The 2008 Environmental Protection Agency and Department of Energy's fuel economy guidebook lists out this year's most fuel efficient vehicles. It is long list across many categories. Here are the category winners, .ie. the most efficient vehicles in each class:

[Class Model City/Highway MPG]

Two Seater - Audi TT Roadster (2 liter engine,auto) 22/29
Minicompact - Car Mini Cooper Convertible (manual) 23/32
Subcompact Car - Toyota Yaris (manual) 29/36
Compact Car - Honda Civic Hybrid 40/45
Midsize Car - Toyota Prius (hybrid) 48/45
Large Car - Honda Accord 4Dr Sedan (manual) 22/31
Small Station - Wagon Honda Fit (manual) 28/34
Midsize Station Wagon - Passat Wagon (manual) 21/29
Sport Utility Vehicle - Ford Escape Hybrid FWD 34/30
Mazda Tribute - Hybrid 2WD "
Mercury Mariner - Hybrid FWD "
Minivan - Dodge Caravan 2WD 17/24
                 Chrysler Town & Country 2WD "
Pickup Truck -  Ford Ranger Pickup 2WD (manual) 21/26
                            Mazda B2300 2WD (manual) "
Van (Cargo&Passenger) - Chevrolet G1500/2500 Van 2WD 15/20 (4.3 liter engine)
                                                 GMC G1500/2500 Savana 2WD Cargo " (4.3 liter engine)