Can't understand my MPG

ITD · 10-05-2022, 09:32 AM

Quote:

Originally Posted by SailinAway

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.

DickR · 10-05-2022, 10:18 AM

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.

ITD · 10-05-2022, 11:12 AM

Quote:

Originally Posted by DickR

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.

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.

AC2717 · 10-05-2022, 11:40 AM

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

LikeLakes · 10-06-2022, 01:49 AM

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.

John Mercier · 10-06-2022, 07:17 AM

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...

thinkxingu · 10-05-2022, 10:18 AM

Quote:

Originally Posted by ITD

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.

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

Sent from my SM-G990U1 using Tapatalk

10-05-2022, 11:40 AM	#4
AC2717 Senior Member Join Date: Apr 2007 Location: Maynard, MA & Paugus Bay Posts: 2,587 Thanks: 756 Thanked 356 Times in 268 Posts	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 __________________ Capt. of the "No Worries"

10-05-2022, 10:18 AM	#2
DickR Senior Member Join Date: Apr 2008 Location: Moultonborough Posts: 758 Thanks: 4 Thanked 260 Times in 172 Posts	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.

The Following User Says Thank You to AC2717 For This Useful Post:
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10-06-2022, 01:49 AM	#5
LikeLakes Senior Member Join Date: May 2021 Posts: 339 Thanks: 50 Thanked 92 Times in 66 Posts	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.

10-06-2022, 07:17 AM	#6
John Mercier Senior Member Join Date: Jun 2021 Posts: 3,563 Thanks: 3 Thanked 635 Times in 522 Posts	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...