The Myth of Tire Size

ABSTRACT: Measure how wide your tires actually are when they're inflated on your wheels. Printed sizes on clinchers are usually wrong, and knowing tire size informs bike clearance and tire pressure choice, and lets you establish equivalency between tires of different models and manufacturers.

We get four questions more than any others: What spoke count do you recommend for me? What size tires do you recommend for use with the wheels I'd like to get? What tire pressure should I be using? And... is there a discount code available. Since the answer to the fourth is no, and the answer to the second is the topic of today's blog, I'll briefly address the first and third: we're working on a calculator where you can input your values to the parameters that we feel are relevant and it will give better and more comprehensive guidance than "you weigh x so get this spoke count."

As to pressure, go to the chart on page 5 of this document (which is definitely worth a read when you have a minute), find your appropriate place on the line for the tires you'd like to use, add a few psi for the rear, subtract a few for the front, and voila! Since pressure is actually really important to how your bike feels and rides (in fact, perhaps THE most important thing), it's worth stressing a few things. Most pump gauges aren't very accurate, so an air checker (we use the SKS ones, which are great) is a good idea, but be aware that your pump's 80psi may have not much to do with your buddy's pump's 80psi, or the guy who parked next tou you's pump, either. That's why having a gauge in your gear bag is worth it. Second, be aware of this and feel free to experiment. Be a little scientific about your methods, but try different stuff. It's fun. Third, measure your tires. Seriously.

This "35" is closer to 37mm, actually. Road tires do this, too.

Which brings us to the point of today's blog. What's written on the side of your tire is probably accurate to within about 15%. Tubeless tires are usually pretty close to what's written on them, tubed tires are often nowhere near that size. I've measured "25mm" tires that were 28.5mm wide. So the question I always ask back when people ask about using a specific width of tire is "do you want to ride a tire that's is x width, or do you want to ride a specific tire that calls itself size x?" As with many of the questions I ask, this one makes me sound like a pedantic twat, which is not at all how it's meant. We want to help, seriously!

A lot of pro teams ride 27mm tires these days, or at least 25mm in any case. Here's the thing - they ride tubulars, which are MUCH more size accurate than clinchers and don't set up to different sizes on different rims. It's, I would say, usually the case that a 25mm tire on a wide-ish rim is going to set up bigger than 27mm. After a long long time of experimenting, I've found that I generally like tires that are in the range of 25mm wide. For me, this means I ride Pro4s and GP4000s in 23mm size, because they are actually 25mm on the wheels I use. But in Maxxis Padrones, I use 25mm size because they're really only just under 26mm wide. Knowing how wide any specific tire is going to set up on a given wheel also allows me a much more effective starting point for learning my preferred pressure for that tire. 

A tire that calls itself a 28 may not fit your frame because it's actually 31mm wide. You might get a new tire that says it's the same size as your old one, but it's actually much bigger, so if you use the same psi as the old tire, it will ride like a rock and you'll think you hate the tire when it's really just pressure.