Help Please!

Ok HERES THE DEAL just got my car about a week ago....had about 63 miles when i got it, and now has about 480. Ok my mpg is 12.1. Im finding most of the GT owners around here getting at least 14 mpg. I was thinking it was my tires. I checked and their psi was about 30-32....:?: SO i went to my local gas station and pumped them up to about 40 infront and 39 in back. Is this good..or should i lower the psi alittle? Could that of been my MPG issue???? thankyou!!:wavey