Deciding on the best tire size is going to vary by wheel, and the conditions you plan to ride the wheel in. In terms of speed, each carbon wheel is designed to be most aerodynamic with a specific tire width, which is listed on specific product pages under rim specs.
To summarize some of our findings: With a broader external width, HED Vanquish and Ardennes wheels roll most optimally with a 28mm (actual width) tire. For Jet wheels, we recommend 25mm (actual width) tires, but 28mm (actual width) tires are still an excellent choice. Unless a podium step hangs in the balance, the difference in the speed of the Jet, using a 25mm tire versus a 28mm tire, works out to only 0.05 kph. That’s approximately 3 – 4 seconds over 40km.
For gravel and fat bike wheels, the best tire, in terms of both width and tread pattern, is going to be largely condition-specific.