Tire Size Calculator: Compare Sizes, Speedometer Error

Dimension	265/70R17	285/70R17	Change
Overall diameter	31.61 in (803 mm)	32.71 in (831 mm)	+1.10 in (+28 mm)
Section width	10.43 in (265 mm)	11.22 in (285 mm)	+0.79 in (+20 mm)
Sidewall height	7.30 in (186 mm)	7.85 in (200 mm)	+0.55 in (+14 mm)
Aspect ratio	70%	70%
Rim diameter	17 in	17 in
Circumference	99.3 in	102.8 in	+3.5 in
Revolutions per mile	638	617	−21
Revolutions per km	396	383	−13

How a tire size is written

Bridgestone’s guide breaks a size such as P225/70R16 91S down like this:

P, LT or nothing. P is a P-metric passenger tire and no letter a Euro-metric one; LT is a light-truck tire, and a C after the rim (235/65R16C) a Euro-metric commercial one. T is a temporary spare and ST a trailer tire.
225 is the section width, sidewall to sidewall, in millimeters.
70 is the aspect ratio: the sidewall’s height as a percentage of that width.
R is radial construction (ZR for a speed rating above V, RF for a run-flat).
16 is the rim diameter in inches, and 91S the load index and speed rating, which don’t change the size.

A flotation size, common on off-road tires, works in inches instead: 33x12.50R15LT is 33 inches tall and 12.5 inches wide, on a 15-inch rim.

How the dimensions are worked out

Sidewall = section width × aspect ratio. For a 265/70R17 that’s 265 mm × 0.70 = 185.5 mm, or 7.30 in.

Diameter = rim + 2 × sidewall = 17 + 2 × 7.30 = 31.61 in. A flotation size gives its diameter, so its sidewall is (diameter − rim) ÷ 2.

Revolutions per mile = 63,360 ÷ (π × diameter): there are 63,360 inches in a mile, and each turn covers the circumference. A 265/70R17 turns 638 times.

Speedometer: true speed = what it shows × new diameter ÷ old diameter. Going from 265/70R17 to 285/70R17, 31.61 to 32.71 inches, 60 mph on the speedometer is 62.1 mph:

Speedometer shows	True speed	Difference
25 mph	25.9 mph	+0.9 mph
35 mph	36.2 mph	+1.2 mph
45 mph	46.6 mph	+1.6 mph
55 mph	56.9 mph	+1.9 mph
65 mph	67.3 mph	+2.3 mph
75 mph	77.6 mph	+2.6 mph

Clearance. The vehicle rises by half the change in diameter, and the tread comes that much closer to the fender above. On the same wheel, half the change in width goes to each side: closer to the suspension inside and to the fender lip outside. A new wheel with a different offset moves the tire again, so measure before buying.

Checked against tire makers’ tables

Tire makers publish each size’s inflated diameter, measured on a set rim. The calculator was checked against all 354 rows of four of their tables: three of Toyo’s and Nitto’s mud-terrain Trail Grappler. A sample:

Tire	Size	They print	Calculator	Revs per mile printed	Calculator
Toyo Extensa A/S II	205/55R16	24.9 in	24.88 in	835	811
Toyo Proxes Sport A/S	225/45R17	25 in	24.97 in	832	808
Toyo Open Country A/T III	265/70R17	31.7 in	31.61 in	656	638
Toyo Open Country A/T III	P285/70R17	32.8 in	32.71 in	634	617
Toyo Open Country A/T III	33X12.50R17LT	32.5 in	33.00 in	639	611
Nitto Trail Grappler M/T	LT285/75R16	33.03 in	32.83 in	–	614
Nitto Trail Grappler M/T	35x12.50R17LT	34.84 in	35.00 in	–	576

Metric sizes on Toyo’s tires measure within 0.9% of the arithmetic (265 rows). The mud-terrain’s deep tread stands 0.3% to 1.5% taller.
Flotation sizes measure up to 1.5% short of the diameter they’re named for (56 rows): a Toyo 33x12.50R17 is 32.5 inches tall.
Revolutions per mile printed by Toyo run 2.8% to 3.3% above what its own diameters give, because they’re measured with the tire under load.
Three rows didn’t fit their own size, and each fits another: Toyo’s Extensa A/S II table gives the 235/60R16 the 225/60R16’s figures, and its Proxes Sport A/S table seems to swap the 265/30R19 and 285/30R19 rows.

Common tire sizes

Each has a page with its dimensions, what tire makers publish for it, and the sizes around it.

15-inch wheels

16-inch wheels

19-inch wheels

20-inch wheels

22-inch wheels

Common upgrades compared

One step up on the same wheel: 10 mm wider, or 5 points taller in aspect ratio, and each flotation size against the metric size nearest it.

Changing tires? The new ones go on at your vehicle’s lug nut torque, tightened in a star pattern.

Questions

Does a bigger tire make the speedometer read high or low?

Low. A taller tire covers more ground each turn, but the speedometer still counts turns as if the old size were on, so you’re going faster than it shows, by the ratio of the two diameters. The odometer undercounts the miles by the same amount. A shorter tire does the opposite.

Why do tire makers print more revolutions per mile than this?

The calculator works from the inflated diameter. Under a vehicle’s weight the tire flattens where it meets the road and rolls on a slightly smaller radius, so it turns more often: Toyo’s published figures are 2.8% to 3.3% higher than their own printed diameters give. Both sizes in a comparison flatten alike, so the speedometer change is the same either way.

Is LT285/75R16 the same size as 285/75R16?

The same dimensions, yes: Bridgestone’s guide notes that the numbers mean the same thing in every metric size. The letters before and after them tell you what the tire is built for and how its load is rated, so an LT tire isn’t a drop-in swap for a P-metric one, or the other way round, without checking the load index and inflation pressure your vehicle needs.

What size is a 33-inch tire in metric?

The nearest common metric sizes are 285/75R16 (32.8 in), 285/70R17 (32.7 in), 275/70R18 (33.2 in), 275/60R20 (33.0 in) and 275/50R22 (32.8 in). A flotation size such as 33x12.50R17 says 33 inches outright, though measured, tire makers’ 33s come up a little short.

Will a wider tire fit my wheel?

Only if the wheel is within the tire’s approved rim width range, which tire makers list for every size. Toyo’s tables, for example, give a 265/70R17 a range of 7.0 to 9.0 inches. Check that, and the sizes and load index on your door-jamb placard or owner’s manual, before buying.

Sources

Bridgestone Americas: Determining tire size: P-metric, Euro-metric, LT-metric, C-type and flotation sizes, and what each number means (checked October 28, 2026)
Toyo Tires: Extensa A/S II sizes and specifications: inflated overall diameter and width, revolutions per mile (checked October 28, 2026)
Toyo Tires: Proxes Sport A/S sizes and specifications: inflated overall diameter and width, revolutions per mile (checked October 28, 2026)
Toyo Tires: Open Country A/T III sizes and specifications: P-metric, LT-metric and flotation sizes, with revolutions per mile (checked October 28, 2026)
Nitto Tire: Trail Grappler M/T sizes and specs: inflated overall diameter and width, LT-metric and flotation sizes (checked October 28, 2026)

Spotted a number that doesn’t match your manual or a source? Tell us; corrections are logged on the sources page.

Tire size calculator

Work out or compare tire sizes

265/70R17 and 285/70R17 side by side

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