Metric vs imperial tire sizes

The framing in the title is the one everybody uses and it is subtly wrong, so let us dispose of it first.

31x10.50R15 and 265/75R15 are not the same measurement expressed in different units, the way 5 km and 3.1 miles are. They are measurements of different things. One states the tire's overall diameter and its width in inches. The other states its section width in millimetres and the height of its sidewall as a percentage of that width. Convert between them and you are not changing units; you are recomputing one description of a shape from another, and the recomputation does not always land on a size anybody manufactures.

Neither notation is metric

Look at the last number in both examples. Fifteen. Inches, in both cases.

Every tire size in mainstream circulation quotes its rim diameter in inches, whichever system it otherwise uses. So the so-called metric size is metric in exactly one of its three fields. The aspect ratio is dimensionless, the rim is imperial, and only the section width is in millimetres.

Fully metric sizing has been tried. Michelin's TRX system in the late 1970s specified the rim in millimetres — 390 and 415 were the common ones — producing sizes written like 190/65R390. It was a coherent engineering package and it is now a curiosity that keeps a small restoration industry alive, because a 390 mm wheel takes a 390 mm tire and nothing else. The lesson the industry drew was that the wheel diameter is the one dimension that must never change, and so it stayed in inches while everything around it went metric.

What each notation was designed to make easy

The metric passenger series exists to standardise a casing. Load capacity, speed capability and inflation behaviour all depend on the shape of the sidewall — how tall it is relative to how wide it is. So the notation states the two parameters the standards care about and lets the diameter fall out as a consequence.

Flotation sizing comes from the opposite tradition: tires for soft ground and rough ground, where the question is how big an obstacle the tire can roll over and how much wheel is buried when it sinks. Overall diameter is the thing that matters, so overall diameter is what gets printed, along with the section width. Aspect ratio was never a design input, so it never made it onto the tire.

Both notations therefore tell you the truth. They just answer different questions, and each hides the other's answer.

The aspect ratios flotation produces do not exist in metric

Because aspect ratio is a by-product rather than a design parameter, it lands wherever the arithmetic puts it. Take one family of fifteen-inch flotation sizes and derive the ratio for each:

Flotation size	Section width	Sidewall height	Aspect ratio
30x9.50R15	9.50 in	7.50 in	79
31x10.50R15	10.50 in	8.00 in	76
32x11.50R15	11.50 in	8.50 in	74
33x12.50R15	12.50 in	9.00 in	72

Seventy-nine. Seventy-six. Seventy-four. Seventy-two. Metric sizes are sold in five-point steps — 70, 75, 80 — so not one of those four ratios is a number you can buy a metric tire in. The two systems do not merely use different units; they quantise the same space on grids that never line up.

Notice also what is happening down the table. Each step up the family adds exactly an inch of diameter and an inch of width, so the sidewall grows half an inch while the width grows a full one, and the ratio falls steadily. A "30" and a "33" from the same range are not the same tire made bigger. They are progressively squatter. That is invisible in flotation notation and glaringly obvious in metric.

One direction is exact, the other is a compromise

Metric to flotation is pure arithmetic and always works, because everything you need is in the code. Compute the diameter, print it, done — the converter does exactly that and shows the derived ratio alongside.

Flotation to metric is where information gets lost, and the loss is not in the maths. It is in the catalogue.

Convert 31x10.50R15 honestly and you want a 267 mm width with a 76 aspect ratio. Neither exists. The nearest size actually manufactured is 265/75R15 — and here is the cost:

• 31x10.50R15: 31.00 in tall, 10.50 in wide
• 265/75R15: 30.65 in tall, 10.43 in wide
• Difference: 0.35 in shorter, 1.13%

The widths land within a tenth of an inch of each other. The height does not, and 0.35 in is enough to matter if you are matching a spare, keeping a set consistent, or trying to leave the speedometer alone. Put the two codes into the comparison tool and the rest of the consequences come with it.

There is no metric size that lands on 31.00 in with a 10.50 in section on a fifteen-inch wheel. There is only the nearest one, and "nearest" is a decision, not a conversion.

The printed diameter is nominal too

One thing flotation notation is routinely given too much credit for: the diameter it prints is not a measured diameter.

33x12.50R15 says 33.00 in because that is the size's name, and the arithmetic in this article takes the name at face value. A real tire carrying that marking, mounted on a particular rim width, inflated to a particular pressure and carrying a particular load, will measure something else — and two brands of the same marked size routinely differ from each other by a visible amount. The metric side has the same problem for the same reasons.

So flotation notation does not give you a measured height. It gives you a stated height instead of a derived one, which is a real convenience and a smaller one than it appears.

Reading a flotation size without doing any conversion

Three shortcuts that work every time and require no calculator:

• The first number is the diameter in inches. No arithmetic. That is the entire selling point.
• The sidewall height is half the difference between the first and last numbers. For 32x11.50R15, that is (32 − 15) ÷ 2 = 8.50 in per side.
• The middle number is the section width in inches, directly. Multiply by 25.4 for the metric equivalent if you need to compare against a metric size.

The metric equivalents of those shortcuts are all harder, because in metric the diameter is the one figure that has to be built up from the other two.

When you actually have to convert

Realistically: when the tire you want is in one notation and the vehicle's placard, or the tire already fitted, is in the other. That happens constantly on trucks and SUVs, where the factory fitment is metric and most of the aftermarket range for the same wheel is flotation.

At that point what you need is not a units conversion but a comparison of two shapes, with the diameter difference, the width difference and the speedometer consequence stated together. The notation reference has the formal definitions, and section width vs tread width covers the other place where the printed width and the physical tire part company.