highway cruising with 4.10 gears

i have a 2001 dakota V6 3.9. auto trans came with 3.55 gears. in town i was getting 8-9 MPG. OK i drive hard get over it. i was tired of low power so i put 4.1 gear and trak-lok. MPG went to 11-12 MPG. how? the O.D. trans will now go into O.D. at 30 MPH. at 1200 RPM. it seams to be the perfect gear for driving under 40-50 MPH.