A relative of mine recently bought an XF Supercharged and remarked to me that he was getting less than 15 mpg in town. Then he noticed some instructions (maybe on the gas tank door, I didn't get this detail) that the fuel cap is supposed to be tightened at least three clicks, and he had only been tightening one click. He changed to three clicks and immediately got 21 mpg on his trip to work.

What is the explanation for requiring three clicks, and could this make this much difference in mpg without the message center alerting the operator? On my Volvo a failure to properly seat the gas cap gives a warning light and maybe a message.

 

Thanks for the replies. Probably the higher mpg achieved after the cap being tightened three clicks was due to the remainder of that trip being started with a fully warmed-up engine and the trip being mostly freeway. I'm guessing that the low mpg values obtained before that were due to a large proportion of very short trips.