Can't understand my MPG

[ITD]

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.