Had to refuel today...numbers are (aero cap hatch off, ducktail in place) 18.44MPG.
1998 Toyota T-100 extended cab SR5 Gas Mileage (BZP T-100 (2011)) - EcoModder.com
I know it's not definitive...but there is a slight drop fer my setup. I did have to carry some stuff in the back during one trip...so not sure if that skewed my numbers too much or not. I did notice that after I put my new aero cap and hatch together...my Ultra-Gauge did go up by .1MPG on the trip MPG before I refueled today...and that was over the 70 miles after I put the new aero cap and hatch together.