Tire Sizes FAQs

By far the most frequently asked question we have received about sizes of tires are of the kind "Will tire X replace tire Y?" where X and Y represent a certain tire size...




Other tire size questions


Next in frequency are questions about what size tire to use for a certain vehicle. Sometimes people ask if tire size X will work, or if it is safe to use on a certain vehicle, which implies that this is not, or they suspect that this size is not normally used on that vehicle, but for some reason they might like to try.

Following this are questions in which people sense that changing to a different sized tire will affect their driving or vehicle's performance, but they aren't sure how, and they'd like to know.

Next in order of frequency are questions about rims, and how they relate to tires and or the car's performance.

Although not high on the list in terms of the number of times asked, we do receive a surprising number of questions which relate to using different sized tires (and/or wheels) at different positions on the same vehicle. This is an important issue, particularly when 4X4 vehicles are concerned.

Finally we get a grab-bag of assorted questions which don't easily fall into any broad category, but which show that people need some additional information or better understanding about tire sizes than they currently have.

I will tackle all of these broad groups of questions in the order of their popularity but, if you'd rather not know or wish to read through each part to find the specific area which interests you, just click on the link to jump to the part which you want to know about, but before you do, I'd encourage you to at least read "About Tire Sizes" unless you are already familiar with how tires sizes are expressed and what each part means so that you'll better understand the details.

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About Tire Sizes

tire size sectionThe most widely used tire sizing system for passenger vehicles has
3 parts to it. You should be able to find the tire size molded on the side of each tire on your vehicle, unless it has been badly damaged or extremely worn. The size will look something like this:

205/65 R15

It is possible that there may be one or two letters before this sequence, like

P195/50 16, and the letter which is in front of the last two numbers could be missing.

In both cases, the letters really don't tell you anything about the size, but provide additional information. The leading letters refer to the kind of use the tire is designed for, and the letters later on refer to the type of construction of the tire. There are also other letters and numbers on a tire's sidewall,but they do not have anything to do with the size. (See Tire Markings.)

The three numbers from a tire's size are, in order:

  1. The tire width, measured in millimeters.

  2. The tire aspect ratio, and

  3. The rim diameter on which the tire is designed to fit.

Taking each of these in turn, the width of the tire is the measurement of a tire when it has been mounted on a wheel and inflated with the air pressure which it operate with, (or no more than the maximum which is stamped on the side of the tire.) If you are not used to working with millimeters, it may help to remember that one inch is about 25mm, so a 200mm tire would be about 8 inches wide or a 255 tire would be just a little bit more than 10 inches.

The tire aspect ratio is used to calculate the distance from the inside hole of the tire, to the outside of the tire, or the distance shown as "height" in the drawing above. The number which is written on the tire following the diagonal slash is a percentage, so /50 is 50% and that means that the distance from the tire's hole to the tread is exactly 1/2 it's width. Usually these numbers run from 80 to as low as 20, but they can be greater than 100, although this size of tire is not generally seen.

This number is usually a number which can be evenly divided by 5, so it will generally be a 5 or a zero.

Exceptions

There are a couple of exceptions which are also very rare, but I will mention them just in case you stumble upon one of them and start wondering just what is happening. Sometimes, the Aspect Ratio might not be marked on a tire and the size might look like 185 14. If there is no number preceeded by a slash, the missing number is usually 80 (or possibly 82). The other exception might be found in certain tires manufactured in Japan: If the number following the slash is over 200, it may be the actual measurement, in millimeters of the tire's height, instead of a percentage.

The final number is the rim diameter, in inches, which the tire is designed to be used with. Most commonly they are whole numbers like 12, 14, 16, but some specialy wheels and tires to exist with 1/2 inch sizes, such as 16.5 or 22.5.

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Reasons for Changing Tire Size

Before we move on to the broad question areas which people have about tires, it may be useful to look at the reasons why people might want to change their tire sizes in the first place.

In my experience it is a good idea to have a clear idea of the reason why you want to change tire size before you do it, because unless you know why you're doing it, it could easily turn into a nightmare that you'll long regret.

The four most common motives to change a tire size are Convenience, Cost, Appearance and Performance.

Let's examine each of these.

Convenience: It may be that the tire on a specific vehicle is a size which is hard to locate so that whenever the owner of the vehicle needs to buy a replacement tire, or to change from summer to winter tires, etc. it is a difficult and possibly costly exercise. Because a tire may not be easily available it is possible that the vehicle could be forced to be out of use if a tire failure happens when a replacement is not available.

Cost: This may or may not be related to the first reason, but sometimes certain tire sizes are much more expensive than other sizes, usually because of popularity. Some tire sizes are extremely popular and highly competetive in their pricing, and this might prove to be an incentive to consider switching to this size instead of the one which originally came with a vechicle.

Appearance: Some vehicle owners are very particular about the appearance of the wheeles and tires they use. This may be a matter of personal taste or popular trends, but sometimes a car owner will want to customize their vehicle to have a certain look which they cannot achieve using stock tire sizes.

Performance: This is a reason which is debated among car enthusiasts as to the actual benefits which can be achieved in the performance of their vehicles and how these are affected by tire size, most frequently in high profile vs low profile tires. Some users claim that they have better control and performance using low profile tires, while others believe that they give us driving comfort and ease of changing and caring for their tires by making this move.


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Using Size X in place of Size Y

Most people who have asked us questions about tire size are wanting to know if they can use tire X instead of tire Y, or possibly they already have decided they'd like to, but are uncertain as to how this might affect their car or the way it performs. We will deal with these issues here.

In first place, we have created a Tire Size Calculator which can:

  • A) Calculate all currently available tires which are close to the same size as any tire which a user has.
  • B) Compare an existing tire with another specific tire and point out the differences.

When you change the size of a tire, there are three measurements which can be changed: the width, the height and the rim diameter.

Changing just one of these affects how a car might operate, what happens if you change two or three at the same time? You can potentially either multiply or un-do the effects of a single change by changing another. For example, many tire technicians learn a rule of thumb which they apply in Plus Sizing to decrease the Aspect Ratio by a certain amount to offset an increase in rim diameter.

Actually calculating these exact values can be confusing for some people when you have to combine metric measurements with inches, and then apply ratio values to figure out what a given change will do. The Tire Size Calculator does all of these things with the click of a button.

In the first mode, the calculator will present you with a list of available tires which are within a range of 2% of the initial tire size entered. If desired, this range can be increased or decreased, but we have set it at a default level of 2% because this is the range in which the most acceptable tire substuions fall.

What happens when a tire size is changed that affects the performance of the car? Essentially the outside diameter of the tire changes. Imagine a tire as being nothing more than a circle of rubber. If you change it, the circle gets either bigger or smaller.

Now imagine putting that circle (see the yellow circles in the photo below) on your car. If that gets too big, it will go beyond the edges of the space where the wheel fits, if it is too small, eventually it will be so small that the car doesn't even have any room under it and would scrape on the ground. Of course, it is not likely that anyone would go to either extreme, but by imagining the extremes, you can begin to understand what is happening when you put on a bigger or smaller tire.

tire size circles

When we calcualte the effect of a tire size change we always try to say as close as possible to the original outside diameter (the size of the tire circle) as possible.

What happens if the circle gets bigger? Well, imagine that your circle is cut and made into a straight line. That line will be longer than the line made from a circle from the original, smaller tire. And that line shows you how far your car will go everytime the wheel turns.

What does this mean to you and your car?
First that you will use less gasoline because everytime your engine turns over you'll go further, but because your speedometer is set up to tell you that you will travel a shorter distance for each turn of the wheel, you will be traveling faster than the speedometer shows.

There is also an additional effect which isn't talked about very much because it usually isn't an important effect when you choose tires that are only slightly larger than the original: Your car will seem to have less power when you get into situations where you need extra power like passing on the highway or going up a steep hill.

When the circle gets smaller, or, in other words you are putting on smaller tires than you had before, just the opposite happens:

  1. You will use more fuel,
  2. The speedometer will show that you are going faster than you really are, and
  3. Your car will have more power.

I've already mentioned that we usually recommend that people stay within 2% when they are swapping to different sized tires. As you can imagine, a 2% change in fuel consumption, or speedometer reading, or power is not something that will be a major effect and this is why we have this 2% limit as a guideline.

Tire width


Sometimes people will ask about changing a tire's width instead of increasing the height or the rim diameter. For example, someone will ask what will happen if they change from a 195/65 R15 tire to a 215/65 R 15 tire.

Well, let's look at this example. At first all it seems that is happening is that the tire is getting wider by 20mm, and remembering that 25.4mm is an inch, we can see that the tire is getting wider by less than one inch. Surely this can't affect a car very much, can it?

In the first place it helps to recognize that the space into which a tire fits on a car is very seldom a "hole" with square sides. If you look at most car designs, you'll notice that these openings are somewhat rounded at the top, so the further you get away from the center of the tire, the opening gets a bit smaller before it finally ends. This means that making the tire wider, will possibly create a condition where the tire will rub against some part of the body, and even if the tire didn't get taller you might have a potential problem.

In fact, if the Aspect Ratio stays the same, the height of the tire does increase when the width increases, by the same percentage of the change in width. That is to say if you increase the width by 10 and the AR is 65, your height will increase by 6.5 (65% of 10).

So, if a person wishes a tire's outside diameter to remain the same, the AR should be made smaller whenever the width is increased (and the other way round if you decrease the width). As a quick way to get an approximate adjustments, tire technicians usually change the AR by 5 for each change in 10 for the width. That won't give you exactly equivalent changes, but it is pretty close.

Summary and Conclusion

Using our Tire Size Calculator is a simple and quick way to learn if a certain size tire is likely to be suitible for your car, but there are two important things to also consider:

Are the Load Rating and Speed Rating of the new tire you wish to use at least the same, or perhaps even more than your original tire?

The reasons are simple.  If you install a tire which is rated less than your original tires you may experience serious problems either in how much weight your new tire will support or the capacity of the tire to resist the speeds which your original tire was designed for.   Having a higher rating generally doesn't hurt anything (except possibly being more expensive), but a lower rating is asking for problems.


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What Size tire does my Vehicle use and what alternates exist?

Some people write and ask us what size of tire to put on their car. For a person who works in the field, it is sometimes amazing that people even need to ask this question, because the size of every tire is molded right into the side of every tire.

tire size

So, the quickest and easiest way to answer this question is to simply look at what is on the sidewall of the tires which your vehicle is now using.

Of course, there could be complications, especially if you look at one tire and see one size and another one has a different size on it. Then the question might arise, which one is right?

Most cars have the manufacturer's recommended tire size on a sticker which is stuck on the door post, (or possibly the door edge) on the driver's side. A few vehicles have this posted on a sticker which is located inside the glove compartment door or the cover for the gas tank. If neither of these can be found, the next step would be to check the user's manual if you have it.

If all these steps fail, try going online to a business which sells tires (like tirerack.com) and search for your vehicle by make, year and modle to find the most likely size which was used when the vehicle was made. Sometimes, your search results will give you alternate wheel and tire combinations which the car was offered with when they were new.

One note, if you purchased a car outside of the USA and look up the tire size from a US tire seller, you may find a different size from what your car has. This is because some cars are equipped with different sized wheels and tires when they are sold in different countries, however you will usually find that although the rim size varies, the overall diameter of the tires are about the same from one country to the next.

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Effects of Changing Tire Size

So what will happen if you change your tire's size?

In very general terms there are only two changes you can make. Either the tire you change to is bigger, or it is smaller. No other change is possible.

We have created a Tire Size Calculator which will calculate the precise values of any change from one tire to another, but in very few words, making a tire bigger will result in using less fuel, giving you less power and making your speedometer show that you are going slower than your actual speed. If you make a tire smaller, the opposite will happen. Use the Calculator to compare your tires.

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Different sized rims

Sometimes people will want to change the rims they have. This is a common practice called Plus Sizing when larger sized rims are being used.

The normal practice when Plus Sizing is done is to reduce the profile of the tire so that the overall diameter remains a close as possible to the original. Our Tire Size Calculator permits you to compare tires of different sized rims, too, if this is what you are considering.

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