How on earth is the Prius piece of this thread still going?
The Prius is extremely good in congested traffic. I drive one, and have the mis-fortune to drive at least an hour a day at an average speed well under 10mph. Queuing in traffic, the petrol engine seldom runs. The occasional movement comes from the battery and electric motor. Occasionally the petrol engine kicks in and, at efficient revs, re-charges the battery. The efficiency is obvious.
At the other extreme, on the motorway, the battery and electric motor are a waste of space and weight, you just have an under-powered over-heavy car. The inefficiency is obvious.
Because the efficiency varies so much with the conditions, there is no point in comparing individual's mpg achievements. Indeed, the compelling case for Londoners to buy a Prius used to be exemption from the Congestion Charge, not the petrol consumption.