I heard somewhere that some folks were using 25 front and 28 rear for time trials, on wheels with rim widths that were optimal for each (see post above about the rule of 105. That gives you narrower where it is cutting through the wind and more compliant in the back where the aerodynamics are less crucial. I can tell you that the one bike I own that can fit 28s is my fastest bike, but it would be ridiculous to claim the wider tires are THE reason why.