This is pretty simple but I can't figure out what I am doing wrong:

Bought a 2002 Saturn for a kick around town car. It's a Canadian
car, so the odometer reads in kilometers instead of miles. It's a
very basic car and you can't set it to read in miles.

So, the day I picked it up it had a quarter of a tank of gas. I
stopped at the gas station and put in $20 worth. At $3.68/gal that
amounts to about 5.4 gallons. It brought the gauge up to about 3/4
full.

Drove the car around for over a week until the gauge read a quarter
tank again. According to the odometer, I had travelled 700
kilometers.

1 kilometer equals 0.62 miles. So, 700 kilometers time 0.62 equals
434 miles.

Divide 434 miles by the 5.4 gallons I had burned and it says the car
is getting about 80 miles per gallon !!!

I was just trying to get a rough idea of what kind of mileage it gets,
but this can't be right. What am I doing wrong? I feel like an
idiot.