Tire Clearance: How to Measure It and Compare Tire Sizes

Tire clearance is the free space between a mounted tire and the nearest part of the vehicle. That includes the fender lip, inner fender liner, strut, spring, control arms, sway bar and frame. It is measured in millimetres or inches.

To measure it, check the gap at each of these points with the vehicle at ride height. Then check again with the wheels at full steering lock and with the suspension in its travel range.

To compare a different tire size, start with the size code on the sidewall, such as 225/45R17. Calculate how much the new size changes the overall diameter and section width. Then subtract those changes from the gaps you measured:

• Half of the diameter increase comes off the space above the tread.
• If the wheel offset stays the same, half of the width increase comes off each side.

This calculation gives a nominal estimate only. Actual tire dimensions and vehicle fitment have to be confirmed separately.

What Is Tire Clearance?

Tire clearance is the smallest distance between any surface of the tire and any fixed or moving part of the vehicle, under a given condition. It is not one number. A tire can have plenty of room at the top of the wheel arch and almost none next to the strut. A gap that looks fine when parked can disappear when the suspension compresses or the wheels are turned.

It helps to think of tire clearance in five zones:

• Radial clearance: the gap between the tread and the fender liner or arch above and around the tire. Overall tire diameter controls this zone.
• Inboard clearance (inner sidewall clearance): the gap between the inner sidewall and suspension components such as the strut body, spring, spring perch, control arms and sway bar links.
• Outboard clearance (fender clearance): the horizontal relationship between the outer sidewall or tread shoulder and the fender lip.
• Steering lock clearance: the gap between a front tire and the frame, inner liner, bumper cover or sway bar when the steering is turned fully left or right.
• Suspension travel clearance: the gap that remains when the suspension compresses, the body rolls in a corner, or the axle articulates off-road.

Tire rubbing happens when any one of these zones shrinks to zero under some driving condition. That is why clearance has to be judged under motion as well as at rest.

How to Read the Numbers on the Tire Sidewall

A standard metric tire size code gives three dimensions:

• the tire section width, in millimetres
• the aspect ratio, as a percentage
• the rim diameter, in inches

The other letters and numbers in the code describe construction, intended use and performance ratings. They do not describe size.

The table below breaks down the common passenger tire marking P225/45R17 94V.

The metric size code mixes units. Width is in millimetres, but rim diameter is in inches. Before calculating overall tire diameter, convert one of them: 1 inch = 25.4 mm.

Why the Aspect Ratio Is Not a Fixed Sidewall Height

The aspect ratio is a percentage of the section width, so the same aspect ratio gives different sidewall heights on tires of different widths. For example:

• A 225/45 tire has a nominal sidewall height of 101.25 mm (225 × 0.45).
• A 245/45 tire has a nominal sidewall height of 110.25 mm (245 × 0.45).

Comparing aspect ratios alone, without the width, does not tell you which tire is taller.

Flotation (Inch) Sizes on Light-Truck Tires

Some light-truck and off-road tires use an inch-based flotation size such as 33x12.50R17 LT. Read it as follows:

• 33: nominal overall diameter, in inches
• 12.50: nominal section width, in inches (about 317.5 mm)
• R: radial construction
• 17: rim diameter, in inches

No aspect ratio appears because the overall diameter is stated directly.

Sidewall Markings That Are Not Dimensions

Several sidewall markings say nothing about whether a tire physically fits:

• UTQG treadwear, traction and temperature grades
• The DOT code and manufacturing date
• Maximum load and maximum inflation pressure
• XL (extra load) or reinforced markings
• M+S and the three-peak mountain snowflake symbol

Load index, speed rating and load capacity still matter when choosing a replacement size, because the new tire must carry the vehicle's load. They do not change the physical gap between the tire and the vehicle.

How to Calculate Sidewall Height, Overall Diameter and Circumference

You can calculate the nominal sidewall height, overall diameter and circumference of a metric tire from its section width, aspect ratio and rim diameter with three formulas:

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

To convert the overall diameter to inches, divide the millimetre value by 25.4.

Worked Example: 225/45R17

1. Sidewall height: 225 × 45 ÷ 100 = 101.25 mm.
2. Rim diameter in millimetres: 17 × 25.4 = 431.8 mm.
3. Overall diameter: 431.8 + 2 × 101.25 = 634.3 mm, which is about 24.97 in.
4. Circumference: 634.3 × 3.1416 ≈ 1,993 mm.

Comparing Tire Sizes: Diameter, Width and What Changes at Each Gap

The table below compares four common alternatives with a 225/45R17 baseline. It assumes the wheel offset does not change.

• Radius change is half the diameter change. It is roughly how much the tread moves toward the arch above it.
• Width change per side is half the section width change. It is how much each sidewall moves outward, toward the fender lip on one side and toward the suspension on the other.

The table shows two separate effects.

Plus sizing means moving from a 17-inch to an 18-inch rim with a lower aspect ratio, as with 225/40R18. It can keep the overall diameter almost unchanged while shortening the sidewall. In that case radial clearance barely changes.

Section width is independent of diameter. A 245 tire takes 10 mm more space on each side than a 225 tire on the same offset, whether its diameter grows or not.

What the Circumference Difference Means

Circumference is the distance a tire travels in one revolution, ignoring the slight flattening at the contact patch. The circumference changes by the same percentage as the overall diameter.

For example, a 245/40R18 is about 3.0% larger in circumference than a 225/45R17. On the 245/40R18:

• The tire covers about 3% more distance per revolution.
• It makes about 487 revolutions per kilometre, compared with about 502 for the 225/45R17.
• If the speedometer was accurate on the original tires, it will read about 3% low. A displayed 100 km/h would correspond to roughly 103 km/h.

Circumference does not affect clearance directly. It is one of the dimensional differences a size comparison reveals.

Light-Truck Example in Inches

Larger off-road tire sizes show how quickly diameter grows. All three sizes below use a 17-inch rim (431.8 mm):

• 265/70R17: sidewall 185.5 mm, overall diameter 802.8 mm (about 31.61 in)
• 285/70R17: sidewall 199.5 mm, overall diameter 830.8 mm (about 32.71 in)
• 285/75R17: sidewall 213.75 mm, overall diameter 859.3 mm (about 33.83 in)

Moving from 265/70R17 to 285/75R17 has these nominal effects:

• Overall diameter grows by 56.5 mm (about 2.2 in, or 7.0%).
• The tread moves about 28 mm (about 1.1 in) closer to the arch.
• Each sidewall moves 10 mm outward if the offset is unchanged.

Calculated Geometry vs Actual Tire Dimensions

A tire size calculation gives nominal dimensions. A real tire mounted on a real wheel can measure differently. The size code is a standardized label, not a precise measurement of the specific tire you buy. The main reasons for the difference are listed below.

• Brand and model: Two tires with the same size code can differ in overall diameter and section width. Tire manufacturers usually publish measured overall diameter and section width for each model, along with the measuring rim width used. Use these published figures in place of the calculated ones when they are available.
• Rim width: Section width changes with the width of the wheel it is mounted on. A wider rim spreads the tire wider, and a narrower rim pinches it. A commonly quoted rule of thumb is about 0.2 in (roughly 5 mm) of section width for every 0.5 in of rim width away from the measuring rim. Treat this as an approximation.
• Overall width vs section width: Rim protector ribs, raised lettering and aggressive shoulder lugs on off-road tires can make the widest point of the tire wider than the stated section width.
• Tread width and shoulder shape: The tread is narrower than the section width. Square-shouldered and rounded-shoulder tires fill the space near a fender lip differently.
• Load and inflation: Under vehicle weight, the distance from wheel centre to ground (loaded radius) is less than half the overall diameter, and the sidewall bulges near the contact patch. Inflation pressure changes both effects.
• Tread wear: A new tire has its full tread depth, so it is slightly larger in diameter than the same tire when worn.

Fitment compatibility is a separate question from geometry. A new size can clear every component and still be unsuitable. It may not meet the vehicle's required load capacity. It may fall outside the approved rim width range for the wheel. It may differ from the sizes approved for the vehicle. The calculation only tells you how much space the tire should take up.

How to Measure Tire Clearance on Your Vehicle

To measure tire clearance, record the smallest gap between the tire and each nearby component. Do this at ride height first, then repeat the checks at steering lock and with the suspension's remaining travel in mind.

You will need:

• a steel ruler or tape measure
• a straightedge or plumb line
• a flashlight
• a notebook
• calipers or a set of feeler gauges for narrow gaps (optional)

If you lift the vehicle at any point, support it on jack stands, never on a jack alone.

1. Prepare the vehicle. Park on level ground with the tires at the recommended pressure and the wheels pointing straight ahead. Note how the vehicle is loaded. Passengers and cargo lower the body and reduce the gaps.
2. Record ride height. Measure from the centre of the wheel hub straight up to the fender lip. This gives a reference you can compare against later, for example after a suspension change.
3. Measure radial clearance. Measure from the tread to the fender liner or arch at the top of the tire. Then measure toward the front and rear of the arch, because the arch is curved and the tightest point may not be at the top.
4. Measure outboard fender clearance. Hold a straightedge or hang a plumb line vertically from the fender lip. Measure the horizontal distance to the outermost point of the sidewall or tread shoulder. If the tire sits outside the fender line, record the value as negative.
5. Measure inboard clearance. Find the narrowest gap between the inner sidewall and the strut body, spring, spring perch, control arms and sway bar links. Measure at several heights. The closest point is often near the strut or spring seat.
6. Check steering lock. Turn the steering fully left, then fully right. At each lock, measure from the leading and trailing edges of each front tire to the frame rail, inner liner, bumper cover and sway bar.
7. Consider suspension travel. Measure the remaining space between each bump stop and the part it contacts. This shows roughly how far the wheel can still move upward. Because the wheel moves in an arc, it can also tilt toward the liner as it rises. Accurately checking clearance through full travel usually requires cycling the suspension with the springs removed, which is a job for an experienced technician or shop.
8. Record everything. Write down each measurement with its location, the side of the vehicle and the condition (ride height or full lock). Measure both sides, because vehicles are rarely perfectly symmetrical.

Where Tire Clearance Is Usually Tightest

The table below shows the common tight spots, which tire dimension reduces each gap, and which wheel change affects it.

Tire Clearance to Suspension Components

Tire clearance to suspension components depends on two things: the tire's section width and where the wheel places the tire relative to the hub. Wheel offset and backspacing describe that position.

• Wheel offset is the distance, in millimetres, from the wheel's centreline to its hub mounting face. A higher positive offset places the wheel and tire further inward, toward the suspension.
• Backspacing is the distance, usually in inches, from the mounting face to the inner edge of the rim. More backspacing also means the wheel sits closer to the suspension.

With these definitions, you can estimate how each gap changes:

• Change in inboard gap = −(width increase ÷ 2) − (offset increase)
• Change in outboard gap = −(width increase ÷ 2) + (offset increase)
• Change in radial gap = −(diameter increase ÷ 2)

All values are in millimetres. An offset increase means a more positive offset.

Example: Wider Tire, Then a Different Offset

Suppose you measured 15 mm between the inner sidewall of a 225/45R17 and the strut, on a wheel with a 45 mm offset (ET45). The figures below are illustrative.

• Same wheel, 245/45R17 tire: The width grows 20 mm, so the inner sidewall moves about 10 mm inward. The estimated inboard gap drops to about 5 mm, before accounting for the actual measured width of the chosen tire on that rim, or for any suspension movement.
• 245/45R17 on an ET35 wheel of the same width: The whole tire moves 10 mm outward, so the inboard gap returns to about 15 mm. The outboard gap now loses 20 mm in total: 10 mm from the extra width and 10 mm from the offset change. The space problem has moved from the strut to the fender lip.

This trade-off is why a width increase cannot be judged by the inboard or outboard gap alone. Changing offset to protect one gap takes space from the other.

How Steering and Suspension Travel Change Suspension Clearance

The type of front suspension decides which gaps change when you steer.

• MacPherson strut: The strut body turns with the wheel, so the gap between the inner sidewall and the strut stays fairly constant when steering. The gaps to the frame rail, sway bar and liner change considerably near full lock.
• Double-wishbone: The upper control arm does not turn with the wheel, so the gap between the tire and that arm can shrink as the wheel is steered.

On any suspension design, compression and body roll move the tire upward and can tilt it. A gap measured at ride height is the starting point, not the clearance available while driving.

Larger Tires and Fender Clearance

A larger tire uses up fender clearance in two ways:

• At the arch, it takes about half of its diameter increase.
• At the outer fender lip, if the offset is unchanged, it takes about half of its width increase.

Whatever space remains must still absorb suspension compression, body roll and the forward and rearward swing of the front tires at steering lock.

Because the wheel arch is curved, a taller tire often contacts the front or rear section of the liner, or the bumper cover, before it contacts the top of the arch. This happens most when the wheels are turned and the suspension is compressed at the same time.

Take the hypothetical light-truck example again. If the original 265/70R17 had 40 mm between the tread and the top of the liner at ride height, a 285/75R17 would leave roughly 12 mm at that point when static. That estimate comes from 40 mm minus the 28 mm radius increase. It does not yet account for suspension travel, steering lock or the tire's actual measured diameter.

What a Tire Size Clearance Calculator Can and Cannot Tell You

A tire size clearance calculator does the dimensional arithmetic for two sizes. It usually reports:

• sidewall height
• overall diameter
• section width
• circumference
• the percentage differences between the two sizes

Some calculators also accept wheel width and offset. These estimate how far the inner sidewall moves toward the suspension and how far the outer sidewall moves toward the fender.

To use a calculator for clearance:

1. Enter your current tire size and the proposed tire size, plus wheel width and offset if the calculator supports them.
2. Note the change in overall diameter, the change in section width, and any inward or outward position change.
3. Apply these changes to the gaps you measured on the vehicle, using half the diameter change for radial gaps and half the width change for each side.
4. Replace the calculated tire figures with the manufacturer's published dimensions for the specific tire model whenever they are available.

A calculator does not know your vehicle. It cannot account for:

• the shape and position of your specific suspension components and liners
• the arc of your suspension travel and steering lock
• alignment settings such as camber
• ride height under load
• aftermarket parts
• tire-to-tire variation in actual dimensions

Its output is a starting estimate of how much space a tire size needs, not confirmation that the tire will fit.

Key Points to Remember

• The size code 225/45R17 means a 225 mm section width, a sidewall height of 45% of that width, and a 17-inch rim. The load index and speed rating are performance ratings, not dimensions.
• Overall diameter (mm) = rim diameter × 25.4 + 2 × section width × aspect ratio ÷ 100. Circumference is diameter × π.
• The radial gap shrinks by half the diameter increase. With the same offset, each side gap shrinks by half the width increase.
• Changing offset moves the whole tire, so it trades inboard clearance against fender clearance.
• Calculated sizes are nominal. Actual dimensions depend on the tire model, rim width, load and inflation. Physical fitment also depends on steering lock and suspension travel.