[MeteorGray]

I've always felt that because there are so many variables in "city" driving, comparing fuel mileage is not very informative. These variables include number of start-stop red lights and traffic blockages per mile, average speed, average distance per cold start, minutes of engine idling per trip, acceleration technique, braking technique, etc.

In contrast, I think that comparing highway fuel mileage is much more informative, with the main variables being average vehicle speed and, related, average wind speed and type (direction of wind relative to vehicle direction.) When comparing highway (especially Interstate) mileage, it is relatively easy to note things such as average speed and what the wind was doing, which is important to consider since they can be quite significant to the results.

In my case, my average fuel mileage to date is almost 45mpg for my '15 Mazda3 automatic. The reason it is so high: I have used it almost exclusively as a commuter car driven 90%+ on 200-mile nonstop highway trips with maybe 10 red lights per trip. I don't exceed the listed speed limit, my average speed is probably around 55 mph, and I'm an easy driver overall and don't compete in the highway "wolf packs" that force a driver to constantly accommodate the crowd.

Although the OP's 15mpg is probably at the very bottom of what one would expect from a Mazda3 in bad city conditions, it is not an implausible value. However, if it were me, I'd investigate for any vehicle problems as noted above, such as brake malfunction. If it's not such a mechanical problem, then it has to be city driving conditions and techniques.