Your use of the word "petrol" makes me wonder if you are aware of the difference between US gallons and UK/imperial gallons. UK gallons are larger volume and so give a higher MPG metric for the same vehicle.
There also are not many sub-compact cars marketed in the US, and the range of engine options is usually limited compared to similar vehicles marketed in Europe and the UK. Compacts like a Civic or Corolla would be considered efficient commuter vehicles by most people here, and might get around 40 MPG (US) in highway cruising and somewhere in the 30s as a combined figure (for non-hybrids).
It's not unusual for my 2-year old Elantra on my 48-mile commute to San Jose down 680 to reach 50 MPG. 47 for going back. I do drive earlier/later in the day to avoid the rush hour.
There also are not many sub-compact cars marketed in the US, and the range of engine options is usually limited compared to similar vehicles marketed in Europe and the UK. Compacts like a Civic or Corolla would be considered efficient commuter vehicles by most people here, and might get around 40 MPG (US) in highway cruising and somewhere in the 30s as a combined figure (for non-hybrids).