Road bike tire width: 25, 28 or 32mm?

The evidence here is moderate: real tests, but not many of them. 14 sources, listed at the end.

What the tests say

The trend. BikeRadar measured 26 tires on 13 bikes at the 2026 Tour de France. By label, 12 of the 26 were 30mm (46%), 11 were 28mm (42%), 2 were 25mm (one time-trial bike, 8%) and one was 29mm, according to the article’s text; its table counts differ by one tire. Measured, they averaged 30.29mm, which BikeRadar puts at 1.79mm wider than printed. That gap is calculated against a 28.5mm median label, and against the mean label in the table it is nearer 1.6mm¹. Cycling Weekly had predicted 30mm “across the board” as the balance of aero, rolling efficiency and grip². Rims explain much of the gap. In BRR’s rim-width test, three 25mm road tires averaged 26.4mm on an 18mm internal rim, 28.1mm on 22mm and 29.8mm on 26mm, at 100 psi³.

Rolling resistance. At equal pressure, wider is slightly faster on a drum. At 80 psi with butyl tubes, the Continental GP5000 measured 12.2 W at 23mm, 12.1 W at 25mm, 11.5 W at 28mm and 11.0 W at 32mm (with latex tubes: 10.2, 10.0, 9.2 and 9.0 W)⁴. But riders don’t run equal pressures. At 15% tire drop (butyl tubes), the order reverses (23mm 10.3 W vs 32mm 11.4 W), because the wider tires are softer at those settings. At an equal 4.5mm of tire drop, used here as a comfort proxy, all sizes were within 0.2 W⁴. So on smooth surfaces, width is mainly a comfort choice. On rough roads, wider tires gain speed indirectly: they let you run lower pressure, which cuts vibration losses (see)⁵⁶. Hutchinson says its 32mm tire, at identical pressure, is at least 5% more efficient than its 30mm. The article gives no lab, speed or method, and it’s manufacturer data, not independently verified⁷.

Aero and the “rule of 105”. SILCA’s author says he formulated the rule in 2001, while working at Zipp (1999–2013), from early wind-tunnel work with 21–23mm tires. It says the rim should be at least 105% of the tire’s width so the rim can recapture airflow coming off the tire⁸. SILCA estimates about 1 W per 2mm of extra tire width at low yaw angles⁸.

Hunt’s 2019 wind-tunnel test measured front wheels alone at 45 km/h, between –20° and +20° yaw. Swapping a 25mm Schwalbe Pro One for a 28mm cost 1.54 W on Hunt’s 34mm wide alloy rim and 4.39 W on a Zipp 202 NSW. That’s a manufacturer test⁹. However, AeroCoach’s 2025 test of nine deep wheels, all with the same 28mm tire, found no relationship between rim-to-tire width ratio and drag. The most aero wheel had a 113% ratio and the least aero 112%, while a wheel at only 93% ranked fifth of nine. Rim shape mattered more than the ratio¹⁰.

Comfort. A 2025 accelerometer study found a 32mm tire at 4 bar gave the lowest vibration at 25 km/h, and a narrow, high-pressure tire the highest at 40 km/h¹¹. This matches Turner’s model, in which more vertical compliance cuts roughness resistance⁶.

Cornering grip. Direct, independent data on width is thin. BRR’s wet-grip test of three 28mm tires found lower pressure raised centre grip (average coefficient 0.69 at 108 psi vs 0.78 at 54 psi), on a smooth ceramic surface at low speed, and wider tires are what make lower pressure practical¹². Rene Herse describes a descending comparison where 42mm tires outpaced 25mm. That’s an anecdote from a tire brand¹³.

Limits. UCI rules cap total wheel diameter at 700mm, tire included. On 622mm rims, Elite Wheels calculates that this caps practical road-race tires at about 38mm¹⁴.

The numbers

• 2026 Tour sample (13 bikes, 26 tires): average measured width 30.29mm, which BikeRadar puts 1.79mm over the label; 12 of 26 labeled 30mm¹
• Three 25mm tires grew about 0.43mm per 1mm of extra internal rim width (averages of 26.4mm on 18mm and 29.8mm on 26mm rims)³
• GP5000 at an equal 4.5mm tire drop (comfort proxy): all widths within 0.2 W⁴
• GP5000 at 80 psi: 12.2 W (23mm) vs 11.0 W (32mm), butyl tubes, one wheel on the drum, 29 km/h⁴
• Going from a 25mm to a 28mm tire cost +1.54 W to +4.39 W of drag (front wheel only, 45 km/h, Hunt tunnel test)⁹
• About 1 W per 2mm of tire width at low yaw (SILCA estimate)⁸
• UCI maximum wheel diameter 700mm, which works out to about 38mm nominal tires on 622 rims¹⁴

Myths and disagreements

• “Wider is always faster.” At equal tire drop (a comfort proxy), the one tire model tested rolled essentially the same on the drum⁴, and aero gets a little worse⁹. Speed gains depend on the road surface⁶.
• “The rule of 105 is a law.” It came from 21–23mm tires in 2001⁸. AeroCoach found no simple ratio predicts drag on modern wheels¹⁰. Treat it as a rough guide.
• “A 28 is a 28.” Measured widths routinely run over the label, and wider rims add more¹³. That matters for frame clearance and hookless rules (see).
• Disagreement: grip. Brands and riders widely report better cornering with wider tires⁷¹³, but no controlled independent test of width vs cornering grip exists yet. The pressure-grip link is better supported¹².

Which tire width to run

• For mixed or rough roads, a 28–32mm tire run at a calculator pressure is the evidence-backed default¹⁴¹¹.
• When you go wider, lower the pressure. Otherwise you give up the comfort and grip benefits⁴.
• On aero wheels, use the tire width the wheel maker recommends, and measure it once it’s mounted³⁹¹⁰.
• Check frame clearance against the tire’s measured width on your rim, not the sidewall label¹³.

Sources