Originally Posted by
Dave Mayer
Continental Grand Prix 5000 S TR 25, 28, 30, 32 mm Comparison (bicyclerollingresistance.com)
The results of this excellent test is that the 25mm tire pumped up to max rated pressure has the lowest rolling resistance of all the sizes tested. This test was done on roughened drum surface to simulate actual pavement. The 25mm tire was also 70 grams lighter than the 32, and obviously has a lower frontal area and therefore was more aero.
Regardless, the RR range difference between all of the tires tested was a trivial 2 watts, or much less of an effect than if you dropped your stem by an inch. The aero disadvantage of the fat tires wasn't tested here but likely exceeds that of any minor RR differences.
So do you want speed or comfort?
Spin it how you like, but running the 25c tyres at their max rated pressure is not likely to be a good idea for either speed, comfort or grip. Remember the Silca data showing how much more you lose if you go 10 psi too high vs 10 psi too low around the optimum for real road conditions? It was 10W vs 1W penalty. That’s why nobody recommends running max rated pressure. It’s just shooting yourself in the foot.
Aero on the wide rims the OP is using (see the thread title) was probably optimised around 28c tyres, so I very much doubt 25c tyres would be an advantage. The external rim width is 35 mm on the front wheel. Moving up to 30 or 32c might be a minor aero penalty, but I very much doubt it would be disastrous.