My gas gauge acts crazy as well, I can fill up drive maybe a few miles and the needle drops like 1 dot on the guage, look at it a few minutes later and it's back on full. This has been going on awhile and it does it over the entire range of the guage not just when it's full. Might be a problem with the guage or sending unit, or something in the wiring. Not sure, I'm too lazy to put the effort into diagnosing it. So I just reset my trip meter everytime I fill up.