Hi Sue....welcome to the Club
You certainly like your convertibles then?
That does sound a bit of a low economy figure, have you tried calculating the fuel consumption e.g. fill up with a full tank, reset the odometer, drive some miles and refill with fuel, measure the quantity in gallons taken to refill and how many miles driven, divide the two and then you have an accurate reading. It may be the readings from the MPG display are wildly out.
Alternatively, you may have issues with the engine/fuel system so worth getting checked out as it should give better MPG than what you are getting.
Let us know how you get on with the actual calculations
Cheers, Trevor