So I'm doing preflight. Normally I do it alone, but due to motor car issue my CFII picked me up at home and was walking and talking while I did the preflight. [Not germane to the story but it's good to live 2 miles from your hangar, easier to get rides ] I get my highly calibrated stick and measure 12 gals in each tank. I turn on the master switch and highly calibrated gauge indicates 12 gals each tank, 24 total. CFII and I jump in, taxi out, he gives me a psuedo clearance, I psuedo read back, I punch it in to the 430W, confirm that was all done properly, delete the flight plan, put in the flight plan we're actually going to do, run-up, take off still climbing he asks "what's that red light?" it's the low fuel on the right tank. But hey, there's 12 gals in there! We both saw it. I hadn't seen this before, but thought, maybe when I level off it will get its act together I say, nope, we're calling this sortie off. switch to the left tank and return to land. The whole time I'm thinking gauge problem. Get her back in the hangar, get out the highly calibrated stick, 6 gals. The gauge is correct. So where did that 6 gals go? I give myself high marks for ADM. It would have been so easy for me to blame the gauge, which had just been crossed check with the stick, but I said nope, ain't right - no flight.