DEAR CAR TALK: Here’s a question you haven’t answered yet.

I recently used a mobile mechanic – a guy who came to me where my car was, rather than me bringing my car to him at his shop. How much of a tip should I give to a mobile mechanic? – Mark

RAY: Good question, Mark. I would say that, unlike a server at a restaurant, who gets paid 75 cents an hour and counts on tips to afford his or her daily gruel, tips for mechanics really are optional. They’re a “thank-you” for particularly good service.

So if you call a mobile mechanic, and he goes above and beyond what he has to do, that’s when you would say “thanks” with a tip.

For instance, if he arrives right away, is friendly, fixes your car in the rain, takes the time to give you some good advice about how to avoid problems in the future, charges a very fair price and doesn’t break anything else or clean out the change in your cup holder, those all can be reasons to tip him.

I would not base it on a percentage of the bill, as you would at a restaurant. Instead, I would offer something between $5 and $20, depending on just how grateful you feel.

Giving a mechanic an extra $5 is a nice way to say, “Thank you for getting here on time and fixing the problem.” Giving a mechanic an extra $20 is an unmistakable way of saying, “Wow, this was great service, and I am very thankful for your extra effort.”

Of course, nothing beats a pan of fresh, warm brownies, but not everybody drives around with one of those, Mark. That’s why the $20 bill was invented.

Got a question about cars? Email Car Talk’s Ray Magliozzi by visiting the Car Talk website, www.cartalk.com.