By the decrease of 17% in the mileage of Jack's car, the new mileage is only 83% of the previous mileage. To get the current mileage, we multiply the mileage by the percentage. That is,
new mileage = (41 mpg)(0.83)
= 34.03 mpg
Thus, the car's mileage in the headwind is only 34.03 mpg.