How much gas would I use weekly?

I'm thinking of buying a new car. Here's the info:

MPG = 20 City / 31 Hwy / 23 Combined (average)

Gallons = 18.5

Gas = $3.60 (my area)

My Miles = 1,800 Monthly / 450 Weekly

This is what I understand:

Gallons X Gas = Price of Full Tank >>> 18.5 X $3.60 = $66.60

Gallons X MPG = Full Tank Miles >>> 18.5 X 23 = 425.50 mi

So every week I would spend about $70 to fill up the gas tank? Excuse my terrible math skills, but given the info above how much gas will I use in a week and how long can a full tank last me?