The top up with windows up will get the best gas milage. Top down with windows up will get the second and top down with windows down will get the worst, but will be the most fun. Don't forget the sunblock if the weather cooperates and enjoy the ride.

 

My brother's corvette has an instant MPG reading. His corvette MPG decreased by about 5 MPG on the interstate with the target top off and windows down vs. on.

 

I believe this may attempt to answer the question a little more quantitatively.

I once read an article about cooling a car with AC versus open windows at different speeds given the fact that opening windows increases the drag coefficient and worsens fuel economy as the other posts have accurately stated. The article stated that for the average car (whatever that is), the drag on the car increases significantly at about 45 MPH such that the advantage of not running the AC is defeated.

Therefore, the take home message is:

if you are traveling in excess of 45 MPH it is usually more efficient to roll up your windows and turn on the AC.
Conversely, if you are traveling below 45 MPH then it is more economical to roll down the windows.

Obviously, if cooling is not an issue, then windows (or top) up is more economical... which defeats the purpose of having a wonderful convertible on a georgeous day.

I can only guess that a similar MPH threshold would exist for a convertible.

 

Watch mythbusters...just go behind a semi and ull see about 35% increase in MPG (drafting) i believe thats what its called...

 

milage be dammed, I like topless.