Looks like your questions got answered brucelidat, but just echoing Mike, the shift in mpg will be enitirely dependent upon your driving style and the type of driving you do. If it is a lot of city driving with stop and go, I would not expect much help if any from these pulleys. If it is exclusively highway, they can be good for a nice 2 mpg. Mixtures in between will give you a result within that range.
