MPG/Idle issue!!
 

Hello all.
So, it's a 2014, 40k on it, 3k by me. I was getting 9 or so litres per 100 km (I haven't figured out how to display in MPG.) All of a sudden, this...
I started getting awful mileage, like 12, 14 L/100k. This badness occurs while idling. If I reset, it automatically shoots to 50 L/100km. As I begin to drive, it will come down, but while stopped in traffic, drive through, etc., it rises quickly..12.1, 12.3, 12.4, and doesn't stop until I drive again. I have yet to manually calculate my mileage, (started with a full tank today), but it doesn't seem good.:(
I hope someone can help.

You say only 3k miles by you, is it safe to assume that the battery may have been disconnected/replaced shortly before you got the car? The mpg (in US here so I'm just familiar with mpg, not this L/km) calculated by the car will reset when the battery is disconnected, and it does vary quite a bit. I have a '17 AT and I average something like 23 mpg with just regular commute (mine is a short commute so nothing even gets up to temperature) but can easily get into the 30s with extended highway drives.

Possible disconnect, but not by me, and it was not doing it before. Watching the "mpg" climb steadily, and seemingly infinitely while stationary is very painful. 9 L/100 is very good. I don't get 9 anymore. And L/100km is sillyness. When I change km/hr to miles/hour with the button, you would think the mileage display would change too. It doesn't.

Stop resetting it and let it get a true average. By starting it over all the time you are skewing the average to the high end because it is counting the fast idle at start up as a higher consumption since it is cold out and it idles longer. Don't go by the live numbers at all just use the average setting. The live numbers show only what you are using at that point in time and since you are not moving there is no miles so they shoot up until you start moving again. You will see an average 1 to 1.5 L/100km increase in the winter and that is normal. Leave your average set for at least 100kms and you should see all is fine. Nothing wrong with the car you are just misinterpreting the data.

The l/100km is based off your odometer not your speedometer. The odometer can not be changed to miles.

Quit worrying about what the car display is telling you. Keep track of you distance and how much fuel you put in your car. This will need to be done over multiple tanks to get an average and reduce the errors of not filling up to the same amount every time.

(NOTE: My comments are in MPG, but they are appropriate with either measurement)

@12foot I assume you know that while idling you are getting zero miles per gallon, correct? That is why it shoots up.

As others have said, it will vary wildly if there are very few miles/km since the last time it was reset, or if you are watching the "actual".

Also the calculation on the car is not as accurate as measuring it by the tank. If you watch the "real time" numbers one of the things you'll notice is it tops out at a certain number (I think it's 99 MPG, but not sure).

I have measured my MPG since the beginning (check my Owner's Journal or Fuelly in my signature. What I've discovered over time is that the car calculates it a bit higher than what my calculations do but its usually within about +-1 MPG. I never reset the car measurement intentionally (I think its been reset twice, once with a new battery and recently with the valve spring service). It is usually steady as a rock when driving because it has thousands of miles on its calculation.

The gauge is reasonably accurate but will differ from the manual calculations since it is measuring mileage during more types of driving than the straight calculation. If you fill it with gas and just let it sit and idle until empty both the gauge and the calculation will read 0 MPG. If you let it sit and idle for half a tank and drive for 100 miles the other half the gauge will calculate an average based on the total usage which will differ from the calculation by a considerable amount since you do not calculate that gas used idling but the gauge does.
In this case the OP is plan and simple using the wrong gauge. The real time gauge will shot up and down dramatically but it is totally meaningless data. If left alone the average gauge will come surprisingly close to the calculated mileage. It needs time to set a stable average though. I never reset mine and it spends all summer parked firmly on 6.8. During the winter it slowly climbs to 7.4 before dropping back when it warms up. Now in fairness my driving pattern is almost precisely the same every single day so there is not much change but even if you do varied driving your average gauge shot not be shooting up and down. At worst it should slowly creep up or down.

I reset the avg mpg and trip odometer A every time I fill the tank, and also keep a running spreadsheet calculating actual miles driven divided by gallons required at fillup. Over three years and abot 50K personally-driven miles (currently car has 71k on it...bought it used), the car-calculated display has been consistently slightly more than 1 mpg optimistic than actual. My mpg has also decreased slightly, from a best of 40mpg hwy/33mpg routine use during th first year of ownership to about 35mpg hwy/28 mpg routine use this past year. No explanation.

Every fill up is a reasonable time to reset since it give you enough mile/kilometers to get a decent average. As long as you are doing the same driving each time though. If I got gas and reset and drove around stop and go traffic it will give me totally different reading than if I reset and went on a 300 mile highway road trip. Both would be accurate according to the type of driving but may not be my "normal" average.

My Tacoma displays "Average" and "Tank Average". They are usually very close, unless the current tank has been through a lot of idling, or a lot of highway time.

I also find the live fuel display useless, although I would love to be getting the 152mpg or whatever it displays on decel.

I also hover right around 27 on the average display, manual trans with DD/spirited driving use.


And since this is in the Warranty category... We deal with this complaint all the time at work. "My car doesn't get the fuel economy printed on the sticker!" Do you drive around with extremely light and even throttle? With the windows up, AC off, radio off, lights off, absolutely nothing turned on but the engine? Probably not. So just drive the car and be happy.

That's why I keep the spreadsheet... it keeps a running tally of tank averages since I purchased the car in chronological order. Helps identify any trends (specifically, the decline in overall mileage that happened a little over a year ago) and unusual spikes up or down can be explained by comparing the date with the type of driving I was doing during that time period/date.

I agree the gauge is reasonably accurate. As most are now. They generally have them programmed to be a little optimistic. Mine is about +1. If he does the real gas mileage way he can eliminate any issue the car is having calculating. Not sure how it would be off but it is possible.

Not sure if it is a programing thing or simply the fact that it measures more accurately when the car is not moving. You of course can not calculate idle time manually so instead of being reflected in the result it just sort of gets buried in the total which would of course be reflected by slightly worse mileage readings.