I cannot speak specifically about I-75, but we went to my cousin's wedding on one of the barrier islands off the Georgia coast weekend before last, flying in and out of Jacksonville, FL and driving a rental car for the three days we were there. We drove north on I-95 into GA. Used just under a half-tank of gas total. On our return trip, I should have filled up in GA just before we crossed the state line, since this is close enough to JAX airport that the gauge would still read full. Gas prices were $1.16 or so in GA and $1.66 or so just outside the airport (which may be higher than if I had filled up somewhere other than along the access road into the airport, but too late now). I think it cost me something between $3 and $4 extra. Funny thing is that on the drive out of the airport I noted the gas prices and remarked to Pam how much lower they were once we crossed into GA. I was just too lazy to make the stop before we left GA.
Bottom line, I would fill the tank in GA.
Ralph