Strange problem. Filled up with Phillips 66 gasoline in Wichita, drove to Hutchinson (approx. 45 miles) and showed that I was getting 33 mpg. Mileage dropped to 25.6 mpg driving around town in Hutchinson for several days. Got back on the highway and the mileage increased to 26.6 mpg. . . but went no higher. This is a recurring theme for my 2007 Avalon. Good mileage after filling the tank, but after the expected decrease in mileage driving in the city, why doesn’t the mileage go back up with the same tank of gas when I get back on the highway?
My 2006 Sienna calculates mpg two ways, cumulative and instantaneous. The cumulative mpg is calculated from the last time the mpg readout was reset regardless of how many miles driven or the driving conditions. The instantaneous mpg is calculated once a second and jumps around a lot. I still prefer to calculate mpg over a 3 or 4 fill-ups based on miles and gallons used.
In your case, if the mpg was reset at the fillup the first mpg was highway only, the second was a combination of highway and city, and the third was highway, city, and highway.
The owner’s manual will probably explain it better than I can.
Use the tried and true gallons used divided by miles traveled and don’t just use the data from on tankful for your result.
The variables in this method are, you may not be able know exactly how much gas you used or exactly how far you traveled, any way you can improve on the accuracy of these two variables will give you a more accurate result.
What margin of error is acceptable to you?
This is like the 3rd one of these this week. Didn’t anyone take any math classes in high-school???
It’s IMPOSSIBLE to get an accurate reading is such short miles…IMPOSSIBLE. Second…the MPG monitoring systems are NOT accurate…NOT even close.
The ONLY way to really tell what your mpg is…is to take the average over several gas tanks. Fill the tank up…record the mileage. Next fill up…record the mileage again and write down how many gallons it took to fill it up. This will give you the gas mileage for ONE tank full…Do this for several fillups and you’ll get a much better reading. Also the more mileage you can put on the car for each fillup the more accurate a measurement you’ll get.
Actually I’ve been pleasantly surprised by the cumulative mpg readout on my 2006 Sienna. Over 3 or 4 tanks of gas the cumulative mpg is consistently only 0.5-0.6 mpg higher than the actual mpg I calculate the old fashioned way.