3.73 Gears Mustang Gt Installation?
Ring and pinion swap shouldn't cost too much, definitely no more than $500.
As for mileage, I have 3.73s in my 98 Cobra (previous owner put them in...shoulda put in 4.10s), and I will say, I do quite well on mileage. A 98 cobra was originally rated at 16/24mpg city/highway, and I average about 23mpg mixed city/highway, and I've gotten as high as 34mpg (really!) on the highway. So you shouldn't even notice a difference. Especially if you don't try to do 80+ everywhere. When I cruise at the speed limit (65-70) is when I get the best mileage. Slower or faster and it drops.