" I keep my gas cap key on the same ring as the ignition key, then I don't
 leave the cap behind by mistake."

I have done this for twenty-five years ever since April 7th 1984 when I
bought my first new and last Westphalia. Still going strong with one rebuilt
manual transmission and a second boxer motor with 304,000 miles on the
clock! Never lost a gas cap yet!

Rey