A wider tire can roll more easily than a narrow one at the same pressure, yet the mountain bike's chunky rubber is usually slower on pavement. Width is not the whole story. The apparent contradiction rests on how a tire bends, what its tread is made to do, and where the bicycle is ridden.
The contact patch is where the narrow tire pays its price. Given comparable construction, wheel diameter, load and pressure, both tires support roughly the same weight over a similar area, but the wider tire generally forms a shorter, broader patch. Its casing flattens less along the direction of travel. That can reduce energy lost through hysteresis, the repeated bending and recovery of rubber and fabric as the wheel turns. Less flex means less heat. Tire construction still matters: a stiff casing can erase the advantage of width.
Mountain bike tires make a different bargain. Their knobs grip loose ground, but on pavement those blocks squirm under load and dissipate energy; tougher casings can add further losses. Wider tires can also increase aerodynamic drag, especially at road speeds, while heavier wheels make acceleration more demanding. Pressure changes the bargain too. Off pavement, a tire with less pressure can conform to bumps and preserve traction, rather than forcing the rider and bicycle upward over every irregularity. On a smooth road, the faster tire is often the one that wastes least in the conditions at hand. The same patch of rubber meets two very different worlds.
