Check out this explination on tires and there psi to weight figures
http://auto.howstuffworks.com/tire4.htm
it says u can guess pretty close to the weight of your vehicle just by looking at the area of your contact patch and yes i understand that your not going to be exact or anything with that calculation but i think you would get pretty dang close. I don't think that friction with the road is all RR is about but if you increase the tire pressure not only are u decreasing the contact with the road but the amount of deforming the tire does therefore less energy converted to heat in the tire now if you could get class D tires with 80Psi max and with the low rolling resistance material then you'll have really good FE gains. i got the idea from watching mythbusters the other night they took tires that had a max psi of 35 and increased it to 40 and got a 6.5% increase in FE and thats 2 points better then the best LRR tire i've seen so far but if you find a better one then let me know ok.(and yes i know i'm not goin psi to psi ok but it steal works)