According to DT Swiss, the answer is 35km/h. Above that, the aero loss of a 28C tyre outweighs the faster rolling speed. Which explains why most pro’s are on 25’s. But most riders can’t average 35km/h, so for most riders, a 28C will roll faster.
These numbers come from detailed testing on their new ERC 1100 Dicut road wheels, which they tested with Continental GP4000S II in both 25C and 28C widths.
DT Swiss are pretty keen on wide. They are calling it #roadrevolution18.
Read the nitty gritty over at roadrevolution18.dtswiss.com/endurance
Frame material, geometry, and layup have been the main influences on rider dynamics for a number of years. Stiffness and compliance are the main measures of a frames performance, while weight is a critical sales metric.
With the migration to road discs, tyre width becomes a new variable. Most race oriented road bikes are shipping with 23C or 25c tyres, endurance bikes with 25C or 28C. I predict both of these will widen over the next few years, not a lot, but I think 23C will become uncommon, and 28C will become more common. Why, because tyre witch is low cost comfort, and carries a minimal weight penalty. Perhaps the small weight increase can even be recovered in the frame design.
Can super stiff race frame with 28C tyres handle better than a good all round frame like the Specialized Tarmac with 23's?
While pros race on tubular tyres, we are not going to see innovations from the peloton. It will take some handmade bicycle makers to reset the definition of the modern road bike. I look forward to seeing what the future will bring!
The trend to wider tyres on road bikes started some years ago. A few short years ago 23C tyres were the standard, but some people had discovered the extra comfort of 25C tyres. Many road bike frames from as recent as 2014 can't fit 25C tyres, but today, 25C is becoming the norm, and Mainstream brand endurance bikes such as the 2017 Specialized Roubaix are being factory fitted with 28C tyres, and the 2017 Trek Domane Disc moving to 32C.
The migration to disc brakes on road bikes means the size constraint of the brake calliper is becoming less of an issue. While we see segmentation of the road bike into Aero and climbing bikes, at the same time we see an N-1 movement where a road bike can be ridden off the tarmac onto unkept country roads and gravel back roads. With road rage on the rise, riders are finding the appeal of getting away from cars onto the outskirts of the city and into the country. Then a revelation occurs. Sometimes the best roads are not the best roads.
Though the name is not locked down yet, this segment will continue to grow. Some call it all-road, some adventure road, in the US the gravel racing scene is dictating the segment be called gravel bikes, and WTB tyres have adopted the term roadplus for their wide road tyres. Whatever you call them, the small weight gain of riding larger tyres is outweighed by the lower rolling resistance, the added comfort, and the versatility. Keep them coming!