Here is an inexpensive way to replace the gas cap seal as they tend to deteriorate over time. Spin off the cap and clean out any remnants of the old seal. Take an old "front flasher lamp rubber body" where the outer edge is still in decent shape and cut off the raised 1/4" or so of the outer edge all the way around. It's the same width and almost the same circumference as the original seal but you will have to cut an inch or so off to make it fit the cap groove. Put a bead of silicone sealant in the groove and insert the rubber strip butting the two ends together. Make sure the flat side faces up. Close the gas cap to seal the rubber in place. Wait till dry and you now have a good gas cap seal.