I have been observing significant discrepancies between the MPG reported by the Energy Monitor and that calculated manually by dividing total no. miles by the number of gallons to refill my tank. Wondering if anyone else has been observing this issue.

Today, when I went to fill my gas, my Energy Monitor reported 27.1 MPG (and that only covered the miles since my previous refill). However, the manual calculation gave me only 25.2 MPG. That's 7% lower!

I did have my A/C and/or fan on for about half of the total miles travelled, but I'm really surprised that that would result in a 7% reduction in my FE. Especially since I had the setting on low for the times the A/C and/or fan were on.

Has anyone else measured such a large discrepancy? I also had my CD player playing for almost the entire miles covered. And for about 20 miles, I had the butt warmers on. But, again, I would expect that to affect the FE by nominal amounts (~1%)