Try burning 12 and leaking 38.
My wife back when she was a school teacher, would close down her classroom in mid-June and head to the airport. She's help the mechanic (cleaning up the shop, opening access panels, etc...) to get all the backlog ahead of our plane out and then to the grunt work on our annual. I'd come over on the weekends to help. One year we were bumping right up on the beginning of Oshkosh. The mechanic finished the "annual" and signed off the books and headed off for her IA continuing education. Me, Margy, and the other (non-IA) mechanic spent the afternoon putting the plane back together. I did a quick test flight, opened up the cowl, etc... and looked for leaks not seeing any topped off all the tanks for our departure the next morning.
Well Margy was flying and we had departed VKX and made it to about the Potomac river when I noticed the main fuel gauge was not pegged anymore. Odd, I thought, I'm sure I topped it off (now it's possible if you fill a Navion too fast that you can over flow the filler neck before the tanks get full, but I've not made that mistake after the first time it happened years earlier). I point it out to Margy (who is flying) and said I'ld keep an eye on it.
Margy at this point is negotiating with Dulles approach for a class B transition and flight following to our first stop which was intended to be Bluffton Ohio. Just about the time she got the clearance, I conclude that we really are burning fuel at an ungodly rate. Margy looks to the left (Leesburg) and right (Frederick), I point out Leesburg looks closer. We decline emergency assistance from IAD after we tell them we're diverting. I figure we still have 80 gallons left aboard.
We park the plane, and I take off to find a mechanic. We had had the boost pump overhauled during the annual process so we figured that might be a good place to start. It's in a bear of a location on the Navion but Margy knows where it is so when I come back with the mechanic, Margy's sitting there removing the access panels to get to it. "How do you get her to do that?" the mechanic asked. I tell him she was the one who just did the annual.
Anyway we open up the cowling and kick the boost pump on. Fuel geysers out of not the boost pump but the hose that runs from the boost pump to the engine driven pump at the fitting on the engine pump (oh, and we replaced all the hoses at that annual, the one that was on that part had a date stamp of 1948...two years older than the plane!). Turns out it never got a wrench put to it. We tighten it up and all is good. I get the fuel truck and put 20 gallons back in the main, what was consumed in our 20 minute flight from VKX to JYO.
Regardless of what everything else tells you, if the gauges are doing something they haven't done before, FIGURE OUT WHY. Fuel leaks can be very difficult to find. Fuel evaporates pretty darned quickly so we didn't see any signs of it after the test flight.
