This week on The Best of Car Talk, why are Helen’s Mercedes’ tires growing mold? Tom and Ray say it’s because Helen drives the car infrequently, to which Helen replies, “Au contraire Piston Pusses!" Elsewhere, Linda’s fiance insists on washing his car before a long trip, to improve its gas mileage. Is he right, or just falling prey to the old "clean underwear" theory? Also, Scott’s wife gave him a homework assignment—write a paper explaining why he should keep the ‘77 MG that’s been sitting untouched in their garage for the last year; and you’ll be shocked, shocked to learn that John’s “cheap and reliable” Ferrari is making a loud, and likely expensive, noise. All this and more, this week on The Best of Car Talk.