How to Measure Wheel Radial Runout, Starting With the Numbers on the Tire Sidewall

To measure wheel radial runout, mount the wheel on a true-running hub or balancer and place a dial indicator perpendicular to the rim bead seat. Rotate the wheel slowly through one full turn and record the difference between the highest and lowest readings. That difference is the total radial deviation, usually given in millimetres or thousandths of an inch. Compare it with the vehicle or wheel manufacturer's specification. Before you measure, read the tire sidewall. The size code tells you the nominal rim diameter, which locates the bead seat. It also gives the sidewall height, which helps you separate tire effects from wheel effects.

What Is Wheel Radial Runout?

Wheel radial runout is how far the wheel's rolling surface or rim bead seat moves toward and away from the axle centre during one revolution. A wheel with zero radial runout is perfectly round and perfectly centred on its hub. Radial runout above the specified limit can cause a vertical shake or a hop that rises with road speed. Causes include an out-of-round rim, a bent rim, a tire that is not uniform, or a wheel that is not centred on the hub.

Lateral Versus Radial Runout

Lateral runout and radial runout are measured in different directions. Wheel roundness checks usually cover both.

Feature	Radial runout	Lateral runout
Direction of movement	Up and down, toward and away from the axle centre	Side to side, along the axle line (wobble)
Where the dial indicator is placed	On the rim bead seat or tread, pointing at the axle centre	On the rim flange face or sidewall, parallel to the axle
Typical symptom	Vertical shake or hop, often felt through the seat or floor	Side-to-side wobble, often felt through the steering wheel
Common causes	Out-of-round rim, flat spots, non-uniform tire, poor centring on the hub	Bent rim flange, hub face runout, uneven lug nut torque

What Do the Numbers on the Tire Sidewall Mean?

The tire size code on the sidewall describes the tire's nominal dimensions, its construction and its performance ratings. A common example is 225/45R17 94W. The numbers do not all use the same units. Some are dimensions, one is a ratio, and two are coded ratings that are not measurements of size at all.

Breaking Down a Typical Size Code

Marking	Type	Unit	Meaning in plain language
225	Dimension	Millimetres	Nominal section width: the widest point of the tire from sidewall to sidewall, on a specified measuring rim
45	Ratio	Percent	Aspect ratio: sidewall height is 45% of the section width
R	Construction	None	Radial construction; it does not refer to radius or runout
17	Dimension	Inches	Rim diameter: the nominal diameter of the rim bead seat the tire is built to fit
94	Performance rating	Coded index	Load index: a code for the maximum load per tire, read from a load index table
W	Performance rating	Coded letter	Speed rating: a code for the maximum sustained speed under specified test conditions

The code mixes metric and imperial units. Width is in millimetres, while rim diameter is in inches. Convert the rim diameter to millimetres (1 inch = 25.4 mm) before combining the two.

Why the Rim Diameter Matters for Radial Runout Measurement

The rim diameter in the size code, such as 17 inches, is the nominal diameter of the rim bead seat. The rim bead seat is the ledge where the tire bead sits. It is also the surface that wheel radial runout is checked against. If the bead seat is out of round, the tire sits off-centre on that section of the wheel, and the error carries through to the tread.

How to Calculate Sidewall Height, Overall Diameter and Circumference

You can calculate a tire's nominal geometry from the size code with three simple formulas. Lengths are in millimetres.

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

Worked Example: 225/45R17

1. Sidewall height: 225 × 0.45 = 101.25 mm
2. Rim diameter: 17 × 25.4 = 431.8 mm
3. Overall diameter: 431.8 + (2 × 101.25) = 634.3 mm
4. Circumference: 634.3 × 3.1416 ≈ 1,992.7 mm

How Width, Aspect Ratio and Rim Diameter Change the Result

Each number in the size code affects the geometry differently. Section width and aspect ratio together set the sidewall height. Rim diameter sets the inner diameter. Overall diameter and circumference depend on all three.

Size	Sidewall height	Overall diameter	Circumference	Diameter vs 225/45R17
225/45R17	101.25 mm	634.3 mm	≈ 1,992.7 mm	Reference
225/50R17	112.5 mm	656.8 mm	≈ 2,063.4 mm	+22.5 mm (≈ +3.5%)
205/55R16	112.75 mm	631.9 mm	≈ 1,985.2 mm	−2.4 mm (≈ −0.4%)

The comparison shows three practical points:

• Raising the aspect ratio from 45 to 50 at the same width adds about 11 mm to each sidewall and about 22.5 mm to the overall diameter.
• A smaller rim with a taller sidewall can give almost the same overall diameter. The 205/55R16 and 225/45R17 differ by only about 2.4 mm.
• A taller sidewall gives a larger, more flexible section of rubber between the rim and the road. This can make a non-uniform tire more noticeable. For this reason, measuring at the rim bead seat isolates the wheel from the tire.

Calculated Geometry Versus Actual Dimensions and Fitment

Sidewall calculations give nominal values, not exact measurements of a specific tire. A tire's real width and diameter vary with the manufacturer, tread design, rim width, inflation pressure, load and wear. The tire is also measured on a specified rim width, so mounting it on a wider or narrower rim changes the actual section width.

Similar calculated diameters do not mean a size fits a vehicle. Fitment also depends on:

• load index
• speed rating
• wheel width and offset
• suspension and body clearance
• the vehicle manufacturer's approved sizes

A wheel radial runout measurement checks roundness only. It does not confirm that a wheel and tire combination is compatible with the vehicle.

What Tools Do You Need to Measure Wheel Radial Runout?

You need a dial indicator, a rigid way to hold it, and a way to spin the wheel on an accurate centre.

• Dial indicator with a fine graduation, such as 0.01 mm or 0.001 in, and enough travel to cover the expected deviation.
• Magnetic base or rigid stand that will not flex while the wheel turns.
• Wheel balancer, runout stand or the vehicle hub with good bearings, for spinning the wheel.
• Roller-tip attachment (optional), which helps on painted or slightly rough bead seat surfaces.
• Tire pressure gauge and a torque wrench, if you are measuring a mounted assembly.
• Chalk or a paint marker for marking high and low points.

How to Measure Wheel Radial Runout Step by Step

The steps below cover measuring the wheel at the rim bead seat. An optional check of the complete tire and wheel assembly follows.

Measuring Radial Runout at the Rim Bead Seat

1. Clean the rim. Remove dirt, corrosion, old weights and bead lubricant residue from the bead seat area. Debris under the indicator tip shows up as false runout.
2. Mount the wheel accurately. On a balancer, use the correct centring cone or a hub-centric adapter. On a vehicle, check that the hub bearing has no play. Tighten the lug nuts to the specified torque in a star pattern.
3. Position the dial indicator. Point the indicator plunger straight at the wheel's axis of rotation, perpendicular to the rim bead seat surface. An angled plunger under-reads the true radial deviation.
4. Preload and zero the indicator. Press the tip against the surface so the plunger has some travel in both directions, then set the dial to zero.
5. Rotate one full revolution slowly. Watch for the maximum and minimum readings. Note any sudden jumps from the valve hole, a weld seam or a damaged spot. These are local defects rather than overall out-of-roundness.
6. Record the total indicator reading. Subtract the minimum from the maximum. This total, not the largest reading on one side of zero, is the radial runout value.
7. Mark the high spot. Put chalk or paint on the rim at the highest reading so it can be used later for tire matching.
8. Repeat on the other bead seat. Measure both the inboard and outboard bead seats, because a rim can be bent on one side only.
9. Repeat the rotation. Turn the wheel two or three more times and confirm the readings are consistent before trusting the result.

Checking the Mounted Tire and Wheel Assembly

An assembly check shows the combined effect of tire and wheel on radial runout.

1. Drive the vehicle briefly, or warm the tire, to remove temporary flat spots from parking.
2. Inflate the tire to the vehicle's specified pressure.
3. Place the dial indicator, or a roller-tip runout gauge, on the centre of the tread. On tires with deep grooves, tape over the tread or use a wide roller.
4. Rotate one full revolution and record the total reading.
5. Compare the assembly reading with the rim-only reading. A large assembly reading with a small rim reading points to the tire. Similar readings point to the wheel or to how it is mounted.

How Do You Interpret Radial Runout Readings?

Compare the total radial runout reading with the limit published by the vehicle or wheel manufacturer. Limits differ between vehicles, wheel types and the surface being measured, so there is no single universal value. If the reading is above the limit, find out which component is responsible before replacing anything.

Observation	Likely source	Typical next step
High reading at the rim bead seat on both sides	Out-of-round rim or poor centring on the hub	Re-check the mounting and centring; check hub runout; straighten or replace the wheel if confirmed
High reading at one bead seat only	Localised bend or impact damage	Inspect for damage; follow the wheel manufacturer's repair guidance
Low rim reading, high assembly reading	Tire non-uniformity or bead not fully seated	Re-seat the bead; match-mount; check the tire
Reading changes after driving	Temporary flat spots	Re-measure after the tire has warmed up
Readings jump or are not repeatable	Loose mounting, worn bearing, unstable indicator stand	Fix the setup and measure again

Match Mounting to Reduce Combined Runout

Match mounting means rotating the tire on the rim so that the tire's high point lines up with the wheel's low point. The two deviations then partly cancel each other. Some tires carry a manufacturer mark showing their high or low point, and some wheels are marked too. Follow the markings and procedure that apply to the specific tire and wheel.

Common Errors in Wheel Radial Runout Measurement

Most misleading runout readings come from the setup rather than the wheel. Check for these before blaming the rim:

• Indicator not perpendicular: an angled plunger reads less than the true radial deviation.
• Measuring the wrong surface: the outer flange lip or a cosmetic edge may not run true to the bead seat that holds the tire.
• Hub or flange runout: a hub face that is not true makes a good wheel look out of round.
• Poor centring: the wrong cone, a missing hub-centric ring, or lug nuts tightened unevenly or out of sequence can shift the wheel off-centre.
• Flat-spotted or cold tires: these give falsely high assembly readings.
• Unseated bead: a bead that is not fully seated looks like tire or wheel runout.
• Single-revolution readings: without repeated rotations, one bad reading can mislead you.

What the Sidewall Numbers Can and Cannot Tell You About Runout

The tire sidewall markings tell you the nominal bead seat diameter, section width and sidewall height. They also give the load and speed ratings. You can use these numbers to compare sizes and to calculate nominal diameter and circumference. They also help you understand how much tire sits between the rim and the road.

The sidewall markings cannot tell you whether a particular wheel is round. They cannot show whether a tire is uniform or whether a size is approved for your vehicle. Only a careful wheel radial runout measurement with a dial indicator can confirm roundness, compared against the manufacturer's specification. Fitment must be confirmed separately using the vehicle manufacturer's approved sizes and ratings.