Ryland,
You've asked for an estimate in the improvement in fuel economy by changing to Low Rolling Resistance (LRR) tires. Probably the best way to describe the situation is with an analogy:
Imagine I put you in a car and have you drive very conservatively around a long loop of the city and measure the fuel consumed. Then I have you start at the same spot and instruct you to drive like a mad man over the same route. Would the fuel consumed go up? Of course it would! But the tires on both trips consumed the same amount of fuel. They only cared about the distance traveled - so the percent the tires contributed to the fuel consumed changed between the 2 trips, even though the amount didn't.
But tires vary widely for Rolling Resistance (RR) - as much as 60% - and what you change from and what you go to is going to have a HUGE effect on the amount of fuel consumed per mile.
- BUT -
RR is not without its penalty. Treadwear and traction are both adversely affected by the things done to improve RR. It is quite conceiveable that you would have to replace tires twice as often - and that, of course, is going to affect the cost calculation.
- PLUS -
There is no standard as to what "LRR" means.
Bottonline: There are just too many variables and too many unknowns to give you even an estimate. Clearly, LRR tires will improve fuel economy compaored to High Rolling Resiance tires, but trying to calculate the $$'s is next to impossible.
BTW, I cover a lot of the engineering aspects of RR here:
Barry's Tire Tech