Welcome to GA.
Siphoning out of filler caps and fuel drains is unlikely to give you the loss rate you experienced.
It has been 10 years since I flew a 172 and seven since I flew a cessna 200/400 series, but the following spring to mind.......
1. They are crap guages. Float and potentiometer sensor, 1960's DC electrics and cheap ammeter calibrated in gallons. Not the capacitance gauges you may have been used to. You really need to know that aircrafts hourly burn from the club records, not from the book, and work to that plus a fuel log. Compare with guages. Believe the worst figure.
2. They might be bladder tanks and not lying flat due to the bonding of the bladder to the wing having come adrift, or perhaps only flat when a certain weight of fuel is holding them down. Also possible problem with vents being blocked or partially blocked and bladder is 'deflating' rather than allowing air in and fuel level going down relative to float sensor.
3. Was it filled on a level surface?
4. I'd believe a fuel dipstick, however crude, over the guages alone.
5. Try filling to known points. Cant remember on the 172, but the 206 and 210 had 'tabs' in the filler inlet. You could go a combination of full as a known quantity, tabs as a known quantity, and one full/one tabs as a third option for a known quantity.
6. If you rented it from a flying school/club, they have to keep fuel burn records for each type. Ask to see them. Also, anything on the maintenance release?
7. How did the start fuel + fuel added from bowser reading compare with the final indicated fuel load? I flew a C206 once with total capacity of 280 litres. I thought I had landed with 50 minutes (30 litres per side) and then proceeded to fill it to capacity for the empty leg home. BP bowser reading? 260 litres!
I agree, there are a number of things you should be concerned about.
In the meantime as our colleague notes... Never trust a Cessna gauge!
Last edited by ITCZ; 4th Dec 2006 at 13:04.