Doesn't anyone read the POH any more? For example, the Cessna 172 has a fine (and accurate) cruise performance table. Set an RPM and lean in accordance with the guidance in Normal Operating Prcedrues and you'll get a pretty accurate fuel burn. Add 10% and 45 mins if, like me, you like to adopt IFR criteria and you will never need to stare longingly at notoriously inaccurate fuel gauges.
PS. I agree about the poor standard of training on this during PPL; we appear to have resorted to filling the tanks and assuming a pilot will be bored, out of cash or in need of a pee before the tanks run dry.