What I have found is that parking in a hot place causes a significant fuel loss.
Avgas expands about 0.1% per degC so if you take on fuel (to max capacity) from a say +10C tank and then park in the sun and the wings reach +40C, you can expect to lose 3% of your tank volume.
If your wings are black then all bets are off. You could lose 5-6%.
The most I have seen is about 2 USG i.e. 1 USG from each tank. That is about 2.3% (my usable fuel is 86.2 USG). The drop in the fuel level was about 1" and on a later occassion I measured how much that is worth, by filling up 1" short and noting the pump reading, then filling right up.
You get bigger errors from 3rd World pumps though. I recall one bowser which was 6% out. I vaguely recall that was in Hania (LGSA) in Greece. Funnily enough the error is
always against you
Well, except one, which was a bowser on the IOM which under-read by 7%
In the UK and Germany the pumps are always within 1%.
I have a fuel totaliser which is accurate to about 1% so I see all kinds of funny stuff. In practice it is difficult to get better than 2% of what you think you have, due to fuel expansion and resulting losses. And the engine power is
mass flow dependent whereas fuel metering is
volume dependent