What's a good/fair tip to give the guys at the AutoBell car wash * [View all]

[Oopsie Daisy]

* for vacuuming the floors and trunk, hand-drying, cleaning glass surfaces, shining the tires. After driving through the automatic car wash, I will sometimes treat myself to having the interior cleaned as well. Often it's a team of 2 or 3 per car (depending on how busy).

If the drive thru premium wash/wax is $40... what's a fair tip for the workers? Perhaps 20% for the team to split? That's $8... or round-up to $10? Or should I tip each one individually?