Yep, what Mikey said. When I asked Vans this same question, they told me because of the wing's dihedral angle, and because the senders are at the lower end of the tank, you'll get quite a bit of air in the wingtip end of the tank before the float starts down. The person I talked to, might have been Gus (its been several months now), said you'll burn about half a tank before their analog gauge comes off the peg. They also suggested if I really need to know what is in the tanks, to use a fuel computer, either stand alone instrument or as part of an engine analyzer. If you look at the pics of the completed and flying aircraft, they are all using engine analyzers. No steam gauges in the engine department. Seems like when I surfed the NTSB reports before commiting to building a Vans, the big killers were the usual suspects: dry tanks and cumulo-granite clouds. I figure the engine analyzer will pay for itself in maintenance costs if it doesn't save my life.