They obviously change top speed by lowering it and will most likely cause a decrease in gas mileage. Shifting RPM's may not change, but you will get to those RPM's MUCH quicker.
I have the 3.73 and love them. I also can still get pretty good mileage - 19-20 around town and suburbs, 24-25 on hiways. I have even gotten as high as 29 on the hiway before. I must say though that my transmission is geared a little higher than the stock T-5, so my set up may be a little closer to a 3.55 gear with the stock T-5. I assume you have a T-5 or T-45, in which case you will probably be happy with either the 3.73 or 3.55.
Without doubt one of the best bang for you buck performance mods you can make! Enjoy
