What do the Various Size Specifications on Bicycle Tires Mean?

Everyone is talking about tire size in road biking and gravel grinding. We have started to discover that wider tires don’t mean less speed. They can bring more speed and, possibly, more importantly, they bring more comfort.

The problem is how do you know what size of tires you need? What tires will fit the rims that you ride? What exactly is the size of my tires? Tires have a lot of sizing information written on their sides and knowing what it means can be elusive. We’ve thought about this and decided to create you a handy guide for tire sizing.

What size tires do I need?

If you want to be comfortable the easy answer here is the biggest tires that will fit on your wheel rim and give you clearance for your frame or fork. You’ll find that many manufacturers will tell you the maximum size that you can fit.

Is there a cheat sheet for road bike tire sizing available?

Yes. If you don’t want to know any more about tires just check our quick guide to which tires will fit the width of your rim.

Image is taken from www.sheldonbrown.com

The above chart shows the internal width of your rim and the corresponding tire size that’ll fit on that rim. You will want to avoid fitting tires in the red sections. Some people have gotten away with putting unsuitable tires on but it is a risk you don’t need to take.

As you can see rims will fit a variety of tire sizes, and it is worth picking a rim that suits your riding. We use an 18.25 internal width of our Aero and Fast and Light wheels as it allows you a big range that fits most modern riding styles. 

What is ERTO?

Do you know how everyone likes to mention that bike manufacturers love to create new standards? The idea for this was born during the period when the cycle industry was giving us a new standard on a weekly basis. Thankfully we seem to be moving away from that.

What though does this have to do with ERTO? Well, back in the day every country had its own way to measure tires. Some made more sense than others. All these standards were proving to be a nightmare to consumers.

It was decided that to make everything easier that there should be an international standard guide for tire sizing. The new method was originally named the "E.T.R.T.O." system, and you may still see that around. The E.T.R.T.O stood for the European Tire and Rim Technical Organization.

The system that was adopted is a part of the ISO system, the International Organization for Standardization. The particular name given to the ERTO standard is ISO 5775. We can thank this rule for allowing us not to have to navigate the fact that there used to be six types of 26” tire.

What does ERTO tell us about tire size markings?

The ISO/ERTO standard system uses two numbers. The first number you see will be how wide the tire inflates. It is also worth noting that this is a guide for most companies. Different rims will affect this width. Your rim can also change the shape of your tire, so if your rim has recommended sizes follow that guide.

The second number is the wheel/tire/rim size, and this is the number that confuses most people. The reason for the confusion is that this number is not the number most people use in their everyday conversations on bike wheels and tires.

You’ll probably use the French system for road wheels and call your wheel a 700c. Under the ERTO system, your wheel is actually 622 mm. 622 is also the same as a mountain bike 29” rim. Mountain bikes generally use inches instead when talking about a wheel and tire size.

Except, there is always an exception, for the new standard of 27.5”. It is sometimes referred to by its size in the French system, which is 650b. So that you know there is also a 650a and 650c. 27.5” and 650b under ERTO are 584 mm. A 650 tire with no letter is narrow, as you go up the letters the tires get wider in the French system. 

The French system in case you were curious uses the outer diameter of the tire to measure its size. The ISO/ERTO system uses the inner diameter of the tire. The inch system is not as a great measurement of size as it isn’t as specific as either the ERTO or French system.





Size Marking


28 x 1.40

28 x 1 5⁄8 x 1 3⁄8

700 x 35C

Outer diameter


ca. 28 inch

ca. 700 mm

Inner diameter

622 mm



Tire width

ca. 37 mm

ca. 1 3⁄8 inch

bzw. 1.40 inch

ca. 35 mm

Tire height


ca. 1 5⁄8 inch


So, if you see 622 x 35mm. You’ll be looking at a road bike sized tire, more likely to be a gravel or cyclocross variant though. If you are buying tubes giving an ERTO number will make your life a lot easier and avoid accidently getting the wrong tube, provided you know what valve style you need.

What size tires can fit the ICAN FL and AERO rim?

Our Aero wheels and our Fast and Light wheels share the same aerodynamic wheel rim shape. The only difference is that the Aero range uses Toray T700 and T800 carbon fiber in its construction. The Fast and Light series only uses Toray T700 in its construction. The mix of carbon fiber allows our Aero wheelsets to be around 100g lighter than our Fast and Light rims.

All of the rims in both series feature an internal width of 18.25mm. The internal width here will allow you to run anywhere from 25mm wide tires up to around 40mm wide tires. 25mm wide has become the defacto modern road racer size. It fits well on our rim and helps to reduce drag being created between the tire and rim interface, giving you free speed.

40mm will be more than enough for the majority of gravel riders. You’d need to be doing some serious off-roading if you want to go more than 40mm wide and you might be better looking at our 29er XC wheels.

In between these sizes, you’ll be able to run the UCI maximum width cyclocross tires. You’ll be able to easily get those 33mm wide tires onto our Aero and FL rims. By the way, did we mention that they are tubeless ready?


  • Apr 22, 2019
  • Category: Articles
  • Comment: 1
1 comment
William King May 08, 2020

Have 20 by 2.15 tires rear on trike

Want to replace with 20 by 3.0 tires. Will they fit on original rims ??

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