Just purchased a used 2014 Mazda 3 S Grand Touring a couple weeks ago. Purchased at 56537 Miles. Full tank of gas.

I noticed it was eating way more gas than I was used to so I've been keeping track.

Now at 56840 I'm just had to fill it up for the second time (Gas light indicator has turned on). Filled it up with 10 gallons to max, I think I did about 9 the first time.
That's 303 (56840-56537) miles driven on roughly 18-20 gallons. 300/19 means I'm getting about 15.8 miles per gallon here... 60% city 40 % highway driving.

What's going on? What do I need to do? The engine indicator is not on.


Edited to adjust for how much I've been filling it up with.

 

What does the gas tank size have to do with your fuel economy?

What was the mileage on the car when you filled the car for the first time?
What was the mileage on the car when you filled it the second time?
How much gas did you put in the tank following your second fill up?

 

YEah, what he said.

The proper way to determine the gas mileage is to fill the tank, note the mileage. Drive the car normally. Fill the tank again and record the fuel you filled in. Note new mileage.
Mileage change divided by fuel you added equals Miles per gallon.

Based on what you stated above, the light is coming on early and you are putting in far less than 13 gallons each time. Probably more like 10, but still that would be horrible mileage.

Check it again.

 

I agree with the above statements, however, unless you are putting around 5 gallons each time you fill up, your mileage is way low.

The buy-gas-now light in my 2015 2.5 3 comes on when it says it is down to 25 miles left until empty and I am able to put in only 10 gallons at most, so there is either 3 more gallons in the tank (which would be good for 90 extra miles) or 3 gallons of air when it is full. I have not figured out which.

Unless you know the actual miles you are getting per tank and the number of gallons you are putting in per tank, you cannot come to any conclusions. The pump could be cutting off at 5 gallons and the problem could be a defective pump.

 

Apologies. Yes, filled it up twice for around 10 gallons each time, maybe even 9 the first time. I just now filled it up and the pump cut off at 10 gallons.

That would leave me at about 300/19 = Roughly 15.7 miles per gallon. This is with 60% city, 40% highway driving.

So, still horrible mileage.

What steps do I need to go through to try and fix this? I guess now I know why the previous owner only owned it for a total of 5k miles...

 

First step is to rule out your traffic environment; take the car on a long freeway trip and log the fuel economy.

If fuel economy for the trip isn't up into the mid-30 mpg range, the the first step is to figure out what is causing the issue; bad O2 sensor, clogged catalytic converter, dragging brakes..., it could be a lot of things.

 

I concur, could be anything. We should setup a poll and bet what it could be.

Personally, I am voting front (main) O2 sensor or temp sensor.

Could be a bunch of things though.