if your backhalfing I would go with ford 9 in. tire size & gear ratio are two separate things, you need to choose the combinationof the two to achive the the desired rpm best for your set up.
Old tire rollout divided by new tire rollout=x.Old gear ratio divided x= new gear ratio.Old tire 90 divided by new tire 102=.88 4.56 divided by .88 =5.18.Put your numbers in.Bill.