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305/40R21 in inches

A 305/40R21 tire is 30.6″ tall and 12.0″ wide, and fits a 21″ wheel. In inches it is written 30.6×12.0R21.

Diameter30.61″
Width12.01″
Sidewall4.80″
305/40R21 drawn to scale: 30.61 inch diameter, 4.80 inch sidewall, 21 inch wheel, 12.01 inch tread width21″ wheel4.80″30.61″ overall12.01″ wideSide view
Drawn to scale. Nominal dimensions calculated from the size code.
Manufacturer data

How tall 305/40R21 tires really are

Heights published by Michelin for the one tire in this size. Their published revolutions per mile work out to 29.7″ under load (below).

The one tire listed, the Michelin Pilot Alpin 5, measures 30.59″: 0.02″ shorter than the size code's 30.61″.

Height under load29.7″

Published revs per mile (680) mean the tire rolls like a 29.7″ tire on the vehicle. Use this for speedometer calibration.

1 tire in 305/40R21

Sort by the spec you care about.

LoadRevs/miSource
Michelin Pilot Alpin 5WinterXL · 113V30.5937.710.52,535680Spec page

Is 305/40R21 a 31-inch tire?

≈31″

Close. The size code works out to 30.61″, and the 1 tire here publishes 30.59″, so people call it a 31.

All 31-inch tire sizes, on every wheel size.

Compare with nearby sizes

Differences in overall diameter, calculated from each size code.

SizeDiameter ″vs 305/40R21Difference
285/40R2129.98-0.63″-2.1%
255/45R2130.04-0.57″-1.9%
235/50R2130.25-0.35″-1.2%
295/40R2130.29-0.31″-1.0%
265/45R2130.39-0.22″-0.7%
275/40R2230.66+0.06″+0.2%
275/45R2130.74+0.14″+0.5%
315/40R2130.92+0.31″+1.0%
255/50R2131.04+0.43″+1.4%
285/45R2131.10+0.49″+1.6%

How to read 305/40R21

How to read any tire size

305/40R21 113V as printed on the tire sidewall, with each part numbered305/40R21 113V123456
How 305/40R21 113V is printed on the sidewall. It's the most common code among the 1 tire here; tires in this size come in other load and speed ratings too.
1305Section width

305 mm across the widest point, or 12.01 inches. Every 305 size

240Aspect ratio

A percentage, not a measurement: the sidewall is 40% as tall as the tire is wide, which is 122.0 mm or 4.80 inches.

3RConstruction

Radial construction.

421Wheel diameter

Mounts on a 21-inch wheel. Approved wheel widths run 10.0″ to 12.0″.

5113Load index

Each tire carries up to 2,535 lb (1,150 kg) at its maximum pressure.

6VSpeed rating

Rated for up to 149 mph sustained.

XLExtra load

Printed after the size.

SpecificationInchesMetric
Overall diameter30.61777 mm
Section width12.01305 mm
Sidewall height4.80122.0 mm
Circumference96.22,442 mm
Revolutions per mile (calculated, unloaded)659.0409 per km
Revolutions per mile (published by manufacturers)680423 per km
Approved wheel width10.0–12.0254–305 mm

Speedometer error after switching to 305/40R21

Your actual speed– mph

Questions about 305/40R21

What is 305/40R21 in inches?

30.61 inches tall, 12.01 inches wide, on a 21-inch wheel. Written in inches, that is roughly 30.6×12.0R21.

What wheel width do I need for 305/40R21?

Manufacturers approve 10.0–12.0″ wide wheels for this size.

How many revolutions per mile does a 305/40R21 make?

Published figures run 680 revolutions per mile. The calculated figure is 659.0, but manufacturers measure the tire under load, where it rolls on a smaller radius.

How much taller is 305/40R21 than 265/45R21?

0.22 inches taller on paper (30.61″ vs 30.39″), which raises the axle about 0.11″. Your speedometer will read about 0.7% slow.