Plus sizing: What's it all about?

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Want to know about plus sizing?

Grab a doughnut and a coffee and I'm going to explain what Plus sizing is all about.

Just hold off on eating the doughnut a bit because I'm going to use it in the explanation. Later you'll get to have it.

If you look at your doughnut it should be easy to see that it has the same shape as a tire (unless you grabbed a jelly doughnut). The doughnut I'll be talking about in my explanation looks like the one in the picture.  If your's is different imagine it looks like the one above.

The basics of Plus Sizing

To understand plus sizing we need to consider the numbers which tell you about a tire's size. 195/65 R15 is typical. The first number before the slash is the width of the tire measured in millimeters. On your doughnut it would be how wide you'd have to open you mouth to take a complete bite.

The last number, preceded by the R -- 15 in our example is the equivalent to the size of the hole in your doughnut. It is also the diameter of the rim, in inches, on which the tire will fit.

The number following the slash is a percentage which is used to calculate the height of a tire's section. You may hear tire people call this the aspect ratio.

On your doughnut, it is the distance between your thumb and finger if you pick up the doughnut putting your thumb in the hole and your finger on the outside of the doughnut. If you wanted to calculate this distance you multiply the width "195" by 65% to learn that our example tire has a section height of 126.75mm.
 

There's one other measurement we need to be aware of.  If your doughnut is not terribly big you could probably pick it up by placing you thumb on one side and your fingers on the other side -- the longest distance in your doughnut.  This is its diameter and, if you notice, there are three parts to it.  First you have a section of doughnut, then the hole and another section of doughnut.  This is the same in a tire and when you are doing Plus sizing your objective is to keep this distance the same.

The reason for keeping the diameter of the tire the same is to maintain the same power, speedometer readings and fuel consumption for which the car was designed.  It also helps in allowing the new tire to fit in the space allowed by the car's body, but see the precaution we note below.

The thing that seems to make this complicated, even for people who are experts is that we are given measurements in both inches and millimeters in the tire size code. But, when you make the conversions to equivalent measurements the process becomes much more simple.

If we convert 15 inches to millimeters it is 381. Now add the two sections and we have [381+126.75+126.75] 634.5 millimeters for the tire's diameter (634.5/25.4= 24.98 inches).

If all these calculations give you a headache, don't sweat.  I have an easy-to-use Excel spreadsheet that does it all.  Get it by clicking on PlusSize Calculator and downloading the zip file.

Two flavors of Plus Sizing

Just like doughnuts, we have more than one version. There is a Plus 0, that's pronounced "Plus zero" and there are Plus 1, Plus 2, Plus 3 etcetera.

In Plus 0 the rim stays the same size and the width of the tire changes. In the other Plus flavors the number tells you how many inches bigger the rim becomes. So, if you start with a 15 inch rim and do a Plus 1, the rim would be 16 inches.

If we wanted to Plus Size our example tire we'd look for width and rim combinations which would keep the same diameter. There could be several possibilities.

would be some Plus 0's for our 195/65 R15 tire.

If you wanted to do a Plus 1

   and you'd look for tires which have these sizes.

Caution

• Not all these tire sizes exist. These are theoretical calculations to show what would keep the diameter the same, you'd then have to find suitable tires and wheels which would fit the vehicle you want to use them on.
• Another limiting factor is the space you have in your vehicle's wheel well. Even though you have kept the diameter the same, installing a wider wheel and tire might not work because there could be difficulties with turning and when hitting bumps in your travels.
• Sometimes switching to a wider tire will also require a wider rim.  There is some tolerance allowed, but only within certain limits.  You need to check the specific tire's specifications to learn the rims that are acceptable.
• If you do the calculations the numbers will not be exactly what we've shown in the examples.  Tire widths and aspect ratios are always a multiple of 5, so the final result is adjusted to be the closest number which ends in 0 or 5. This is usually an acceptable approximation because 5mm is less than 1/4 inch, and that amount of variation does not result in a significant difference.

Conclusion

Many people like Plus sizing because it enhances the vehicle's stability and appearance, and while calculating the equivalent sizes is fairly simple once you know the process, finding available tires and installing them so they won't provoke problems on a specific vehicle is something you don't want to leave to luck.

If you want to do this, I'd recommend that you either rely on people who have had proven results on the same vehicle as you have, or rely on shops who are specialists in this process and know what they are doing from experience.

And if you want my Excel calculator get it by clicking on PlusSize Calculator and downloading the zip file.

Oh, I almost forgot:  Enjoy your doughnut!

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