Having a spare tire in the back saves you from the trouble of calling an expensive towing service in the middle of nowhere. Since most car brands offer an additional tire, you never bother to learn to read a tire size chart. It isn’t necessarily a big deal until you’re at the mechanics, trying to understand the obscure symbols and finalize the correct size.
To decode the hieroglyphics without embarrassing yourself, you end up with the wrong size. Since it seems all the same to you, you’re potentially risking losing the balance of your car. So before it all goes off the rails, we’ve curated a guide on reading tire size. The article lists the different dimensions and has a few charts attached to help you gain a quick understanding.
But before jumping into the technical aspect, let’s understand why it is necessary to find the right size.
Importance of finding the right size
Choosing the correct size isn’t only about making the driving experience smoother but looking out for the safety of passengers and those on the road. Opting for a slightly smaller or larger size may not seem like a big deal. But we assure you, even the slightest difference in size could have the most severe repercussions. If your tire isn’t identical to the ones the company suggested, you’re endangering the lives of those on the road with you.
In addition to a serious safety concern, here are a few other factors affected by the use of the wrong size:
Additional cost for replacing with a bigger tire
Greater surface area results in an even distribution of pressure. This is why bigger ones provide a smoother driving experience. You feel like your vehicle is gliding along the road. But you can’t expect to go against the manufacturer’s instructions and face no harm.
Installing bigger tires means higher sidewalls. It damages your vehicle’s suspension system, stability system calibrations, and the wheel. But that’s not all. A taller tire influences the speedometer ratings to show a figure much slower than the actual speed. Incorrect speed reading can be detrimental to everyone involved. Not only will you collect a bunch of speeding tickets, but you also risk the lives of those around you.
Stability and traction deterioration
You may have heard a direct correlation between stability and tire size. So a bigger ones should result in excellent stability, right? Except that’s not always the case. Using the wrong size messes with the center of gravity of your vehicle. So every time you take a turn, the risk of completely losing control of your vehicle increases.
Higher risk of accidents with smaller tires
Choosing a smaller tire is just as damaging as opting for a bigger one. A small one also influences the speedometer ratings, giving inaccurate speed readings. As a result, there is a higher risk of the anti-lock brakes malfunctioning, the engine light fusing and the tire bursting.
Tire dimensions explained
If someone handed you a paper with the code ‘P285/70R17 120R’ written on it, it would take ages before you could even tell that this is related to a vehicle. But as a car owner, it’s vital to understand the code and what each value stands for.
However, you don’t have to enroll in a class to understand this. Instead, just read the following guide. Here is each dimension of a tire size explained in detail:
The first alphabet of the size designation indicates the class of the tire. There are three types, based on class division.
P – The letter P stands for Passenger Vehicle. P-type ones are used for regular cars, crossovers, SUVs, minivans, and small pickup trucks.
LT – The letters LT stand for Light Truck. L-type tires are associated with vehicles carrying heavy loads, towing trucks, and cars looking for a heavy-duty option. Typically, they’re used for 1-ton trucks and SUVs.
ST – The letters ST stand for Special Trailer. ST-class ones are solely used for heavy-duty travel trailers, fifth wheels, utility, and boat trailers.
After the first letter, you’ll observe a string of three numbers. The three-digit number refers to the width, measured in millimeters. The width is measured at the widest point, from one sidewall to another.
The two-digit number following the forward slash divide is the aspect ratio, a measure of the sidewall as a percentage. The aspect ratio is calculated by expressing the height of the sidewall as a percentage of the width. The higher the aspect ratio, the bigger the sidewall.
For example, an aspect ratio of 70 means the sidewall is 70% as high as the tire is wide.
After the two-digit aspect ratio, the single letter represents the construction type. Essentially, it’s the interior design. The construction type is either R or D.
R – The letter R describes radial tires, the industry tire standard today. Radial tires are celebrated for their better road grip, excellent stability, and higher durability. In addition, the radial tires lower the rolling resistance. Therefore, they offer much better gas mileage.
The radial construction consists of three layers running perpendicular to the direction of motion. The outer layer is cap ply, made of a parallel arrangement of nylon cords. The inner layer consists of steel belts and fabric blend, added to strengthen the structure.
D – The letter D describes diagonal plies or bias-constructed tires. You might have heard conventional, cross-ply, or x-ply used to describe construction type D tires. However, they are not the industry standard for traditional vehicles. Instead, only motorbikes and trailers use them. Regarding providing stability and better ground contact, construction type D is far inferior to type R.
The last two digits in the tire size designation refer to the wheel diameter measured in inches. So it’s the distance between one-bed seat area of the wheel to another.
The size designation ends at the wheel diameter measurement, but another code follows with a bit of space. The first two or three digits of the second code form the load index, indicating the weight capacity. However, the load index is not to be taken at face value. Instead, it would be best if you referred to a chart to convert it into its counterpart loading capacity in pounds.
For instance, the code above shows a load index of 120. Referring to a conversion chart, we can deduce the loading capacity is about 3,086 lbs.
The last letter of the second code refers to the tire’s top speed limit. You can use a speed rating chart to convert the alphabet into a numerical value. Speed beyond that numerical value will damage the tire and the engine.
For instance, the code above has a speed rating of R. Referring to the chart below. We can deduce the maximum speed is around 106 mph.
What do tire size numbers mean?
Understanding tire size is an essential skill for all adults. But we don’t blame you if you are puzzled by a string of numbers.
For the tire size ‘315/70R17 120R,’ here’s the breakdown of all the numerical values:
315 – The three-digit number represents the width in millimeters. It is measured at the widest point, between two side walls.
70 – The two-digit number following the forward slash represents the aspect ratio, which is the height of the sidewall expressed as a percentage of the width.
17 – The last number in the size designation is the wheel diameter, expressed in inches. The diameter is the distance between the wheel’s bed set areas.
120 – The second part of the tire size starts with a number representing the load index. The value can be used to derive the maximum loading capacity of the tire. All you need to do is, refer to a conversion chart, and you’ll have a corresponding value for the load index.
Tire size chart
Chart for 13” diameter
|21″ 185/60R13 = 21.7×7.3R13 215/50R13 = 21.5X8.5R13 225/45R13 = 21×8.9R13 225/50R13 = 21.9×8.9R13 255/40R13 = 21x10R13||22″ 155/80R13 = 22.8×6.1R13 175/70R13 = 22.6×6.9R13 205/60R13 = 22.7×8.1R13 22X8R13||23″ 165/80R13 = 23.4X6.5R13 185/70R13 = 23.2×7.3R13|
|24″ Diameter 175/80R13 = 24×6.9R13 185/80R13 = 24.7×7.3R13 24.5X8R13 24.5X9R13|
Chart for 14” diameter
|19″ 105/70R14 = 19.8X4.1R14||20″ 115/70R14 = 20.3X4.5R14 125/70R14 = 20.9X4.9R14||21″ 165/55R14 = 21.1X6.5R14|
|22″ 135/80R14 = 22.5X5.3R14 165/65R14 = 22.4×6.5R14 185/55R14 = 22×7.3R14 185/60R14 = 22.7×7.3R14 195/55R14 = 22.4×7.7R14 205/50R14 = 22.1×8.1R14 205/55R14 = 22.9×8.1R14 225/50R14 = 22.9×8.9R14||23″ 175/65R14 = 23×6.9R14 175/70R14 = 23.6×6.9R14 185/65R14 = 23.5×7.3R14 195/60R14 = 23.2×7.7R14 205/60R14 = 23.7×8.1R14||24″ 185/70R14 = 24.2×7.3R14 185/75R14 = 24.9X7.3R14 195/65R14 = 24×7.7R14 195/70R14 = 24.7×7.7R14 215/60R14 = 24.2×8.5R14 225/60R14 = 24.6×8.9R14|
|25″ 185R14 = 25.7×7.3R14 195/75R14 = 25.5×7.7R14 205/70R14 = 25.3×8.1R14 215/70R14 = 25.9×8.5R14 235/60R14 = 25.1×9.3R14 245/60R14 = 25.6×9.6R14||26″ 195R14 = 26.2×7.8R14 205/75R14 = 26.1×8.1R14 215/75R14 = 26.7×8.5R14 225/70R14 = 26.4×8.9R14 26X8.5R14||27″ 205R14 = 27×8.2R14 27X8.5R14 27X9.5R14|
|28″ 28X10R14||30″ 30X10R14|
Chart for 15” diameter
|20″ 105/70R15 = 20.8X4.1R15||21″ 115/70R15 = 21.3X4.5R15 125/70R15 = 21.9X4.9R15 165/50R15 = 21.5X6.5R15 195/45R15 = 21.9×7.7R15||22″ 125/80R15 = 22.9X4.9R15 145/65R15 = 22.4×5.7R15 155/60R15 = 22.3×6.1R15 165/60R15 = 22.8×6.5R15 175/55R15 = 22.6×6.9R15 195/50R15 = 22.7×7.7R15 22X8R15 22.5X4.5R15 245/40R15 = 22.7X9.6R15 275/35R15 = 22.6×10.8R15|
|23″ 135/80R15 = 23.5X5.3R15 165/65R15 = 23.4X6.5R15 175/60R15 = 23.3×6.9R15 185/55R15 = 23×7.3R15 185/60R15 = 23.7×7.3R15 195/55R15 = 23.4×7.7R15 205/50R15 = 23.1×8.1R15 205/55R15 = 23.9×8.1R15 225/45R15 = 23×8.9R15 225/50R15 = 23.9×8.9R15 295/35R15 = 23.1X11.6R15||24″ 135/90R15 = 24.6X5.3R15 155/80R15 = 24.8X6.1R15 175/65R15 = 24×6.9R15 185/65R15 = 24.5×7.3R15 195/60R15 = 24.2×7.7R15 205/60R15 = 24.7×8.1R15 225/55R15 = 24.7×8.9R15 235/50R15 = 24.3×9.3R15 24X4.5R15 24X5R15 24.5X8R15 245/50R15 = 24.6×9.6R15 345/35R15 = 24.5×13.6R15||25″ 165/80R15 = 25.4X6.5R15 185/70R15 = 25.2×7.3R15 195/65R15 = 25×7.7R15 195/70R15 = 25.7×7.7R15 205/65R15 = 25.5×8.1R15 215/60R15 = 25.2×8.5R15 225/60R15 = 25.6×8.9R15 265/50R15 = 25.4X10.4R15 275/50R15 = 25.8×10.8R15|
|26″ 205/70R15 = 26.3×8.1R15 215/65R15 = 26×8.5R15 215/70R15 = 26.9×8.5R15 235/60R15 = 26.1×9.3R15 245/60R15 = 26.6×9.6R15 26X10R15 26X10.5R15 26X11.5R15 26X6R15 26X7.5R15 26X8R15 26X8.5R15 295/50R15 = 26.6×11.6R15||27″ 205/75R15 = 27.1×8.1R15 215/75R15 = 27.7×8.5R15 225/70R15 = 27.4×8.9R15 255/60R15 = 27x10R15 27X10.5R15 27X9.5R15 27.5X4.5R15 295/55R15 = 27.8X11.6R15 325/50R15 = 27.8×12.8R15 E78-15 = 27×6.5R15||28″ 225/75R15 = 28.3×8.9R15 235/70R15 = 28×9.3R15 235/75R15 = 28.9×9.3R15 245/70R15 = 28.5X9.6R15 255/65R15 = 28.1x10R15 275/60R15 = 28×10.8R15 28X10.5R15 28X11.5R15 28X12.5R15 28X13.5R15 28X4.5R15 28X7.5R15 28X9R15|
|29″ 215/85R15 = 29.4X8.5R15 255/70R15 = 29.1x10R15 265/70R15 = 29.6×10.4R15 29X10.5R15 29X11.5R15 29X12.5R15 29X15.5R15 29X18R15 29X18.5R15 29X4.5R15 29X8.5R15 29.5X11.5R15 29.5X13.5R15 29.5X9R15 315/60R15 = 29.9×12.4R15 7R15 = 29.5×7.7R15||30″ 255/75R15 = 30.1x10R15 265/75R15 = 30.6×10.4R15 295/65R15 = 30.1×11.6R15 30X10R15 30X10.5R15 30X11.5R15 30X12.5R15 30X13.5R15 30X7R15 30X9R15 30X9.5R15||31″ 31X10.5R15 31X11R15 31X11.5R15 31X12.5R15 31X13R15 31X13.5R15 31X14.5R15 31X16.5R15 31X18R15 31X18.5R15 31X9.5R15|
|32″ 32X10.5R15 32X11.5R15 32X14R15 32X16R15 32X17.5R15 32X18.5R15 32X9.5R15||33″ 33X10.5R15 33X11.5R15 33X12.5R15 33X13R15 33X13.5R15 33X14R15 33X14.5R15 33X15.5R15 33X16.5R15 33X19.5R15 33X21.5R15 33X9R15 33X9.5R15||34″ 34X10.5R15 34X12.5R15 34X9.5R15|
|35″ 35X10.5R15 35X12.5R15 35X13.5R15 35X14.5R15 35X15R15 35X15.5R15 35X16R15||36″ 36X12.5R15 36X13.5R15 36X14.5R15||37″ 37X12.5R15 37X13R15 37X13.5R15 37X14R15 37X14.5R15|
|38″ 38X12.5R15 38X13R15 38X14.5R15 38X15.5R15 38.5X11R15 38.5X13.5R15 38.5X14.5R15 38.5X15R15 38.5X16R15||39″ 39X13.5R15 39.5X13.5R15 39.5X15R15 39.5X18R15||40″ 40X16R15|
|42″ 42X14R15 42X15R15 42.5X13.5R15||44″ 44X18.5R15 44X19.5R15 44X21R15||47″ 47X17R15|
Chart for 16.5” diameter
|29″ Diameter 8.75R16.5 = 29.5×8.8R16.5||30″ Diameter 9.5R16.5 = 30.6×9.5R16.5||32″ Diameter 32X12R16.5|
|33″ Diameter 33X11.5R16.5 33X12.5R16.5 33X14R16.5 33X14.5R16.5 33X15.5R16.5||35″ Diameter 35X12.5R16.5 35X14.5R16.5 35X15R16.5 35X15.5R16.5 35X16R16.5 35X9.5R16.5||36″ Diameter 36X12.5R16.5 36X13.5R16.5 36X14.5R16.5 36X16R16.5|
|37″ Diameter 37X12.5R16.5 37X13R16.5 37X14R16.5||38″ Diameter 38X11R16.5 38X12.5R16.5 38X15.5R16.5 38.5X11R16.5 38.5X13.5R16.5 38.5X14.5R16.5 38.5X15R16.5 38.5X16R16.5||42″ Diameter 42X14R16.5 42X15R16.5 42.5X13.5R16.5|
|44″ Diameter 44X18.5R16.5 44X18.5R16.5 44X19.5R16.5 44X21R16.5|
Chart for 25” diameter
|14″ Wheel 185R14 = 25.7×7.3R14 195/75R14 = 25.5×7.7R14 205/70R14 = 25.3×8.1R14 215/70R14 = 25.9×8.5R14 235/60R14 = 25.1×9.3R14 245/60R14 = 25.6×9.6R14||15″ Wheel 165/80R15 = 25.4X6.5R15 185/70R15 = 25.2×7.3R15 195/65R15 = 25×7.7R15 195/70R15 = 25.7×7.7R15 205/65R15 = 25.5×8.1R15 215/60R15 = 25.2×8.5R15 225/60R15 = 25.6×8.9R15 265/50R15 = 25.4X10.4R15 275/50R15 = 25.8×10.8R15||16″ Wheel 135/90R16 = 25.6X5.3R16 145/80R16 = 25.1X5.7R16 195/60R16 = 25.2×7.7R16 205/60R16 = 25.7×8.1R16 215/55R16 = 25.3×8.5R16 225/55R16 = 25.7×8.9R16 245/50R16 = 25.6×9.6R16 265/45R16 = 25.4X10.4R16 275/45R16 = 25.7×10.8R16 315/40R16 = 25.9X12.4R16|
|17″ Wheel 115/95R17 = 25.6X4.5R17 135/80R17 = 25.5X5.3R17 145/70R17 = 25X5.7R17 155/70R17 = 25.5X6.1R17 205/50R17 = 25.1×8.1R17 205/55R17 = 25.9×8.1R17 215/50R17 = 25.5X8.5R17 225/45R17 = 25×8.9R17 225/50R17 = 25.9×8.9R17 235/45R17 = 25.3×9.3R17 245/45R17 = 25.7×9.6R17 255/40R17 = 25x10R17 265/40R17 = 25.3×10.4R17 275/40R17 = 25.7×10.8R17 295/35R17 = 25.1×11.6R17 315/35R17 = 25.7×12.4R17||18″ Wheel 135/70R18 = 25.4X5.3R18 155/60R18 = 25.3X6.1R18 215/45R18 = 25.6×8.5R18 225/40R18 = 25.1×8.9R18 235/40R18 = 25.4×9.3R18 245/40R18 = 25.7×9.6R18 255/35R18 = 25x10R18 265/35R18 = 25.3×10.4R18 275/35R18 = 25.6×10.8R18 285/35R18 = 25.9×11.2R18 295/30R18 = 25×11.6R18 305/30R18 = 25.2x12R18 315/30R18 = 25.4×12.4R18 335/30R18 = 25.9×13.2R18||19″ Wheel 115/70R19 = 25.3X4.5R19 125/70R19 = 25.9X4.9R19 225/35R19 = 25.2×8.9R19 235/35R19 = 25.5×9.3R19 245/35R19 = 25.8×9.6R19 255/30R19 = 25x10R19 265/30R19 = 25.3×10.4R19 275/30R19 = 25.5×10.8R19 285/30R19 = 25.7×11.2R19 315/25R19 = 25.2×12.4R19|
|20″ Wheel 215/30R20 = 25.1X8.5R20 225/30R20 = 25.3×8.9R20 235/30R20 = 25.6×9.3R20 245/30R20 = 25.8×9.6R20 275/25R20 = 25.4X10.8R20 285/25R20 = 25.6×11.2R20 295/25R20 = 25.8×11.6R20|
Where to find tire size?
Reading your tire size may seem like a complicated science. If you don’t understand the meaning of a single element, the entire picture won’t make sense. But complexity is only limited to understanding the size. When figuring out the size for your car, you won’t have to jump through these many hoops.
You can find your car’s size in two places. First, look at the information sticker on the driver’s side door. Alternatively, you can refer to the vehicle owner’s manual. You can find the manual in the glove department. Both the sticker and the manual will have complete information about each dimension of the size.
How to read a tire size?
Reading and understanding the size isn’t a skill that only yields a theoretical value but offers practical application. Unlike geometry classes from high school, we assure you that this knowledge will be tested in the real world. If you’re unable to read tire size, there’s a possibility that you may end up with the wrong size. A tire that doesn’t compliment your car’s settings will deteriorate the car’s performance and stability.
So before you spend hundreds of dollars on a tire only to find out it’s the wrong size, here’s how you can read and understand each element:
We’ll use the tire size ‘P225/70R16 91S’ as an example to break down the meaning of each dimension.
P – The first letter of the code indicates the class to which the tire belongs. Using the example above, we can deduce that the tire is designated for a passenger vehicle. It’s suited for conventional cars, minivans, and SUVs.
If the code had read LT, it would be suitable for a light truck. In contrast, ST-labeled tires would be ideal for special trailers.
225 – The first 3-digit number in the code refers to the tire’s width, measured in millimeters from one sidewall to another. For this particular tire size, the width is 225 mm.
70 – Following the width is the aspect ratio, also known as the profile size. The aspect ratio measures the height of the sidewall as a percentage of the width. For example, an aspect ratio of 70 suggests the sidewall height be 70% as high as the tire is wide.
R – The second alphabet in the code refers to the construction type. R stands for radial structure, made of radial plies running perpendicular to the movement’s direction. They’re made of a mix of fabric, steel, and rubber. The only other type is type D, which is made of plies forming a crisscross pattern.
16 – The two digit-value next to the construction type is the wheel’s diameter, measured in inches.
91 – The last number in the code is the load index or service description rating. The value indicates the weight capacity. You can find the corresponding weight in pounds by using a conversion chart.
S – The last element in the code is a letter representing the speed rating. The alphabetical value can be traced to a corresponding speed in mph. For an S speed rating, the maximum speed is 112 mph.
What is the tire aspect ratio?
The tire aspect ratio measures the sidewall’s height relative to the width. The height is the distance between the wheel rim and the top of the tread. In contrast, the width is measured between two sidewalls.
The height is expressed as a percentage of the width. For instance, an 84 aspect ratio suggests that the sidewall’s height is 80% as high as the tire is wide.
What does 265/70R7 mean?
The tire size ‘265/70R7’ provides information about multiple elements, which is crucial to understanding the size. The number 265 refers to the width in millimeters. Followed by 70, which is the aspect ratio. It suggests that the sidewall’s height is 70% as high as the tire is wide. The letter R refers to the construction type. Lastly, the single digit 7 measures the wheel’s diameter in inches.
To an inexperienced driver, all the tires look the same. But you may be surprised to find a wide variety in the market. Depending on the function and the terrain, you’ll have to choose accordingly. So here are all the types you need to know of:
If you’re looking to improve the overall driving experience, there’s no better type than performance tires. They’ve distinct tread patterns designed to enhance the grip and responsiveness to the road. With these on, you’ll feel like you’re gliding on an icy lake.
Touring tires are perfect if you’re looking for an option that doesn’t betray you when it gets too cold out. They’re designed with better wet/dry traction, allowing long-distance travel smoother than ever. In addition, you won’t have to worry about switching to another type once the snow settles with their high all-season rating.
Despite all-season tires’ best efforts, it might not be suitable for you. Winter tires are specifically created to improve traction in snow and ice. With deeper treads and irregular edges, winter tires guarantee a smooth experience even when the road is covered in thick snow.
Driving on smooth surfaces requires no particular skill or equipment, but things get much more complicated when driving on rough terrains. If you’re worried about your vehicle’s performance on uneven roads, we suggest investing in all-terrain tires. Their complex tread patterns can handle different driving conditions, from sand and gravel to broken roads.
Special service tires are your best bet if you’re scared of getting stuck in a quicksand situation. They’ve outward and pointy treads designed to carefully handle damp soil conditions. The design prevents any mud from clogging the structure and inhibiting smooth movement.
For race fanatics, regular tires don’t provide the same thrill. So if you’re looking for speed, track tires will perform tremendously. They offer much better grip and ensure a safe driving experience even when you’re going at 30 mph.
It means the tire has a 265 mm tread width. The number 70 refers to the ratio between the height of the sidewall to tread width. The inner rim mounting diameter is 17 inches. It’s a radial tire.
It means it has an inner rim mounting diameter of 20 inches and a width of 12 inches. The sidewall is 50% of the tread width.
It depends on the car, but it’s sometimes possible.
Yes, it should work fine in most cases.