Hi Guys,
I was going to put some 30mm GP 5000 sTR’s on my Reynolds AR58 wheels, they are 28mm external wide. But Someone told me it would slow me down and be worse of aerodynamically. I race road and have an up coming 120km road race that will be on pretty smooth surface.
Should I run 30mm tyres or stick with 28mm that I use now?
In a recent cycling news story the difference between 30-32mm tires was 3w at 45kph. People seriously love making comments like that without looking at data. Also if you’re mostly riding in a group, throw that teeny tiny disadvantage out the window.
What you were told is technically true, but for some reason loads of people interpret the 105% rule as though aerodynamic performance drops off a cliff and you might as well be using box section rims once the tire is wider than the rim. As far as I know, no test has ever shown anything remotely like that.
Yes it’s true that you need the rim to be 105% as wide as the tire to achieve the absolute optimum performance of the system, but the gravel world has recently shown us that deep section rims still give a significant aero advantage even with MTB tires fitted that are nearly double the width of the rim.
So you’re giving up a tiny bit of aero with the wider tires, but as long as you stand to gain more in comfort, RR, and suspension from them than you’re losing in aero you’re still coming out ahead. And since every tire/rim combo behaves differently and every rider, course, and surface is different, you’ve got to make an educated guess about where your particular inflection point comes and wider tires start to make you slower instead of faster.
I liked NorCal cycling’s video on this - it’s incredibly un-scientific, but probably about as good as most of us are going to get without access to a wind tunnel. IIRC his fastest combo was the first tire size that inflated wider than the rim. Seems like that’s what a lot of WT pros are settling on at the moment too. Not sure whether we should take that to mean “1 size bigger than the rim” is ideal or if that relationship is coincidence and it’s really just that 30-32 inflated width is the best absolute size for most road riders in 2025.
Flo Cycling has a blog post on this that found 30mm tires on 28 mm wide wheels is roughly the same as the 28mm tire due to changes in rolling resistance.
I think the 30mm tires were about 1 watt faster based on their data. It’s basically an N=1 data set but it was enough for me to consider 30mm tires.