Hi Jai! Yes, driving at a high speed with the windows down can affect the fuel economy due to increasing aerodynamic drag. Thus, the car consumed more fuel compared to driving at high speed with the windows down. Other external features such as roof racks and spoilers also affect the aerodynamics of a vehicle.
To find out more tips on fuel efficiency, you can click on this article - https://www.motorist.sg/article/168/8-fuel-efficiency-tips-to-help-you-save-more-money