http://www.google.org/recharge/experiment/
"The RechargeIT driving experiment consisted of three types of trips;
City trips driven exclusively on surface streets,
Combined City/Highway trips that were a mix of surface streets and highways, and
Highway trips that were almost exclusively on highways.
All trips from each trip type were grouped together to allow us to measure fuel efficiency specific to each type of trip. The graph above shows the overall MPG for the complete set of trips in the experiment, and the the graphs below show the MPG numbers for each specific trip type."
Note 30mpg US = 36 mpg UK = 7.8 L/100km