Gas mileage decreases when parked

[Elbass74]

Picked up a new CX-5 in late Feb and one curious thing I have noticed is that the avg gas mileage reported on the dash seems to decrease every time I park. For example I take a half hour trip and when I reach my destination the mpg might read 28.4. I park and 8 hours later come back to my car and start it up, right away my mpg reads 27.9.

Has anyone else noticed this with their Mazda? I came from a 15 year old vehicle without a mpg readout so I'm not sure if this is normal or not but it seems funny that the mileage would go down while the car is sitting.

 

[MikeM.]

It shouldn't go down when sitting. What you're probably seeing is the rounding error that happens.

Because the average MPG is only updated every mile it's not always displaying the most recent figure. If you shut your vehicle off 0.6 miles from the last update, it probably updates and stores the new number (which is what you would see when you turn the ignition on).

 

[TommyW]

Currently I am averaging 30.3 awd. I love this car. Doesn't gi down overnight.

 

[GAXIBM]

The gas gage likely uses a variable resistor that may be affected by temperature change. If the temperature changes while parked it may indicate less fuel remaining in the tank and thus a lower average mileage.

 

[mazdadude]

Picked up a new CX-5 in late Feb and one curious thing I have noticed is that the avg gas mileage reported on the dash seems to decrease every time I park. For example I take a half hour trip and when I reach my destination the mpg might read 28.4. I park and 8 hours later come back to my car and start it up, right away my mpg reads 27.9.

Has anyone else noticed this with their Mazda? I came from a 15 year old vehicle without a mpg readout so I'm not sure if this is normal or not but it seems funny that the mileage would go down while the car is sitting.

The area that displays the mpg you are talking about... using the INFO button on the steering wheel, you should be able to toggle between the different readouts in the display. Such as the: Instant or current MPG, the Average MPG since last reset, the average speed since last reset.

Is it possible you are looking at your average SPEED, and not MPG?

When I fill my tank, I reset the AVG MPG, and AVG Speed by holding the INFO button in for each item I want to reset.

 

[Elbass74]

Thank you all for the replies. It's definitely avg mpg that have been noticing. It's possible it is a temp thing as here in WI the temperature changes a lot throughout the day. I'll have to keep a log starting with my next fill.

Thank you for the replies so far. Interesting that no one else seems to notice the same behavior.

 

[piotrek91]

Thank you all for the replies. It's definitely avg mpg that have been noticing. It's possible it is a temp thing as here in WI the temperature changes a lot throughout the day. I'll have to keep a log starting with my next fill.

Thank you for the replies so far. Interesting that no one else seems to notice the same behavior.

Are you looking at individual trip MPG?
Or the lifetime MPG?

Say you drove 45MPH for 10 mi and you got 40MPG, but during the last 1/4 mi of your trip you had 2 stop signs and only got 10MPG.
You parked your car before the MPG display had a chance to include the two stop signs in the average, so the car is still showing 40MPG. When you start your car the next day the car will show your MPG as 37.3 because it now calculated the last, inefficient 1/4 mile into the average.