Originally Posted by maartendc

Well I would say the rolling resistance is ALWAYS there, no matter what speed. So lower rolling resistance is going to be a constant improvement. Rolling resistance between a slow and fast tire can easily be in the range of 10 watts per tire, or 20 watts for 2 tires. (GP5000 vs Gatorskins for instance).

That being said, the difference between 25mm and 28mm of the same tire is likely 5 watts or less. You can already get a 5 watt difference between 60 PSI and 120PSI on the same tire. Reading Bicyclerollingresistance.com reviews gets you a good feel for the wattages.

The aero benefits are only at higher speeds, and I doubt the difference between a 25mm and a 28mm tire is more than 1-2 watts, if you see how small the differences even are between a 40 mm and 60 mm wheelset... I wouldnt worry about it!