Each tyre was set up to a pressure determined using SRAM’s online pressure calculator, inflated using a digital track pump and verified by a standalone tyre pressure gauge. Both the Panaracer tyre casing test cohort and the Pirelli models were all 40c in width, so an identical pressure was used for a great many of the tyres on test.
Even if we don’t take the 33c option out, the fastest tyre width at both 7m/s and 9m/s was the giant 2.1” option, with a saving of 5.9 watts per tyre at 7m/s and 6.5 watts per tyre at 9m/s, comparing the 31c to the 2.1”. That’s an 11.8-watt difference for a pair, or 13 watts at 9m/s for two tyres across the whole range.
In aerodynamic terms, while we did not take these setups into the wind tunnel ourselves we are able to draw on the work of others. Dylan Johnson, professional gravel racer and YouTuber, also recently undertook his own tyre testing at the SSE Hub, and while his tests involved different parameters, speeds, and setups to ours, he did also put his own race bike in the wind tunnel with varying tyre sizes. At 36km/h he found a 5.5 watt penalty moving from a 40mm to a 50mm Pirelli Cinturato H, but was also at pains to point out that his tests of the 2.2in (56mm) Continental RaceKing actually tested faster than the 50mm Pirellis, perhaps due to the sidewall texture, at least showing there is more to aerodynamics than simple frontal area calculations.
Cumulatively then, accounting for choosing a faster tread and the fastest width, we are currently looking at a total rolling resistance difference of around 20 watts on a smooth surface, and likely more on a rough one, moving from a narrower knobbly tyre to a larger, more slick option.
At more reasonable speeds - below 30km/h, so essentially everything below top-end gravel racing - the gains will be even greater, as the impact of aerodynamics, while still important, will play a lesser role in the overall picture than it does at fast road riding speeds. Essentially, as a rule, you should fit the widest tyres you can, and if you’re buying a new bike then the maximum tyre clearance should be a key consideration. Not only will you be faster in almost all scenarios, but you will be more comfortable, and have better grip too