Back to what Denny said, what sight gauges may indicate
Or fuel computers are reading, may mean nothing. The tryed and true method that works in ALL Cubs regardless of the fuel system. Jump into an unfirmilar Cub, after you fill it full of fuel. T/O
And when climbing thru 500' switch to "One" tank at that time, and 'Note the time' on your watch exactly! Set your rpm to an exact amount, you use for cruise flight. Fly the Cub until it quits at altitude, and switch to the full tank. Look at your watch to know EXACTLY how long it ran on the first tank, Now you know within a very few mins how long it will run on the other tank, assuming you maintain that same rpm. Engines very wildly, in the time on one tank: I have flown lots of Cubs rigged up about the same, that were as short as 1:50 min! : To say as long as 2:30 min, before they quit on standard 18 gal tanks. So until you have established exactly how long a tank last with one perticular Cub with one engine, one carburetor, one prop, etc. You just don't know what it does exactly. You do this and land the airplane regardless of fuel system; 10 min "before the first tank quit" and you will likely NEVER run outta fuel.....( But you will be dam close) If your cutting it this close, you can't use a long nose down decent to land, unless you have a 'front outlet port' on that tank your coming in on.
Good luck.
E
This is what all the old Polar Bear guides did before headed out over the ice for Russia.
