He used his household computer to post the question to a geeky BBS?

None of the above.

All he needs to do is take a glass jar (like a mayonnaise jar) and using the same yardstick he used to stick his fuel tank, measure and mark the round end of the jar at 1/4 the diameter. Now, put water in the jar and by trial and error add enough so that when the jar is lying on its side the water level is at the mark.

Next, and here's the clever bit, stand the jar back on its end in the normal upright position. Measure the distance from the bottom of the jar to the water level, and figure this as a percentage of the total height of the jar. For example, the jar might be nine inches high, and the water level at 1.4 inches. 1.4 divided by 9 = about 15% filled. (I am not saying that these would be the actual numbers, but are just given as an example).

The same ratio would hold for the fuel tank. If the tank held 100 gallons, then when the fuel level was one quarter of the way up the diameter of the tank, there would be about 15 gallons in it.

I have to confess that when this puzzle appeared on Car Talk, I was unable to solve it. But I certainly appreciated the ingenuity and intuitiveness of the people who did.

tanstaafl.