Distinguish minimum dual spacing, wheel centerline distance and sidewall clearance

The short answer

Minimum dual spacing is the smallest permitted distance between the centerlines of the two wheels in a dual (side-by-side) pair. It is measured from the center plane of the inner wheel to the center plane of the outer wheel. It is not the air gap between the two tires' facing sidewalls, and it is not the front-to-rear distance between axles. Bridgestone's Truck Tire Data Book (Version 24.1) defines it in exactly these terms: the minimum distance between wheel centerlines in a dual arrangement.

One condition changes the number: the published spacing applies to the tire mounted on its design rim. If your wheels are a different width, the listed figure no longer applies directly, and the tire manufacturer has to confirm the adjusted value.

This matters for anyone with a dual-rear-wheel vehicle, including Chevrolet dually pickups and cutaway vans. The term comes from tire-maker data, not from a Chevrolet-specific measurement. For a Chevy, the relevant value comes from the tire manufacturer's data for your exact tire size, read alongside GM's wheel and tire specifications for your model year and configuration.

Three measurements that are often confused

Measurement	Endpoints	Direction	What it tells you
Dual spacing (wheel centerline distance)	Center plane of inner wheel to center plane of outer wheel	Side to side, across the dual pair	Whether the dual pair is spread far enough apart for that tire on its design rim
Sidewall clearance (facing-sidewall gap)	Outer face of the inner tire's sidewall to inner face of the outer tire's sidewall	Side to side, in the space between the tires	The physical air gap left between the tires, which shrinks as load makes the sidewalls bulge
Fore-and-aft axle spacing	One axle centerline to the next axle centerline	Front to rear, along the vehicle	The spacing of axles in a tandem or multi-axle group; unrelated to how a dual pair is set up

When a data book lists a minimum dual spacing, it gives the first of these three. The second is a consequence of the first plus the tire's actual loaded width. The third is a different axis entirely.

Why the spec exists and how it works

A tire carrying load flattens at the bottom and its sidewalls bulge outward. In a dual pair, the two tires bulge toward each other. If the wheel centerlines are too close, the loaded sidewalls can touch or rub. The minimum dual spacing sets how far apart the wheel centers must be so that this tire, on its intended rim, keeps the two tires apart under load.

The sequence works like this:

• 1. Tire size and rim width determine the tire's section width. A wider rim pulls the sidewalls outward.
• 2. Load makes the sidewalls bulge, so the loaded width is greater than the unloaded width.
• 3. Wheel offset and hub design determine where each wheel's center plane sits, which sets the centerline distance of the pair.
• 4. The gap between facing sidewalls is whatever remains after the loaded tire widths occupy that centerline distance.

This is why the spec is stated as a centerline distance rather than a gap. The centerline distance is a fixed property of the wheel and hub setup. The gap varies with load, inflation and rim width.

Reading a real data-book entry

Bridgestone's data book lists 11R22.5, a common commercial-truck size, with approved rim widths of 7.50 and 8.25 inches. The 8.25-inch rim is shown in bold, which marks it as the design rim. The minimum dual spacing listed is 12.5 inches, which converts to 317.5 mm.

Read this as follows. On 8.25-inch rims, the centers of the two wheels in a dual pair must be at least 12.5 inches apart. Because the spacing value is tied to the design rim, mounting the same tire on the 7.50-inch rim means the 12.5-inch figure cannot simply be carried over. The adjustment has to come from the manufacturer.

The same data book publishes tire dimensions measured on a stated measuring rim. For example, the R268 Ecopia 11R22.5 load range G is listed on an 8.25-inch measuring rim. Loaded dimensions in these tables are taken at the maximum dual load. Note which rim and which load a dimension was measured at before you compare it with anything else.

The 11R22.5 example is a heavy-truck size. Chevrolet dually pickups use light-truck (LT) sizes, which have their own spacing values. Use the line for your exact size and load range, not this example.

Why subtracting tire width does not give an approved gap

It is tempting to take the dual spacing, subtract the tire's section width, and call the result the sidewall clearance. Here is an illustrative calculation with a hypothetical tire to show why that does not work:

Hypothetical example, not a real tire
Minimum dual spacing (centerline to centerline):  12.5 in
Unloaded section width (hypothetical):            11.0 in
Half of inner tire + half of outer tire:          5.5 + 5.5 = 11.0 in
Apparent unloaded gap:                            12.5 - 11.0 = 1.5 in

The 1.5 inches is only an unloaded geometric figure. Under load the sidewalls bulge into that space, so the real gap is smaller. Neither the spacing table nor this subtraction establishes an approved sidewall gap or approves a particular wheel and tire assembly. Treat the minimum dual spacing as the controlling spec, and treat any gap you calculate as an informal comparison.

Metric data books and twin-fitment labels

Some manufacturers publish dimensions in millimetres while still expressing rim diameter as an inch code. Confirm the units before comparing figures from different sources. Continental's car, 4x4 and van databook, for example, defaults to millimetres. It marks fitments with S for single and T for twin, and its van tables separate new dimensions from maximum operating dimensions. Those labels and widths are not a dual-spacing value. That databook directs twin-fitment spacing questions to Continental's truck tyre databook.

How to interpret a dual-spacing spec for your vehicle

What you need: the tire manufacturer's current data book or technical sheet for your exact tire, your vehicle's wheel and tire specifications from the owner's manual or door placard, and the actual rim width of your wheels, usually stamped on the wheel.

• 1. Identify the exact tire. Record the full size, load range and model. Spacing can differ between sizes and load ranges.
• 2. Find the dual-spacing line and read its definition. Confirm it is described as a wheel-centerline distance, and note the units.
• 3. Identify the design or measuring rim. Look for the boldface or flagged rim width in the approved-rim list.
• 4. Compare it with your actual rim width. If they differ, write that down. The listed spacing does not apply directly, and you need the manufacturer's adjusted figure before going further.
• 5. Keep the table footnotes. They often state the load, inflation or rim conditions the numbers depend on.
• 6. Record sidewall clearance and axle spacing separately. Do not substitute a width, a calculated gap or an axle-to-axle distance for the dual-spacing value.
• 7. If you need to measure the vehicle, note that with identical tires and wheels on both positions, the distance between matching points, such as the outer face of the inner wheel to the outer face of the outer wheel, geometrically equals the centerline distance. Use the manufacturer's stated measuring method where one is given, and measure with the vehicle safely supported on level ground.

Completion check: you should have one figure for dual spacing in stated units, tied to a specific tire, load range and rim width, with any rim mismatch flagged. If you still have a mismatch, spacers, aftermarket wheels or a different tire size, stop there. Get confirmation from the tire manufacturer and the vehicle or wheel manufacturer, or have a qualified tire shop check the assembly. A data-book value does not certify the fitment of a particular wheel, hub and tire combination.

A simple worksheet

Field	Dual spacing row	Sidewall gap row	Axle spacing row
Manufacturer and document version/page
Exact tire size, model, load range
Wheel arrangement (single/dual)
Actual rim width
Design or measuring rim
Measurement endpoints	Wheel centerline to wheel centerline	Facing sidewall to facing sidewall	Axle centerline to axle centerline
Direction	Side to side	Side to side	Front to rear
Units
Specified minimum
Stated conditions (load, inflation, footnotes)

Keeping three separate rows makes it hard to mix up the measurements, and it shows at a glance whether your rim matches the basis on which the spacing figure was published.