rear defogger / average mpg

Bought a used 2004 Camry 2.4 XLE with 80K miles last week. It rides nice and is quiet. There are two minor things about it that bother me.

1. The digital readout for the average mpg has steadily risen from

28.0 to 28.9 since I've owned it. After filling up today and doing the calculation of miles driven divided by gallons pumped I came up with 24.7 mpg. Why such a disparity?

1. The rear defogger seems quite anemic to me. We haven't had much frost but it can't seem to clear even dew from the back window in the ten minutes or so to work. I mean there's no separation occurring between the filaments at all. The thing is on and you can see it barely working, but its weak and slow.