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Hello,
A week ago I bought Mazda 3 2015 2.0 L. The car runs perfectly, everything is fine except the gas mileage. On fueleconomy.gov (2015 Mazda 3 4-Door) city mpg is 30(=7.8l/100km). For this one week I mostly drove in the city and at the end average mpg was 15(=15l/100km), I was shocked because I didn't drive like a maniac, my average speed was 30mph.
I know that the bad gas mileage can be caused by bad filters (actually I already bought new ones, just waiting for delivery), wrong tire pressure and ect. but I don't think that they can cause that much difference ( from 30mpg to 15mpg ).
Also This particular model doesn't have an i-STOP system, it might be the main reason of all this, but still it's 15mpg loss.
So, if someone have had the same problem or if someone knows the reason, please help!



P.S. sorry if my grammar is wrong)
 

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15l is not good, but it's also very possible. If you sit static in traffic, then you are burning fuel and traveling 0km. Totally depends how long you sit for and how heavy the traffic and how often you accelerate and brake.
 

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Mine has i-stop (which hasn't worked for at least a year) and on highway with cruise control set to 90km/h I get 20.2km/l (translates to 47mpg). I do have the 2.0L 2012 Mazda 3. However when my foot feels heavy or when using my paddles to get through traffic fast, this gets halved.

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that's definitely a little high. despite what people tell you, the filters don't really have much of any effect on fuel economy. they might if they were REALLY trashed but that would take a lot. filters used to have an effect with carburetors but as soon as we started using fuel injection and sensors got better, all that happens is you lose some power in the higher RPM range.

worth knowing if you have manual or auto trans. tell us if there is anything unusual going on. noises? strange behavior? have you checked for ECU codes? any mods we should know about?

you're generally looking for anything that would waste engine power. a slipping belt, dragging brakes (the rear calipers are known to seize), fuel leak, misfires just to name a few. whats your mileage? was regular maintenance kept up? badly worn spark plugs could be a problem.

although very unlikely, the intake valves could be getting clogged up with carbon. i would leave that to last on the list to check. it's not really a problem on these cars but is not impossible. i'd guess usually with only driving short trips in the city would this ever be a problem and even then that would be rare.

give us all the information you can in as much detail as you can. fuel economy problems are difficult to sort out.
 

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that's definitely a little high. despite what people tell you, the filters don't really have much of any effect on fuel economy. they might if they were REALLY trashed but that would take a lot. filters used to have an effect with carburetors but as soon as we started using fuel injection and sensors got better, all that happens is you lose some power in the higher RPM range.

worth knowing if you have manual or auto trans. tell us if there is anything unusual going on. noises? strange behavior? have you checked for ECU codes? any mods we should know about?

you're generally looking for anything that would waste engine power. a slipping belt, dragging brakes (the rear calipers are known to seize), fuel leak, misfires just to name a few. whats your mileage? was regular maintenance kept up? badly worn spark plugs could be a problem.

although very unlikely, the intake valves could be getting clogged up with carbon. i would leave that to last on the list to check. it's not really a problem on these cars but is not impossible. i'd guess usually with only driving short trips in the city would this ever be a problem and even then that would be rare.

give us all the information you can in as much detail as you can. fuel economy problems are difficult to sort out.
Actually a few days ago I noticed that the car is pulling to the left, it might be a seized break caliper but it also can be just a bad alignment, I will definitely check both of those as soon as possible.

For more details I can add that my Mazda came with automatic transmission. Before I bought this car I check it in local Mazda service, there were no ECU codes. Mechanics listened to the engine, they didn't hear anything strange neither, everything seemed fine. The car has 50 000 miles on it and as I said, I bought it a week ago so I didn't really maintained it yet, but last owner did every maintenance it needed, everything was checked and changed regularly.
P.S. It doesn't have any mods installed, everything is stock. It's just a car for everyday driving, which gets 15mpg in the city)).
 

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You have only had the car a week...how are you determining your fuel consumption? Don't use the numbers on the dash display. Fill the tank and do the math.
 

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Actually a few days ago I noticed that the car is pulling to the left, it might be a seized break caliper but it also can be just a bad alignment, I will definitely check both of those as soon as possible.

For more details I can add that my Mazda came with automatic transmission. Before I bought this car I check it in local Mazda service, there were no ECU codes. Mechanics listened to the engine, they didn't hear anything strange neither, everything seemed fine. The car has 50 000 miles on it and as I said, I bought it a week ago so I didn't really maintained it yet, but last owner did every maintenance it needed, everything was checked and changed regularly.
P.S. It doesn't have any mods installed, everything is stock. It's just a car for everyday driving, which gets 15mpg in the city)).
that would be a classic symptom of a seized caliper. if you put your hand near the wheels (don't touch!) after a 15 minute drive i'm going to guess one will feel hot.

if you find this is the case, here's some info. feel free to pass this on to your mechanic if you're not doing the work yourself. this is IF it is a rear caliper seizing.

- there's a recall for the parking brake. the cable that actuates the calipers has a rubber boot at the caliper end. the original boot can leak, make sure this recall has been done as it can cause a caliper to seize

- the rear brakes like to jam up anyway. root cause is corrosion buildup UNDER the pad shims. this pinches the pads and causes them to seize. this is not to say that other causes such as stuck slide pin or stuck piston shouldn't be investigated. fixing it is actually pretty easy. knock the pads out, you'll probably have to pound them with a hammer. before installing new pads/shims, use a heavy metal file (bastard file) on the channels where the shims go and the pad tabs ride. your goal should be to widen them a little. the corrosion has to be removed anyhow. next put a light coat of caliper grease there before putting the shims in. this will help prevent future corrosion and give it some room if it does start to form.

if it turns out not to be brakes, we can start exploring other possibilities. i'm not expecting much since you have no CEL. modern ECUs are very good and picking up small faults.
 

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You have only had the car a week...how are you determining your fuel consumption? Don't use the numbers on the dash display. Fill the tank and do the math.
Of course I'm not looking at the dash numbers, 5 days ago I filled the gas tank all the way up, drove the car for 100km(=62mi) and again filled it up until it was completely full. I did this to see how much fuel it consumes per 100km and the number was 16 litres.(=4.2gallons). Right now I'm trying another method. last time when I fully filled the car I also reseted a trip odometer. as you know Mazda 3 has 9 "Big Squares" and 6 "Small rectangles" on dashboard for fuel indication. I already drove 220km and Squares went down from 9 to 2. Mazda 3's tank size is 50L, each of "Big Squares" are equal to 5L. 5 x 7 = 35 | 35 / 220 = 0.159 | 0.159 x 100 = 15.9. and this is the math I'm frustrated with.
 

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that would be a classic symptom of a seized caliper. if you put your hand near the wheels (don't touch!) after a 15 minute drive i'm going to guess one will feel hot.

if you find this is the case, here's some info. feel free to pass this on to your mechanic if you're not doing the work yourself. this is IF it is a rear caliper seizing.

- there's a recall for the parking brake. the cable that actuates the calipers has a rubber boot at the caliper end. the original boot can leak, make sure this recall has been done as it can cause a caliper to seize

- the rear brakes like to jam up anyway. root cause is corrosion buildup UNDER the pad shims. this pinches the pads and causes them to seize. this is not to say that other causes such as stuck slide pin or stuck piston shouldn't be investigated. fixing it is actually pretty easy. knock the pads out, you'll probably have to pound them with a hammer. before installing new pads/shims, use a heavy metal file (bastard file) on the channels where the shims go and the pad tabs ride. your goal should be to widen them a little. the corrosion has to be removed anyhow. next put a light coat of caliper grease there before putting the shims in. this will help prevent future corrosion and give it some room if it does start to form.

if it turns out not to be brakes, we can start exploring other possibilities. i'm not expecting much since you have no CEL. modern ECUs are very good and picking up small faults.
I'm definitely going to check if any of the wheels are getting hot and thanks for the info, it's really helpful.
 

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You can't really figure fuel consumption by driving a few miles or counting the lines on the fuel gauge. Neither way is real accurate.
The way the rest of the world has figured fuel consumption for years goes like this -
Fill the tank, record the odometer reading. Drive the car until the tank is getting towards empty again, fill it and record the odometer reading. Figure out how many miles the car drove between fill-ups. Divide the distance traveled by the amount of fuel used.
If you still only get 15 mpg, there is a problem. A bad coolant temp sensor could do this, or an O2 sensor that isn't working right, or a malfunctioning MAF sensor, or a number of other things or a combination of things, including poor quality gasoline.
Its also possible that sitting in traffic will lower your mileage considerably, more than you may realize. The reality is 30 mpg is a bit of an overstatement for your car unless you do some highway driving too. Constant stop and go is a killer for Skyactiv gas mileage.
 

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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.
 
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