i have a pair of front and rear diffs for any cosworth/xr car, these are a lower ratio thus improving the accelleration of the car.
using the front 3.9 as well as the rear means no changes are required to the gearbox step offs.
if you are concerned about the topend loss, then as an example if you are on 225/40/17 ( as an average wheel size) then in fifth at 7500rpm in fifth you will still be pulling over 165mph

i want £200 for the pair ovno plus delivery is extra, they will be on ebay next week if i dont get any interest !!