How to Convert a Flotation Tire Size to Metric

You find a tire listed as 33x12.50R17. Another shop sells a tire as 315/65R17. Are they the same size? Almost. This guide shows how to convert one to the other, step by step, and why the result is close but never exact.

"Flotation" is the older name for tire sizes written in inches, such as 33x12.50R17. These sizes are used on light truck (LT) tires. LT tires are built for trucks, SUVs and off-road use. Light truck metric sizes start with LT, for example LT315/70R17. The conversion math is the same.

You need three numbers and a few short formulas. Every number on this page was checked with a script, which is included at the end.

What does a flotation size like 33x12.50R17 mean?

A flotation size has three numbers:

• 33 is the overall diameter in inches. This is the height of the tire from the ground to the top.
• 12.50 is the section width in inches. This is the width of the tire from one side to the other.
• R17 is the wheel (rim) diameter in inches. The R means radial construction.

The overall diameter is written in the size, so you see how tall the tire is at a glance.

A metric size, such as 315/65R17, works differently:

• 315 is the section width in millimeters.
• 65 is the aspect ratio: the sidewall height as a percentage of the width.
• R17 is the rim diameter in inches.

The metric size does not show the overall diameter. You must calculate it. This is why the conversion needs a formula.

One more word: "nominal" means the size printed on the tire, not a measured size.

How do you convert a flotation tire size to metric?

Use this rule: we keep the width within 10 mm and then pick the aspect ratio that gets the height closest. In steps:

1. Width in mm = width in inches x 25.4. List the common metric widths within 10 mm of it (widths come in steps of 10, for example 275, 285, 295, 305, 315, 325).
2. Aspect ratios to try: 50, 55, 60, 65, 70, 75, 80 and 85.
3. Overall diameter (in) for each pair = (rim x 25.4 + 2 x width x aspect / 100) / 25.4.
4. Pick the pair with the diameter closest to the flotation diameter. The metric size is width / aspect R rim.

The difference between the two diameters, divided by the flotation diameter, is your error. We accept an error up to 2.5%. If no pair is that close, there is no close equivalent.

Example 1: 33x12.50R17

1. Width: 12.50 x 25.4 = 317.5 mm. Common widths within 10 mm: 315 and 325.
2. Diameters with 315 mm: 65 gives 33.12 in, 70 gives 34.36 in. With 325 mm: 65 gives 33.63 in, 70 gives 34.91 in.
3. The closest to 33 inches is 315/65R17 at 33.12 in.
4. Result: 315/65R17, +0.37% from 33 inches.

Example 2: 35x12.50R20

1. Width: 12.50 x 25.4 = 317.5 mm. Common widths: 315 and 325.
2. Diameters with 315 mm: 55 gives 33.64 in, 60 gives 34.88 in, 65 gives 36.12 in. With 325 mm: 60 gives 35.35 in, 65 gives 36.63 in.
3. The closest to 35 inches is 315/60R20 at 34.88 in.
4. Result: 315/60R20, -0.34% from 35 inches. The 325 mm width at 60% is further away (35.35 in, +1.0%).

Example 3: 31x10.50R15

1. Width: 10.50 x 25.4 = 266.7 mm. Common widths within 10 mm: 265 and 275.
2. Diameters with 265 mm: 75 gives 30.65 in, 80 gives 31.69 in. With 275 mm: 75 gives 31.24 in, 80 gives 32.32 in.
3. The closest to 31 inches is 275/75R15 at 31.24 in.
4. Result: 275/75R15, +0.77% from 31 inches. The 265 mm width at 75% is a little further away (30.65 in, -1.13%).

The reverse conversion, from metric to flotation, has its own guide.

Which metric size is closest to my flotation size?

Table 1 uses the rule from above: we keep the width within 10 mm and then pick the aspect ratio that gets the height closest. The aspect ratios tried are 50, 55, 60, 65, 70, 75, 80 and 85. The last column shows how much taller (+) or shorter (-) the metric tire is than the flotation diameter.

Some of these sizes may not exist from every maker. Check that the size is on sale before you decide.

Why is the metric size never exactly the same?

Three things cause the difference.

• Rounding the width. 12.50 inches is 317.5 mm. Metric widths come in fixed steps, so 315 mm or 325 mm is used.
• Rounding the aspect ratio. Aspect ratios come in steps of 5. A change of 2 points on a 315 mm tire changes the diameter by about half an inch.
• Real tire size. The size printed on the tire is nominal. Real tires differ from it by a few tenths of an inch, depending on the brand, the model and the wheel width.

For this reason, treat the table as a guide. If the exact diameter matters, use the published specification of the tire you plan to buy. For the overall diameter of a metric size, see How to convert a metric tire size to inches. To compare other sizes that fit your wheels, see tire size alternatives. See also the site disclaimer for the limits of nominal geometry.

Does the new tire change my speedometer?

Yes. Your speedometer counts wheel turns. It assumes the tire has the original diameter. A taller tire covers more road in each turn, so the speedometer reads too low.

Example: the new tire is 1.8% taller than the old one. When you drive a true 60 mph, the speedometer shows about 58.9 mph. The rule is: true speed = indicated speed x (new diameter / old diameter).

A change of 1% to 2% is small. A larger change is easy to notice. Moving from a 33-inch to a 35-inch tire is a 6.1% change in diameter. See our full guide: How to calculate speedometer error after a tire size change. To compare any two sizes, use the tire size calculator.

Frequently asked questions

Is 33x12.50R17 the same as 315/65R17?

Almost. The overall diameter of 315/65R17 is 33.12 inches, which is +0.37% from 33 inches. Real tires of the two sizes can differ a little because of brand and model.

Can I put a metric tire on a wheel made for a flotation tire?

The rim diameter is the same number in both systems, so R17 fits an R17 wheel. Width is a separate question. Check the rim width range that the tire maker approves for the tire you choose.

Why does the flotation size show the diameter and the metric size does not?

They are two different sizing systems. Flotation sizes state the overall diameter directly. Metric sizes state the width and a ratio, and you calculate the diameter from them.

What if the aspect ratio is not a multiple of 5?

Round to the nearest multiple of 5. For example, 76.19 becomes 75 and 72 becomes 70. In this guide we try only 50, 55, 60, 65, 70, 75, 80 and 85. Then check the new overall diameter with the formula above.

How close does the new diameter need to be?

A smaller difference is better. At 3% taller, a true 60 mph shows about 58.3 mph on the speedometer. Check your vehicle manual or the tire maker for the sizes approved for your vehicle.

Verification script

All numbers on this page come from this Python script.

import math

IN = 25.4
WIDTHS = list(range(235, 336, 10))   # 235, 245, ... 335 mm
ASPECTS = [50, 55, 60, 65, 70, 75, 80, 85]

def diameter_in(width_mm, aspect, rim):
    return (rim * IN + 2 * width_mm * aspect / 100) / IN

def candidates(w_in):
    """Common widths within 10 mm of the converted width."""
    w_mm = w_in * IN
    return [w for w in WIDTHS if abs(w - w_mm) <= 10]

def equivalent(d, w_in, rim):
    """Keep the width within 10 mm, then pick the aspect ratio with the closest height."""
    return min((abs(diameter_in(w, a, rim) - d), w, a)
               for w in candidates(w_in) for a in ASPECTS)[1:]

FLOTATION = [(31, 10.5, 15), (32, 11.5, 15), (33, 12.5, 15), (33, 12.5, 17),
             (33, 12.5, 18), (35, 12.5, 17), (35, 12.5, 18), (35, 12.5, 20),
             (37, 12.5, 17), (37, 13.5, 20)]

if __name__ == "__main__":
    for d, w_in, rim in FLOTATION:
        w, a = equivalent(d, w_in, rim)
        dia = diameter_in(w, a, rim); diff = (dia - d) / d * 100
        flag = "" if abs(diff) <= 2.5 else "  OVER 2.5%"
        print(f"{d}x{w_in:.2f}R{rim}: width {w_in*IN:.1f} mm, candidates {candidates(w_in)} -> {w}/{a}R{rim} {dia:.2f} in ({diff:+.2f}%){flag}")
    for d, w_in, rim in [(33, 12.5, 17), (35, 12.5, 20), (31, 10.5, 15)]:
        print(f"{d}x{w_in:.2f}R{rim}: sidewall {(d-rim)/2} in")
        for w in candidates(w_in):
            print("   ", [(a, round(diameter_in(w, a, rim), 2)) for a in ASPECTS])
    print("0.5 in check: 315 mm x 2% x 2 =", round(315 * 0.02 * 2 / IN, 3), "in")
    print("33 -> 35 in:", round((35 / 33 - 1) * 100, 1), "%")
    print("1.8% taller, true 60 mph shows:", round(60 / 1.018, 1))
    print("3% taller, true 60 mph shows:", round(60 / 1.03, 1))