This is a trick question, because if you don't know how to claculate for the tire size, your mpg is going to look much worse than it really is.
Rate of change: how much larger is the new tire than the old? 34.6/31.5= 1.09
This means the rate of change is 9%. In simple terms, if your odometer says you went 100 miles, you really went 109.
Trip odometer x rate of change/gallons at fill up = mpg. To get the most accurate numbers, fill that dude till it comes out the neck during these tests. It could "click off" at different points giving erroneous results.
287 miles indicated and 22 gallons taken on would be 287x1.09=313 miles, divided by 22 gallons = 14.22 mpg. If you don't allow for rate of change, it would come out 13.04.
Do you see how it's a trick question now?