Originally Posted by grayrest

The 25mph number sounds like it's an educated guess to me. For aerodynamics, the top of the tire is the fastest moving part on the bike (it moves exactly twice the speed of the bike) and drag is area times velocity squared so the wider tire is going to be a noticeable drawback at some speed. Race motorcycles have a small fairing over the top of the front wheel to handle this and there's no reason you couldn't put one on a bicycle (e.g. front half of a close fitting full fender) but racers won't because fairings are banned by the UCI. I've done enough Jan Heine reading to be interested in what speed wide tires turn out to be a drawback and I know it is possible to get fairly accurate cda/crr numbers for a setup using the Chung method. It's on my list of things I want to try if I ever get a power meter.

For your 36 vs 28 example, it could be either the bike or the tires. Given the same bike and tire line, the wider ones should be slower accelerating due to the extra weight but I expect that'd be a smaller effect than the differences between bikes or tire manufacturers.