Tire Fitment Guide: Checking Replacement Sizes for Your Vehicle

To check whether a replacement tire size suits your vehicle, start with the size printed on your current tire sidewall and on the vehicle placard, such as 205/55R16 91V. Then calculate the replacement size's sidewall height, overall diameter and circumference and compare them with the original. Next confirm that the load index and speed rating meet the vehicle's requirement. Last, check that the tire suits your wheel width and clears the body, steering and suspension. The numbers tell you how two sizes compare on paper. They do not prove that a tire will physically fit a particular vehicle.

What Is Tire Fitment?

Tire fitment means a tire size passes four checks on a specific vehicle:

• It mounts correctly on the wheel.
• It turns and moves through the full range of steering and suspension travel without touching anything.
• It carries the vehicle's load.
• It is rated for the speeds the vehicle can reach.

A size fits only when all four are true. Geometry, load capacity, speed capability and physical clearance are separate checks. A size that passes one of them can still fail another.

How to Read the Numbers on a Tire Sidewall

A tire sidewall size such as P205/55R16 91V contains three types of information:

• Dimensions: width and rim diameter, in millimetres or inches.
• A ratio: the aspect ratio, as a percentage.
• Performance ratings: the load index and speed symbol, which are codes rather than measurements.

The table below breaks down a typical passenger car marking.

Dimensions: Section Width and Rim Diameter

Section width is the width of the inflated tire body in millimetres, measured from sidewall to sidewall. It is measured when the tire is mounted on a standard measuring rim. Raised lettering and protective ribs are not included. Section width is not the same as tread width, which is the part that contacts the road and is narrower.

Rim diameter is given in inches and must exactly match the wheel's diameter. A 16 in tire mounts only on a 16 in wheel. That is why a size with a different rim diameter always requires different wheels.

A typical passenger size therefore mixes two units: millimetres for width and inches for the rim. Calculations must convert to a single unit (1 in = 25.4 mm).

Ratio: Aspect Ratio

The aspect ratio is the sidewall height divided by the section width, expressed as a percentage. In 205/55R16, the sidewall is 55% of 205 mm, or about 112.8 mm. A lower aspect ratio means a shorter sidewall relative to width. It is a ratio, not a measurement in millimetres.

Light Truck and Flotation Markings

Truck tires often use one of two systems:

• LT-metric sizes, such as LT265/70R17, read the same way as passenger sizes.
• Flotation sizes, such as 35x12.50R17LT, list everything in inches. Here that means a nominal overall diameter of 35 in, a section width of 12.50 in and a rim diameter of 17 in.

With flotation sizes, the overall diameter is stated directly instead of being calculated.

Performance Ratings: Load Index, Speed Rating and Load Range

The load index is a number that refers to a standard table of maximum load per tire at a specified inflation pressure. Some common values:

• 91 = 615 kg (1,356 lb)
• 100 = 800 kg (1,764 lb)
• 104 = 900 kg (1,984 lb)
• 112 = 1,120 kg (2,469 lb)
• 121 = 1,450 kg (3,197 lb)

A higher number means greater load capacity, but the scale is not linear. Each step adds a different amount of load.

The speed rating is a letter for the maximum speed the tire is certified to sustain under specified test conditions:

• Q = 160 km/h (99 mph)
• S = 180 km/h (112 mph)
• T = 190 km/h (118 mph)
• H = 210 km/h (130 mph)
• V = 240 km/h (149 mph)
• W = 270 km/h (168 mph)
• Y = 300 km/h (186 mph)

Several other markings relate to load:

• XL or Reinforced marks a passenger tire built to carry more load at a higher inflation pressure.
• LT load range letters (C, D, E, F) indicate increasingly strong construction and higher maximum inflation.
• Two load indexes on an LT tire, such as 121/118, give the load per tire in single and dual-wheel use.

Other Sidewall Markings

• Maximum load and maximum pressure: the tire's structural limits. Maximum pressure is not the recommended pressure for your vehicle. The vehicle placard gives that.
• M+S and the three-peak mountain snowflake symbol: indicate mud-and-snow design and certified severe-snow performance.
• DOT code (US) or E-mark (Europe): certification markings. On a DOT code, the last four digits give the week and year of manufacture.
• UTQG grades (US passenger tires): treadwear, traction and temperature grades. They are useful for comparing tires but do not affect size or fitment.

Where to Find the Original Equipment Tire Size

The original equipment tire size is listed in two places:

• The tire placard, usually on the driver's door jamb, sometimes inside the fuel door or glovebox.
• The owner's manual.

The placard also lists the recommended inflation pressure, and often the required load index and speed rating.

Compare the placard with the sidewall on the car. The tires currently fitted may not be the original size if a previous owner changed them.

Optional tire sizes are sizes the vehicle manufacturer has approved for the same model, often because they were fitted to other trim levels or wheel packages. Optional sizes are the most reliable starting point for alternatives because the manufacturer has already considered clearance, load and electronic systems. Alternative or replacement tire sizes that are not on the manufacturer's list rely on calculation and physical checking instead.

How to Calculate Sidewall Height, Overall Diameter and Circumference

You can calculate a tire's main dimensions from its size marking with three formulas:

• Sidewall height (mm) = section width (mm) × aspect ratio ÷ 100
• Overall diameter (mm) = (rim diameter in inches × 25.4) + (2 × sidewall height)
• Circumference = overall diameter × π (about 3.1416)

Sidewall height is counted twice in the diameter because the tire has sidewall above and below the wheel. Revolutions per kilometre equal 1,000,000 ÷ circumference in mm. Revolutions per mile equal 63,360 ÷ circumference in inches.

Worked Example: 205/55R16

1. Sidewall height: 205 × 55 ÷ 100 = 112.75 mm (4.44 in).
2. Rim diameter in millimetres: 16 × 25.4 = 406.4 mm.
3. Overall diameter: 406.4 + (2 × 112.75) = 631.9 mm (24.88 in).
4. Circumference: 631.9 × 3.1416 ≈ 1,985 mm (78.2 in).
5. Revolutions: about 504 per kilometre, or about 811 per mile.

Why Aspect Ratio Depends on Width

Because aspect ratio is a percentage of width, changing only the width also changes sidewall height. Take two sizes on a 17 in rim:

• 225/65R17: sidewall 146.3 mm, overall diameter about 724.3 mm.
• 255/65R17: sidewall 165.8 mm, overall diameter about 763.3 mm.

The wider tire is about 5.4% taller in diameter, even though both show 65. Two sizes with the same aspect ratio number are the same height only if their widths are also the same.

Alternative Tire Sizes for Your Car: Comparing Replacement Sizes

An alternative tire size is a different size whose calculated overall diameter stays close to the original. That keeps the speedometer, odometer and gearing close to how the car was designed.

When a larger wheel is used, the usual approach is called plus sizing. The rim diameter goes up, the aspect ratio comes down and the width often increases. The result is a similar overall diameter with a shorter sidewall.

The table compares calculated values for sizes often considered as alternatives to a 205/55R16. The speedometer column shows the true speed when the speedometer reads 100 km/h, assuming it was accurate on the original size.

What a Percentage Difference in Diameter Means

A percentage difference in overall diameter means the same percentage difference in circumference and in distance travelled per wheel revolution. If a replacement tire is 1.7% larger in diameter, the car covers about 1.7% more distance at the same indicated speed. The speedometer then reads about 1.7% low, and the odometer under-records distance by the same proportion.

A commonly used rule of thumb keeps the diameter of a replacement size within about 3% of the original. That rule is only a screening guide. It does not guarantee clearance, and some vehicles have tighter limits. All-wheel-drive vehicles, for example, can be sensitive to diameter differences between tires or between axles.

What the Table Does Not Show

Two sizes can match closely in diameter and still differ in several ways that matter for fitment:

• Width, and therefore side clearance.
• The wheel width they require.
• Load index. A low-profile plus size may need an XL version to match the original load capacity. For example, 205/55R16 is commonly sold as 91V, while 225/45R17 is commonly offered as 91 in standard load and 94 in XL.

Always compare the load index printed on the specific replacement tire, not just its size.

Calculated Geometry vs Actual Tire Dimensions

Calculated dimensions come from the nominal size marking. A real tire's measured dimensions can differ from them because of:

• Manufacturer and tread design: two tires with the same size marking can differ in actual diameter and width. Each manufacturer's specification sheet lists its own measured overall diameter and section width.
• Rim width: section width is defined on a measuring rim. On a wider or narrower wheel the tire's width changes. A commonly used approximation is about 5 mm (0.2 in) of section width per 0.5 in change in rim width.
• Inflation and load: the calculated diameter is unloaded. Under the vehicle's weight the tire flattens at the contact patch. The loaded radius, measured from axle centre to ground, is smaller than half the overall diameter.
• Tread wear: diameter falls by twice the tread depth lost. Wearing 8/32 in (6.35 mm) of tread reduces diameter by about 0.5 in (12.7 mm). On a 24.88 in tire that is about 2%, which is larger than some of the size differences in the table above.
• Flotation sizes: the first number, such as 35, is a nominal diameter. Actual measured diameters vary by brand and model.

For tight-clearance decisions, use the measured figures from the manufacturer's specification sheet rather than values calculated from the size marking.

Calculated Size vs Vehicle Fitment Compatibility

Matching diameter on paper does not make a tire compatible with a vehicle. Compatibility also depends on the following factors.

Approved Rim Width

Every tire size has a range of approved rim widths. 205/55R16, for example, is typically listed for rims of about 5.5 to 7.5 in. A tire mounted on a rim outside its approved range may not seat or support the load as designed. Check the tire manufacturer's data for the exact range.

Wheel Offset and Where the Extra Width Goes

Wheel offset is the distance in millimetres from the wheel's centreline to its mounting face. A higher offset places the wheel further inward. A lower or negative offset pushes it further outward.

On the same wheel, a wider tire grows roughly equally on both sides. Going from a 205 to a 225 section width adds 20 mm, or about 10 mm toward the suspension and 10 mm toward the fender. Changing the wheel offset by 10 mm shifts the entire wheel and tire 10 mm inward or outward.

Physical Clearance

Tire clearance must be checked in four places:

• Fender clearance: to the fender lip and liner.
• Suspension clearance: to struts, springs and control arms.
• Steering clearance: at full steering lock in both directions.
• Compression clearance: at full suspension compression, when the tire rises into the wheel well.

A larger diameter uses up space above and in front of and behind the tire. Extra width uses up space at the sides.

Load Capacity and Speed Rating

A replacement tire should have at least the original load capacity. It should generally have at least the original speed rating, although local rules and winter-tire exceptions vary.

Load capacity depends on inflation pressure, so an equivalent load index only holds at the appropriate pressure. P-metric, Euro-metric and LT tires follow different load and inflation tables. Use the load and inflation tables for the specific tire type rather than assuming two sizes with similar numbers carry the same load.

Truck Tire Fitment Guide

Truck tire fitment follows the same calculations as car fitment, with two extra checks: the tire type and the load range.

Many pickups and SUVs come with either P-metric or LT tires. Switching between them changes load capacity, recommended pressure and ride characteristics, even at the same size. When a P-metric tire is used on a light truck, its rated load capacity is reduced under industry practice.

Moving to LT tires means checking that the load range suits the vehicle. A load range E tire, for example, is designed for higher pressures than most passenger vehicles use.

For flotation sizes, overall diameter and width are already given in inches. You can compare them directly with a metric size after converting that size's calculated diameter to inches.

Lifted Truck Tire Fitment Guide

On a lifted truck, the suspension or body lift raises the body to create vertical space for a taller tire. It does not by itself create space for a wider tire, more steering lock or a different wheel offset.

The table compares a common original size, 265/70R17, with larger options often fitted to trucks with a leveling kit or suspension lift. The ground clearance column shows how much the axle centre rises, which is half the increase in overall diameter. The last column shows the true speed when the speedometer reads 60 mph, assuming it was accurate on the original size.

How to Read the Lifted Truck Numbers

A taller tire raises the axles, differential housings and the whole vehicle by half the diameter increase. A 35 in tire replacing a 265/70R17 raises the axle centre by about 43 mm. That calculated rise is the only ground clearance a tire size change adds.

At the same time, the top of the tire moves about 43 mm closer to the fender and body. The front and rear edges of the tire also extend further, which affects clearance at full steering lock.

The extra width matters too. A 315 tire is 50 mm wider than a 265, or about 25 mm on each side on the same wheel. That extra width often brings the tire closer to frame rails, control arms or steering components when turned. Lower-offset wheels are commonly used to move the tire outward for that reason, but they change the tire's position relative to the fenders.

Other Changes That Come With Large Size Increases

Large diameter increases affect several systems beyond clearance:

• Speedometer and odometer: a 10.7% increase is far beyond the common 3% screening guide, so readings will be noticeably wrong unless recalibrated.
• Effective gearing: taller tires make the gearing effectively taller.
• Load and pressure: larger LT tires may need different inflation pressures to carry the same load, so check the manufacturer's load and inflation tables.

Calculated dimensions only show the size of these changes. Whether a truck accepts the tire depends on its lift, wheel offset and steering and suspension geometry, which must be checked on the vehicle itself.

How to Check Tire Fitment: Step by Step

1. Find the original tire size. Read the vehicle placard and owner's manual. Note the size, load index, speed rating and recommended pressure, and any optional sizes listed by the manufacturer.
2. Read the current sidewall. Confirm whether the fitted tires match the placard and note the tire type: P-metric, Euro-metric, LT or flotation.
3. Calculate both sizes. Work out sidewall height, overall diameter and circumference for the original and the replacement size, using a single unit.
4. Compare the diameter difference. Express the difference as a percentage. Many drivers use about 3% as a screening limit, unless the vehicle manufacturer lists the size as optional or the vehicle has been modified for larger tires.
5. Compare the width difference. Halve the width change to estimate how far the tire grows toward the suspension and toward the fender on the same wheel.
6. Check wheel compatibility. Confirm the rim diameter matches. Confirm the wheel width falls within the tire's approved rim width range and note the wheel offset.
7. Check performance ratings. The load index, and the load range for LT tires, should provide at least the original load capacity at the correct pressure. The speed rating should generally match or exceed the original.
8. Use measured data for close calls. Take actual overall diameter and section width from the tire manufacturer's specification sheet rather than calculated values.
9. Check clearance on the vehicle. Check fender clearance, suspension clearance and steering clearance at full lock in both directions and through suspension compression. Do this before relying on the size, and again with the tires installed.

Key Points About Tire Size Numbers and Their Limits

• Section width (mm) and rim diameter (in) are dimensions. Aspect ratio (%) is a ratio. Load index and speed symbol are performance codes.
• Overall diameter equals rim diameter plus two sidewall heights, and circumference equals diameter × π.
• The percentage difference in diameter equals the percentage difference in speedometer reading and distance per revolution.
• Calculated values are nominal. Actual dimensions vary with brand, rim width, inflation, load and tread wear.
• A size can match the original geometry on paper and still fail on load capacity, speed rating, wheel width, offset or physical clearance.