If on the rare occasion I use my car for work, I tend to drive it exactly the same, regardless of roof up or roof down. Not heavy driving, but not dithering about either.
Same round trip with the roof up, just under 1/4 of a tank,
Roof down, just over 1/4 of a tank
It works about at just under 2 Litres of burn difference across the two comparisons over 150 mile round trip.
It might also be worth noting that if I don’t have the roof down, it’s usually because it is a lot cooler, or raining, and both of those factors will only make a petrol engine run better, so not entirely down to drag in my case.