I've been trying to figure out why the average MPG display (Ave: ...) doesn't match the MPG I calculate using the odometer and the number of gallons I use at the pump. I've read the related posts here:
- The average is not just over the last 15 minutes
- The average is reset with the Reset button
- The Ford folks say it is a running average over the last 2000 data points
- We assume the 2000 data points are the per minute data points we see on the 15 minute graph
What I can't explain is that the average displayed is significantly lower than the average I calculate at the pump. But I have a theory. I think the average display is calculated based on the last 2000 data points, but each data point is the MPG calculated (and shown in the graph). The data points are not gallons used and distance traveled, they are the MPG calculated for each of the last 2000 minutes. So the running average is an average of the last 2000 per-minute averages. This means that the running average is over time, not distance.
If the running average is over time, then a low MPG while going slower (uphill for example) will skew the average lower than true MPG. I think this is what is happening in my case. We drive uphill 8 miles on a bumpy dirt road to our house, getting low MPG.
I tested this theory by resetting the average before driving home. The drive home is in two segments, the first segment driving 70
MPH on the highway and the second segment driving more slowly up the dirt road. At the end of the first segment, the average reads 31.4 MPG. After the second segment it reads 26.7 MPG. The question is: How many MPG did I get during the second segment alone?
MPG Miles Minutes
31.4 42 36
X 8 20
Ending average: 26.7
If the running average data points are gallons and miles, then X must be 2.0. This isn't possible.
If the running average data points are per-minute averages, then X must be 18.4. This is about right.
If I'm right, to get true MPG the gallons at the pump and the odometer reading should always be used. The average display is not true MPG.
Am I right?
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Here are the calculations I used to get 2.1 and 18.4:
26.7 = ((31.4 * 42) + (X * 8)) / 50
1335 = 1318.8 + (X * 8)
16.2 = X * 8
2.0 = X
26.7 = ((31.4 * 36) + (X * 20)) / 56
1495.2 = 1130.4 + (X * 20)
364.8 = X * 20
18.24 = X