18mpg! What could be causing it?
 

Hi. I just bought a 2008 Mazda 3 with the 2.0l engine and 4sp AT. I love this car, but the onboard computer states an average of 18.7mpg! Oil is fresh, tire pressure is fine, car is in tropical weather, only city driving. I purchased a bluetooth OBD code reader and no codes are stored in the car. What could be wrong? Are there any known culprits for Mazda 3's to develop such a terrible mileage? Thanks!

1) City Driving!
2) Change spark plugs
3) Put fuel system cleaner in
4) Change O2 sensor before catalytic as they get lazy with age
5) Change your PCV Valve
6) Clean your Mass Air Flow Sensor and throttle body.

While you are actually doing the cheap stuff first (don't do O2 sensor), re-set ECU by disconnecting battery (negative terminal) followed by depressing brake pedal

I have basically the same car just one year older. That is terrible fuel mileage as you say. The worst I've ever gotten in city is 25ish and I'm no grandma. My first inclination is faulty reading for the trip computer. Has this mileage been confirmed by filling the car up, driving until you need fuel again, refilling and calculating? Checking/replacing the components listed by Useyournoggin would be a good place to start otherwise. I would only add checking the air filter/ air box for debris and fuel filter replacement.

Low Mileage
 

I used to have below average gas mileage too. Try your tire pressure. When i added the right amount of air to the tires i got a little over 3 mpg. Also this might sound weird but if you have alot of stuff in your car it could be weighing your car down.

Fully synthetic oil, expensive tires, and regular air filter changes have kept my running smooth and about 34mpg combined since day 1. 2008 3i manual, 114k+ miles

Also watch for dragging brakes.