The tires have the same size, yet in fact they are different. How is this possible?

Motorists have long since been arguing about the real size of tires. «My tires are narrower than the width indicated on the sidewall» is a common complaint that comes from even some experienced motorists.

Why it happens and why the real tire size can indeed be different, we will explain in this article.

The real tire section width

There is no point in measuring tires off-rim. Measuring the tread makes no sense at all…
There is no point in measuring tires off-rim. Measuring the tread makes no sense at all…

Almost everybody knows that the 225/40/18 means that the tire section width is 225mm, while the tire’s section height is indicated not in millimeters but as a percentage aspect ratio in respect to that width. On the other hand, there are people out there who are clueless even about this. But these are, fortunately, few and far between. Most drivers are well aware of these numbers.

At the same time, there is a common misconception, even among «seasoned» motorists, that the «tire section width» equals the width of the tread. And this is just not the case.

The «tire section width» means not the width of the tread but the measurement of the tire from sidewall to sidewall in millimeters. And it is implied that the tire is fitted on a rim, and is under inflation pressure.

There are two conclusions that we can draw from this.

  • People who think that their tires are undersized measure the tread, which has nothing to do with the tire section width.
  • Those who look to install wider tires oftentimes expect their tread width to equal the indicated figure… and often get very upset when their expectations are failed…

Tire manufacturers’ tolerances. Why is one and the same size indicated, but the tires look different?

±3% is a common tolerance in the tire industry.
±3% is a common tolerance in the tire industry.

A lot of people ask us: «Why do various tire size calculators allow for a 3% tolerance when you change the tire size?»

Indeed, why 3%? Where does this figure come from?

The answer to this question seems to be quite obvious but sometimes it can baffle even an experienced tire geek.

The first important thing to realize is that tire makers work not with metallic bricks but with an extremely elastic material, which, to make things still more complicated, can be in various states — from lying on a shelf to being mounted on a vehicle, under inflation pressure. Therefore, all tire manufacturers stay within the limits of one tolerance corridor.

And this tolerance must not exceed 3%. Hence, if you buy tires of identical size BUT from different manufacturers or even of different models, etc. — chances are they are not going to be perfectly matched.

The tread width is 200 mm, but the tire size is indicated as being 225/40/18. How is this possible?

Now let’s get back to the first part about the tire section width, where we established that section width does not equal tread width, then add the pressure factor to it, and this «size discrepancy» thing will become even more confusing. Therefore, it does not make much sense trying to «catch» the manufacturers supplying «faulty» tires because this is standard practice of working with tolerances.