RIMLIST

How to Measure Your Current Setup

Before you buy new wheels or size up your tires, it helps to know exactly what you currently have. Your door jamb sticker shows the OEM spec, but if someone has already changed the wheels or tires on your used car, measuring gives you a concrete baseline. What are the actual dimensions of the wheels? How far do the tires stick out relative to the fender? How much inner clearance do you have? Here is how to find out.

Tools You Need

No specialized equipment is required for most of these measurements:

  • Steel tape measure. At least 10 feet for the string method.
  • Ruler or small straightedge. For measuring offset from a flat reference surface.
  • Digital calipers. Not required, but considerably more accurate than a tape measure for measuring wheel width and tight clearances.
  • Floor jack and jack stands. You need to safely support the car with the wheels off the ground for the wheel measurements.
  • A helper optional but useful for checking clearance.

Measuring Wheel Width

One of the most confusing basic measurements is wheel width. Wheel width is measured from inner bead seat to inner bead seat, the flanged edges the tire mounts against, rather than the outer lips of the wheel. This is the nominal width used in wheel specs: when a wheel is described as 19x8.5J, that 8.5 refers to the bead seat to bead seat measurement in inches. As a cheat code to measuring, the wheel width is often stamped into the back of the wheel.

The most accurate way to measure is with the tire off the wheel. Measure across the inside of the barrel from bead seat to bead seat. Digital calipers give you a clean measurement and let you confirm that a wheel labeled 8.5J actually measures 8.5 inches.

If the tire is mounted and you do not want to dismount it, you can estimate wheel width by measuring the outer rim width (outer lip to outer lip) and subtracting 11mm (0.43”) per side — approximately 22mm (0.87”) total. Most passenger car and aftermarket wheels use J-type flanges, which have a minimum flange width of 11mm. This gives a reasonable approximation for a quick check, but for anything precise, dismount the tire.

Measuring Wheel Offset

Wheel offset (ET) is the distance in millimeters from the wheel’s centerline to its mounting face — the flat surface that contacts the hub. Positive ET means the mounting face is closer to the outside of the wheel (wheel sits inward). Negative ET means the mounting face is closer to the inside (wheel sits outward). See Understanding Wheel Offset & Fitment for the full explanation of how offset affects handling and clearance.

To measure offset:

  1. Lay the wheel face-down on a flat surface. (You may want to cushion the face.)
  2. Place a straightedge across the back of the wheel, resting on the outer rim edges. If the tire is still mounted and lifts the straightedge, you will have to account for the difference between the tire and the rim.
  3. Measure from the straightedge down to the mounting face. This is the backspacing.
  4. Measure the wheel’s total width including the rim flanges.
  5. Calculate offset:
ET (mm) = Backspacing (mm) − (Total Width (mm) ÷ 2)

Example: A wheel with 130mm of backspacing and a total outside-to-outside width of 9 inches (228.6mm):

ET = 130 − (228.6 ÷ 2)
ET = 130 − 114.3
ET = +15.7mm ≈ ET+16

If the wheel is installed on the car, measuring offset directly is not practical without removing it. In that case, the string method below gives you the protrusion picture without pulling the wheel.

Checking Outer Protrusion with the String Method

The string method measures how far the outer edge of your tire and wheel extends relative to the fender. This tells you whether your wheel is flush, tucked inside the fender lip, or poking out — and by how much.

You need two lengths of string or fishing line and a way to anchor them parallel to the car:

  1. Park on a level surface.
  2. Stretch a string taut along one side of the car, making contact with the two widest body points at each end — typically the bumper corners or the widest part of the fender flares. The string should be taut and parallel to the car’s side, touching both endpoints.
  3. Measure from the string to the outermost point of the tire sidewall at the front wheel, then at the rear.

A positive number (string sits outside the tire) means the wheel is inside the fender line — you have room to go wider or run a lower-offset wheel. A negative number (tire pushes the string outward) means the tire protrudes beyond the fender.

This method does not measure inner clearance. A tire can be flush at the outer fender and still rub on the strut, inner fender, or suspension components when the suspension compresses or the wheel turns to full lock.

Checking Inner Clearance

Inner clearance is where most unobvious rubbing occurs. The gap between the inner sidewall and the strut housing, plastic fender liners, inner fender metal, ABS lines, or brake lines is what fails first when a fitment is too aggressive.

At full steering lock: Turn the wheel to full steering lock in both directions with the car at rest. Have a helper look and listen for any contact between the inner sidewall and the strut, spring, or control arm at each full-lock position. The gap at full lock and at static ride height is the minimum clearance you will have while the car is stationary. Any road imperfection while turning will compress the suspension and reduce that gap further.

Under suspension compression: Push down firmly on the corner of the car above each wheel to simulate compression. This is approximate, but obvious contact points will reveal themselves. For a more accurate check, compress the suspension using a floor jack placed under the control arm or subframe, which pushes the wheel upward while the car body stays level. Watch the inner sidewall gap and other contact points as the suspension compresses.

A practical minimum is 10mm of inner clearance at static ride height. For a car that will see rough roads or spirited driving, more is better. For a lowered car, the car is already running the suspension in a more compressed state than stock. The remaining upward travel before contact is shorter than on a car at factory ride height. See Suspension Geometry & Wheel Fitment for how lowering and camber change clearance.

Measuring Your Tire’s Actual Dimensions

Listed tire dimensions are nominal. The 255 in 255/35R18 means the tire was designed to the 255mm section width standard, not that your mounted tire will measure exactly 255mm in treadwidth. Actual mounted width varies with rim width. A 255 on a 10J wheel splays the sidewalls outward, while a 255 on an 8J wheel pulls them in. Mounted section width can vary ±10–15mm from the nominal spec.

To measure actual section width:

  1. Inflate the tire to the door jamb spec.
  2. With the tire mounted on the wheel but off the car, measure the widest point from sidewall to sidewall using a tape measure or calipers. Measure the outside of the sidewalls, not across the tread face.

To measure overall diameter:

  • Chalk a reference mark on the tread.
  • Roll the tire one complete revolution on a flat surface.
  • Measure the distance traveled. This is the circumference.
  • Divide by π to get the diameter: diameter = circumference ÷ 3.14159

Alternatively, with the tire mounted on the car and the car parked on flat ground, measure from the ground straight up to the crown of the tread directly above the wheel center. This gives you the radius. Double it for diameter. This method is approximately accurate within a few millimeters.

Rim Width and Tire Fitment Limits

Once you know your wheel width and current tire section width, you can check whether a different tire size is within the approved mounting range for your wheel. The Tire and Rim Association (T&RA) publishes approved mounting widths for each tire size. Mounting a tire outside those limits affects the tire’s profile, handling characteristics, and wear pattern. See Tire and Wheel Sizing Compatibility for the full width compatibility table and how to read it.

You did it. You got to the end. Now you know how to measure your wheel’s actual offset, check your tire’s mounted dimensions, use the string method to see how far your setup sticks out, and find your inner clearance before it becomes a rubbing problem. Have a burning question you want answered in a guide? Email us at hello@rimlist.com.