Here is an A/B test to determine what a fold out RTT does to your economy.
First, 22 mpg is pretty great for that T-reg on At’s. Second, thats a big difference in economy.
Obviously the delta is going to change for a vehicle that it already un-aerodynamic like my Land Cruiser, or a vehicle that is already using a lot of fuel generally because of lack of power (i.e. me). so it would be interesting to test on a few different vehicles. I would guess 15-20% is a good average guess.