Rather than it get lost in the pointless drivel (dribble?) in the Petrol v Diesel (Prius) squabble I'll start this new thread.
One of the many programs I've written over the years to help me in my automotive related business is a spreadsheet which calculates the power and fuel useage a car requires to travel at a given speed. It takes the weight, rolling resistance, frontal area and drag coefficient, works out the wheel bhp and flywheel bhp required at each speed from these and then applies average BSFC (brake specific fuel consumption) data for both petrol and diesel engines to calculate the fuel requirement and mpg. You can also enter the price per litre of each fuel and it will calculate the pence per mile fuel use.
You can also alter the speed in any of the calculation rows to a new value and it'll tell you how much power you'll need to achieve that top speed and what the fuel consumption would then be.
By playing with the car weight and drag data you can find out very quickly what factors affect power requirement and fuel consumption. Basically weight affects rolling resistance which is a big factor at low speed but not at high speed and aerodynamic drag is the reverse. For good economy at low speed you want a low weight vehicle and at high speed a low drag one.
Using it you can easily see what is needed to design a genuine 100 mpg car. Low weight, low drag, efficient diesel engine. A 2000 lb car with low rolling resistance tyres, 17 sq ft frontal area, 0.26 Cd drag (same as a Prius) should do 110 mpg at 60 mph and 90 mpg at 70 mph with an efficient diesel engine. For a one or two occupant vehicle this is not a difficult concept to realise. The average hatchback in the 80s weighed less than that and many cars such as the Mini have been much lighter.
This morning I've tweaked the program to be more user friendly and added some guidelines for the data inputs for various cars. It's hosted here.
Play with the numbers and you'll see what your actual car ought to be giving in mpg terms. If I enter the data for my Focus (3000 lbs, 0.013 RR, 22 sq ft, 0.33 Cd) I get 38 mpg at 70 mph which is what it actually does. You can then alter those numbers to see how easily we could all be driving small, single user 100 mpg cars if the will were there or if fuel prices went up enough.
We don't need hybrids, regenerative braking or any other unusual tactic (not that I'm decrying them) to achieve this. Just common sense, efficient diesel engines, low weight, size and drag.
In no more than 5 minutes I could sketch out a small streamlined car that could carry two people and luggage and get them from London to Aberdeen on 5 gallons of fuel for 25 quid. My Focus needs three times that. If you want to move house it won't be the ideal vehicle for you but as most cars only have one person in them for 99% of the time it would do for most of us for general use and commuting.