I almost fell off my seat when you said the guages on PA-28 are accurate. They are not. The only time they "legally" have to be accurate is when the tank is empty. A situation best avoided in mid-air I think you'd agree!
Basic rules for fule management on the PA-28 are: Fuel Management - you have to do it on a PA-28. Switch between the left and right tanks every 30 mins. You don't have to do this on a gravity fed cross suplied Cessna. Remeber to switch the fuel pump ON before you switch tanks, check pressure, switch tanks, switch pump off, check pressure again.
The "tabs" on the tanks show 17 Gallons. If both tanks are "at the tabs" you have 34 gallons, of which 32 will be useable. 32 gallons is 3 hours endurance with a margin of safety built in for diverts etc.
A hint I was given when I was training was to always fly with full tanks - can make 'experimental' landings less "kangaroo" like ;-)