Inconsistent Mileage

K

k8-5

Member
:
2012 Mazda5 GT
Hey folks,

We have a 2012 GT that has been getting highly inconsistent mileage for the past several months (no CEL). When we purchased it new, we were getting ~27mpg. Today we have 36k on it and are ranging between 20.5 and 26mpg on a 80hwy/20city commute (no variables: we don't have any extra weight in the car, and we travel the same highway 75 miles round trip 5 days per week).

When I first noticed the drop, I checked the tire pressure and air filter - all was well. Regular, full synth oil changes every 7k as well. I ended up changing the air filter and ran a bottle of Castrol Syntec through a tank and saw no change. I brought it to our dealership and they drove it and stated it felt totally normal. IMO, the transmission does not feel the way it used to. I feel it prematurely shifts up to 5th, sometimes at speeds ~35-40mph. The service manager informed me that there would be a CEL if anything was out of spec. I just can't fathom why we'd get 20mpg one week, 23 another and every blue moon, 26mpg with no change in commute or driving style. Any info is appreciated.
 
Have you been filling up at a different gas station? Has your has station changed brands of gas? Different brand of synthetic motor oil or oil filter?
 
Good points and thanks for the response. We typically fill up at the same Mobil. Last week I tried another brand and haven't refueled since. I've had the oil changes done at Mazda, Tires Plus (Kendall oil) and I've changed it myself with Mobil full synth. I would understand the oil/filter issue if our inconsistency wasn't from tank-to-tank.
 
This may be unrelated, but if you are reporting shifting issues, and you are at 36k, you should look into changing the transmission fluid. It is unfortunately a dealer only item, MV type they recommend. I have been having issues with my tranny (110k on an 08) and through my research I have found it is recommended to change the transmission fluid every 30k or sooner. Don't do a flush, that can cause bigger issues. Just a drain and refill, and a new filter while you are at it. It is looking like the PO never changed the fluid in mine, and after only having it since January I am looking at a transmission rebuild.
A quick test would be to pull the transmission dipstick and examine the oil, even smell it.
 
Thanks for the advice - ATF is on my list of things to do.

This morning the trans shifted into 5th at 35mph... I watched the revs as I applied more throttle and it took a significant amount of input to get it to shift down to 4th. Obviously, the more throttle input applied equals more fuel being sprayed through the injectors. I'm curious as to why this wouldn't throw a code, and I feel it's directly related to our poor mileage. Has anyone else experience this?
 
k8-5 said:
Thanks for the advice - ATF is on my list of things to do.

This morning the trans shifted into 5th at 35mph... I watched the revs as I applied more throttle and it took a significant amount of input to get it to shift down to 4th. Obviously, the more throttle input applied equals more fuel being sprayed through the injectors. I'm curious as to why this wouldn't throw a code, and I feel it's directly related to our poor mileage. Has anyone else experience this?
Click to expand...

I wouldn't be surprised if the computer is forcing the upshift in order to access the torque converter lockup feature. IDK if it is available on gears 1-4, but would doubt it. When the converter is not locked, there is a decent amount of engine rotation lost inside the converter, at least enough to affect mpgs, which is why lockup converters started to roll out in the energy-conscious 90s. What rpms are you at when driving 35mph in 5th gear?

Back to the loss of mpgs: Done the usual?
a) change the air filter?
b) used MAF cleaner to clean the MAF (no touching, just like the instructions say)?
c) kept a log of variables like A/C use or heavy traffic/jams/roadside accidents/construction?
d) have you added/subtracted a routine that the car used to make, i.e. soccer practice several miles away that requires you to hit a dozen stop signs each way?
 
It could be something as simple as not pumping to the Full mark at every pump. In my experience, the pump click-stops at different levels from one visit to the next even at the same gas station. Tell me, do you notice the needle of your gas gauge begin dropping at around 30-40 miles from Full with one tank and then the next tank doesn't begin dropping until 60-80 miles?
 
How are you calculating MPG? I suggest fuelly.com as a great place to record accurate MPG information.

The dealer is correct, if there are no issues that the PCM/TCM is reporting, then causation of inconsistent MPG is not something they can determine. I can attest that when traveling on level highway, a good amount of throttle (>50%?) must be given before a 5-4 downshift is performed in our '08. This behavior is directly related to engine load. Manifold pressure and throttle % are monitored by the PCM when calculating load, but unless it determines that feedback from these sensors are not within parameter, then no code will be triggered.

I agree that an ATF exchange is likely due. Since I've always performed my own maintenance, I simply drain/fill our ATF at every oil change. Though this may not remedy your MPG concerns, the life of the transmission can be extended.
 
I have to agree with you that different gas pumps fill to slightly different levels. Sometimes I've noticed that my car will take less gas if parked at the pump facing uphill. When my gas light comes on, typically after about 300 miles on a "city" tank, it will take about 13 gallons. If it's much less or more then I suspect a filling issue, especially if my gauge drops below full before 50 miles on a new tank. For consistency I try to face the car downhill at any gas station I visit, within reason.


kickin_81 said:
It could be something as simple as not pumping to the Full mark at every pump. In my experience, the pump click-stops at different levels from one visit to the next even at the same gas station. Tell me, do you notice the needle of your gas gauge begin dropping at around 30-40 miles from Full with one tank and then the next tank doesn't begin dropping until 60-80 miles?
Click to expand...
 
Last edited:
kickin_81 said:
It could be something as simple as not pumping to the Full mark at every pump. In my experience, the pump click-stops at different levels from one visit to the next even at the same gas station. Tell me, do you notice the needle of your gas gauge begin dropping at around 30-40 miles from Full with one tank and then the next tank doesn't begin dropping until 60-80 miles?
Click to expand...

I rarely drive the 5, but my wife said "this is going to be a bad tank" recently because of the gauge reading. I've been calculating mileage (miles driven/gallons in) for all of our vehicles at every fill up for several years and have never experienced this before, but I guess it makes sense. I suppose I could start putting 10 gallons in for a few tanks to see if this is the culprit. Thanks for mentioning it.

Thanks to everybody for their info as well. I'll report back after a few weeks in case this thread can help others.
 
Well, we did 3 fill ups, 10 gallns each time and saw 24.xx, 24.xx and 23.xx - so that helped the consistency issue, I'd say. Thanks again kickin_81.
 
k8-5 said:
Well, we did 3 fill ups, 10 gallns each time and saw 24.xx, 24.xx and 23.xx - so that helped the consistency issue, I'd say. Thanks again kickin_81.
Click to expand...

Nice. That's right on track with what I've been getting. However now that its colder here in MN, the oil refineries are mixing our gas with higher ethanol mixtures effectively limiting our MPGs. I'm probably gonna guesstimate I'll only get about 21 MPG from the change in the gas mixture and rolling with winter tires.
 
My average is in the 24.x range, but my individual fill-ups vary from 21 to 26mpg depending one when the pump cuts off. For example, just this past weekend I was happy that I got a 26.5mpg tank, but then I quickly realized that the next tank had dropped a lot more than normal within the first 50 miles. The next tank was indeed lower, 20.5mpg. I don't know if it's the shape of the tank or the way the filler neck is, but I get huge tank to tank variation that always averages out.
 
Please log in or register to reply here.

Similar threads

M
Stuttering while lower RPM under load
Replies
0
Views
971
Meppen
M
N
2012 Mazda5 Intermittent Upshift issue
Replies
2
Views
924
n2gadgets
N
possumvan
    • Like
2013 Mazda5 L4 engine maintenance 67k
Replies
2
Views
2K
possumvan
possumvan

Latest posts

Back