Realistic gas bills?

Hi guys!

I'm thinking about buying a pickup, and spotted the F150 (V6 4.2L) since it seems to have an engine as hungry as Chevy/GMC/Dodge's mid-sized pickups, but has a more spacy bed than these other mid-sized pickups.

I'm at the point where I'm actually trying to forecast what would be the gas bills, since this will determine whether I can afford the truck or not.

I'll probably get a used one (95-97) and will make sure it has been well taken care of in the past.

Is the MPG on paper reliable VS reality? Does age affect the MPG even if the truck is in good shape? Does winter affect MPG that much?

I'll probably roll for 13k miles a year with it. That makes 1083 miles a month. Calculating in a 'worse case' scenario, the truck is said to make 20 miles a gallon (city MPG).

Gas is now 0.93$ CDN/L (which equals to 3.52$ CDN US gallon)

With these numbers, that makes a 190$ CDN monthly gas bill.

Am I getting real here?

Thanks!