouldn't many people be tempted to make a purchase decision primarily hinging on this factor alone?
As many of us already know, some hybrids have always been calibrated to slightly overestimate the displayed fuel economy... and now that Honda also appears to have returned to the practice with their 2010 Insight (see our in depth review of the 2010 Insight for details) then why not have it escalate to the next level?
In the case of the new 2010 Prius: How does a 3-7 MPG (US) overestimate sound?
It appears that the discrepancy is larger when highly focused and fuel efficient driving is employed whereas the gap seems lessened when the vehicle is driven in less optimal ways.
How do we determine that "instrumented overestimates" are occurring?
Carefully and patiently top of the tank until you see the fuel level, then reset the trip meter of your choice and drive. At a later date/time, return to your favorite pump and refill to the same level as before and calculate your vehicle's actual fuel consumption by hand. Do this just several times and you will have your confirmation.
NOTE: While topping off an AT-PZEV rated vehicle on a regular basis and in this fashion is not a recommended practice, a few times will certainly not compromise your vehicle's evaporative emissions system.
Wasn't it always like this?
Mostly yes, but still... while some "up or down" deviation is acceptable, this much will inevitably be noticed.