When it comes to wheels and tires, size does matter. If in the MTB sector a balance has been found between 650b and 29″ after years of keyboard battles and hand chronometer comparisons, in the gravel sector, fortunately, the phase of battles over what is better or worse has been skipped over. In fact, the two diameters offer different advantages and disadvantages and each one is better for certain situations and has its own field of application.
How come? Simply because the actual diameter in the end becomes almost the same; what changes is the volume of the tire and its section.
Let’s try to explain better: the tires that are usually mounted on 700c wheels vary between 34/35mm section up to 40/42mm, those that are mounted on 650b wheels range from 42/45mm up to almost MTB sections. It goes without saying that if the section increases, the final diameter of the wheel increases, so they are very close to each other in absolute terms of diameter.
Therefore, a 650b tire has a larger volume, a larger trace and usually a heavier weight. Consequently, we can ride at lower pressures with more comfort and grip, but on the other hand it will be less smooth and generally less fast. In short, it is a tire more suitable for those who pedal most of the time on trails and technical terrain.
With the same type of tire, a 700c tire will have a smaller trace and a lower weight, being consequently smoother, faster and quicker even in changes of pace. Also in downhill on asphalt, a smaller volume represents an advantage in terms of drivability. The main disadvantages are the lower comfort due to the lower absorption and the higher pressure required as well as a lower grip. It is a tire suitable for those who pedal more on fast and smooth terrain or who cycle a lot on asphalt in addition to dirt roads.
The various merits and demerits can be mitigated by adopting various types of rubber: for example, if we mount a smoother rubber on a 650b or if we mount a more treaded one on a 700c to increase grip on mixed terrain.