Joe you need to remember lower RPM's don't mean better milage! If you are happy with your milage now.. then you need to keep the rpm's after the gear change to what you have now or MORE! Since the tires will be harder to spin you will need a few more R's to roll em.
The most common problem I see with installing larger tires is the engine lugs and a lower tranny gear is needed to simply get around town. So you run around in 4th and never see 5th again.
When going up to HUGE meats many more R's will be needed to keep the same or equal "feel" to the ride.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

professional bovine relocation specialist