Question 864058
1287/18=71.5 gallons outgoing 
1287/26=49.5 gallons incoming  

22 gallons difference
11 gallons added against the wind and 11 gallons fewer with the wind

60.5 gallons with no wind 21.27 miles per gallon with no wind
2) 
I would expect usage to average out with the wind and against the wind. Otherwise we have no way to calculate anything. Any differences comes from rounding off.