Originally Posted by Sloth
I'm trying to figure that out myself. On my commute, the answer seems to be "not very", or maybe "it doesn't matter."
The primary driver of lower RR is inflation pressure (and that includes changing tire size, though smaller tires should have higher RR.) but the differences are so minor that it's almost not worth worrying about. At reasonable speed, running a larger tire at 75 PSI is so close to running a rubber band of death at 145 PSI that it's not worth the effort. On rollers, the RBOD is consuming ~30 watts versus ~40 watts (but that's out of 250-300 watts total, less than 10%.) On a real road, they are probably comparable, and my tests seem to bear that out. My commute is very consistently 1:15-1:20. Swapping between those tires makes no discernable difference in commute times.
In comfort and flat resistance, the difference is VAST.
Well, maybe the RR is not that big a difference, but the weight difference certainly must be. Why turn all that extra weight? Why carry that extra weight up hills? As far as comfort and flat resistance... again, 700x23s are comfortable on centuries (and I know many people who use them on double centuries and beyond), so I don't understand the comfort issue. As far as flat resistance, I haven't gotten any flats on my 700x23s in 2 years and several thousand miles of commuting and recreational rides.
Unless one lives somewhere with no hills and lots of cobble-stoned streets, I don't understand the point of carrying the extra weight of anything heavier than a 700x23.