Can't understand my MPG
 

How come my MPG always says 31 mph, no matter what speed or distance I drive? Are you supposed to reset that each time you fill up? I thought the MPG was supposed to adjust itself while you're driving so you can change your speed accordingly. Chevy Sonic.

It probably stays 31 because you've got so many miles and tanks through that the average isn't going to change.

If you want an accurate measure, you should be resetting that at every fill-up AND calculating it against your regular trip computer vs. gallons used.

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I think it's an average, my Corolla always says 41 MPG. It does have a reset button but always ends up back at 41 MPG, maybe it's my driving habits.

My 2010 Corolla
 

My 2010 Corolla doesn't update as often as it did when new, but will still update in smaller increments, sometimes over a period of a week or two (or tankfull or two).

My 2018 Camry, on the other hand, does not update unless I do it through the "Menu".

I can update the menus on the Corolla by pushing the odometer dash button for MPH, Average MPH, odometer, and miles driven. The Camry is not so easy. Newer isn't always easier.

Dave

Fill your car up, record your odometer reading. When you need gas again, fill it up again, record your odometer reading again. Record how much gas it took to fill your tank again. Subtract the first odometer reading from the last, that is how far you went. Divide how far you went by how many gallons it took to fill your tank, that is your mpg. Use that number as a sanity check for your mpg meter. It should be close.

2013 Subaru.
It has two trip odometers.
I use the 'A' one to record miles on the tankful. (no need to record mileage and do the math).
Reset it to zero at every fill up.
The AVG MPG resets when the odometer is reset.

Salin...
You may need to check the owner's manual.

My Accent resets with every fill up.
It will change my average fuel economy up/down about every 1/4 mile or so.
That number moves very fast after a fill up due to limited data for the average, and more slowly as I approach the time for another fill up.

But I only focus on the items that I can readily change... tire pressure and overall driving distances. I can't control the flow of traffic, or how cold it is outside - requiring my engine to heat up or for time to defrost the windows.

Sorry, everyone. Turns out my car does have a reset button. I'll write down the miles the next two times I fill up and compare the result to the mpg on the instrument panel. I think 31 mpg is too low for my conservative driving style. Some people claim 40 mpg with the Chevy Sonic, but I don't believe that at all.

I was amazed at how much leaving your car idling affects mpgs. Sometimes it's unavoidable, but I generally just start up, let it idle for about 5 seconds, then go, driving gently for the first few minutes. Keep that in mind, if you let your car warm up every morning before you drive, that kills your efficiency.

I have a Subaru Imprezza, with CVT for transmission ("continuously variable"). City/Highway estimated mpg was given as 34-37, about 3 mpg better than for the 5-speed manual transmission. I do get that 37, but mainly in warm weather driving, with a lot of long runs where the engine stays warm, and on roads that don't have a lot of steep hills. MPG gained on the downhills does not offset the loss going up the hill. Also, a lot of short runs starting with a cold engine really cuts the mpg. In winter, the mileage over a tank often is just in the low 30s. The sweet spot is summer driving on long country roads, with speed limits of 40-50, when I see mpg up in the low 40s over 40-50 miles. Trips at 65-70 mph show a notable drop in mpg.

I always fill the tank when refueling, rough out the miles/gallons mentally, then reset the trip odometer. This resets the running mpg indicated on the dash. What I see is that actual mileage runs on the order of 2 mpg lower than what the dash number shows. The latter is obtained by integrating what the instrument thinks is rate of fuel to the engine and dividing by accumulated miles, so it's easy to understand how accumulated error can result. Still, it's a good reading to have, as a measure of how things are going, as I can just subtract 2 for a "good enough" estimate of actual mpg. There is one display option that shows "instantaneous" mpg, updated every few seconds, which is interesting to watch, although fairly useless for any purpose. At constant speed over a level road, the instantaneous reading is a lot higher than the average shows. A stretch of road that appears to be level may be imperceptibly uphill or downhill, and that is seen on the instrument, and it doesn't take a super-steep hill for the reading to max out at 99.9 mpg.

Yup, and today's vehicles don't need to be warmed up like those in the past.

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The fuel formulation changes for warm weather driving versus cold weather driving. The summer gas has about 1.7% more energy per gallon than the winter gas. The summer gas is less volatile to help with emissions and vapor lock issues.

The difference in energy between summer and winter means your car will go further on a gallon of summer gas versus winter gas. Summer gas is also more expensive to make...... supposedly.

If you never reset the tripometer, the vehicle calculates the average mileage per gallon based on the total on the tripometer. After 400 miles or so on that tripometer, it is very hard to move it off the average that it is currently sitting

you have to reset the tripomenter to get a new average calculated. Can do it the old fashioned way, fill up the tank, count the miles from that point until you fill up again. divide the miles traveled with how many gallons you put back in the tank

If you truly are interested in your mpg, you need to hand calculate it like ITD said early in the thread. None of the onboard readouts are accurate, and in my experience are always slanted toward the high end, meaning if your dash says 25, you are probably getting 23 or 24 actual.

When I was really interested in accurate mpg numbers, I'd calculate over a series of tanks, maybe take 2 fillups before doing the calculation. That eliminates factors like one day caught in stop and go traffic lowering your mpg or one day with an unusual amount of 50 mph cruising raising your mpg.

My on-board seems to be right on the numbers when I do the manual.
Other than a splash here or there due to how the pumps handle the shut-off...