Q: Car gets very low gas mileage

I would do a little quick research and find out what is typical real-world MPG for others who drive your make and model. That will provide you a realistic base-line expectation that will be more accurate that Mazda or EPA estimates. Now to a few things that can hurt fuel mileage. The wind resistance from the rack will certainly make a difference. But more at highways speeds and less so around town at lower speeds. Tire inflation is a factor. Check them monthly and inflate as per door sticker. Your right foot is probably the biggest variable in all of this.

You could do an experiment! For one week drive like there is an egg between your right foot and the gas pedal. Your mission is to press very gently on the pedal so you don’t break the egg! The results will indicate if your poor gas mileage is due to "enthusiastic" driving. I don’t know if your Mazda has an automatic or manual transmission. If you have a stick shift and you regularly run it up to higher rpm’s before shifting, that will use more gas. If I was looking at scan tool data for an engine performance problem that was hurting gas mileage, I would view fuel trims and misfire data after checking for diagnostic trouble codes (DTC’s). If you would like to have this checked, a certified technician from YourMechanic can come to your home or office to diagnose the mileage issue.