If your mileage is that low at that speed, something is wrong. I get 31+ mpg driving at 65-70 mph; 29+ driving 75-85 mph. I typically get 23-25 mpg driving mixed city/highway during the week to and from work.

My mileage was dropping for a while until I changed out my spark plugs (NGK Iridium) and wires (Magnecor) - as soon as I changed them, the mileage/performance jumped back up. I inspected the old plugs and wires and couldn't see anything wrong, but it sure made a difference.

What RPM's is your engine at when you are at 65? It should be between 2000 - 2500 RPM - maybe you're not shifting into overdrive?