Auto Doing Manual Start

I have a 07 Mustang Gt
-Cold air intake
-Bama Tune
-Pypes cat back
-Auto Tranny

- and i drive it all the time. my gas mileage when im home is an average of 19.5 (including flooring it several times a trip) lately i have played with getting better gas mileage when i am not letting off some stress. i have found that if i lock the car into 3rd and then back to "D" at about 1800rpm, my gas mileage increases noticeably. My question is whether or not doing several set offs from zero in third gear with an auto tranny and just shifting back to "D" will harm the tranny in any way?