DEAR CAR TALK: I’m retired, and at age 70, I hit my midlife crisis and bought my first Mercedes (new).

But being retired and living downtown where I can walk to most things, I find myself doing city driving almost all the time. So is it necessary to get on the freeway once in a while to keep the car engine in good shape?

If so, how frequently, and how far as a minimum? – Jim

RAY: No, there’s no need to drive your car on the highway, Jim.

People used to tell you to take your car out on the highway and “blow out the carbon.” But there is no more carbon.

Modern engines with fuel injection, computerized engine controls and improved gasolines run so cleanly now that if you take your engine apart after 50,000 miles, the tops of your pistons will be as clean as they were when you drove off the lot. So doing only “in town” driving is no problem.

And if you’re driving for at least 15 or 20 minutes on a semi-regular basis, you’re probably getting the exhaust system hot enough to evaporate most of tthe water vapor that condenses in there when the exhaust system is cold.

If you take all five-minute trips, you could cause your exhaust system to corrode prematurely. An exhaust system on this car could run you a few mortgage payments.

On the other hand, the exhaust system on a Mercedes is pretty robust. So even with all those short trips, corroding the exhaust might take 10 years.

And by then, your ne’er-do-well son probably will have talked you out of this car, and you’ll be driving a 2028 S-Class. And we can let him replace the exhaust.

Enjoy your car, and don’t worry about it, Jim.

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