Anybody notice better mileage with a cap or tonneau cover?

I was wondering if anybody noticed better mileage with either, I have a buddy trying to decide to buy a cap or a tonneau and with one if any gives better mileage

Have a Leonard cap and don't notice any difference with or without. I think it's pretty much been determined that the air flow around a pick-up isn't hindered by the bed being open so probably the reason why my mileage didn't change.