Well, we need more info, was the 25.5 tire used when you ran 16's? If so, then 3.73's would be good, if not a 4.10 ratio would be better because the bigger tire than a stock tire, will decrease the final ratio, the 4.10 would compensate for the raito loss and end up being around a 3.73 or so, to get a true 4.10 which I doubt you would need or want because of some streetability issues on interstates and such, you would need a 4.27 raito.