At 65mph, something like 80% of your total drag is aero drag. Wind is directly proportionate to it's speed, so if you're doing 65mph into a 25mph headwind, you have to overcome the drag equal to doing 90mph. About the only thing you have going for you better than if you really were going 90mph is that you still only have the internal engine friction losses, drivetrain, and rolling resistance associated with 65mph. Likewise, if you'd had a 25mph tailwind, you probably could have seen your mileage go up to 70-75mpg or so.