I don't recommend 3.90's. I run those, and it runs more like a 4.10 car than a 3.73 one; and the 3.90 doesn't seem as well matched to a NA engine. I have to shift to fourth in the quarter almost at the end of the track, and that one shift kills it. It drops a bit out of the power band, and I end up at 13.9 everytime. I ran better time in a GT almost identical to mine with 3.73's. My cousin swears by 4.30's. His car runs at a higer RPM on the hwy, out of the dreaded drone zone. The 4.10 should be around 2500 at 70, and the 3.73 about 2250 at same speed. I'm at about 2350-2400 at 70 with 3.90's. For the city the 4.30's actually improved my cousin's GT's gas mileage. He can leave it in second or third the whole time. I'm thinking of moving up to the 4.10.