What car engine do you have? I'd be surprised if there's any car engine that can do 65% of 160 HP all day long, on just 6 liters per hour, where an O-320 (PA28-161) uses about 30 liters to do so - assuming you leaned correctly.
Remember that at typical highway speeds, your car only needs 20% or less of its rated power. If you don't correct for that, the comparison will be meaningless.
All the calculations I have seen suggest that a properly leaned aircraft engine is pretty efficient, considering the power output.