I would think that its OK to be a little heavy in the rear with a full tank of gas since the car is rear wheel drive. You also probably shed a little weight by going to 18's in the rear. I would try going to 120 lbs. I have no idea what it will do to your gas mileage though. You might get a little worse mileage lugging around the added weight. Still it's better to be safe and get a mile less per gallon than end up stuck or worse with such a long commute.