Strangely enough, I answer this question at least once a month. I put a link to an article on this very subject. changing the gears is going to cost $300 in parts alone, so you want to feel something for the money. forget the 3.73 gears. No kick for the money. you will feel something with 4.10, but I like 4.30s. 4.30 has a kick, and mileage is not effected as much as people say (maybe 1-2 mpg less) over stock. On my Ranger it improved around town. If you have hills in your town as mine does, it really helps. I only wish I put in the 4.56 now. as far as top speed goes, well how often do you drive 160, or 140, or 120? your top speed is going to be around 140 basic on engine rpm, but your ecu will limited you before then.