Mini Cooper owners report real world MPG numbers averaging between 25-28 MPG.
The official Mini Cooper website says it will get between 37 MPG city and 28 MPG highway.
It does this with the following setup:
The 2008 Mini Cooper and Mini Cooper Convertible models are fitted with a 1.6 liter four cylinder engine producing 118 horsepower (HP) [...]

Here are is an incomplete list of some vehicle models that would make good hypermiling cars:
Ford Focus
Mercedes-Benz E320 Bluetec
Nissan Sentra
Pontiac G5
Cheverolet Aveo
Geo Prizm
Toyota Corolla
Honda Accord
Toyota Prius
Here are the most (and least) fuel efficient cars as ranked by the Federal Government at FuelEconomy.gov:

2008 [...]