The Jeep Liberty's fuel economy depends heavily on the model year and engine. Generally speaking, the Liberty is
not known for its great gas mileage and is considered to be relatively expensive to fuel compared to many other SUVs, especially newer models. Older models with the V6 engine are particularly less fuel-efficient.
While exact figures vary, you can expect lower MPG than many contemporary SUVs. To get a more precise idea, you should look up the EPA estimated MPG for the specific year and engine of the Liberty you're interested in. Websites like fueleconomy.gov are excellent resources for this information.
