A very good point made about fuel consumption by achimha. According to my POH in the F172H the O-300 will use 7 us gals (26.5 litres) per hour cruising with 60% power (2300rpm) at 2500ft. This is for a new fully loaded aircraft.
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In my experience remembering to lean and careful engine management can better this for general day to day leisure flying between 2 and 3 thousand feet around the local area at 90knots on 100LL AVGAS.
The led in 100LL will occasionally foul the spark plugs causing problems from rough running on a magneto check during the pre-take off engine checks. Just running the engine on both magnetos at full power for a couple of minutes will usually clear this.