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305/25R21 in inches

A 305/25R21 tire is 27.0″ tall and 12.0″ wide, and fits a 21″ wheel. In inches it is written 27.0×12.0R21.

Diameter27.00″
Width12.01″
Sidewall3.00″
305/25R21 drawn to scale: 27.00 inch diameter, 3.00 inch sidewall, 21 inch wheel, 12.01 inch tread width21″ wheel3.00″27.00″ overall12.01″ wideSide view
Drawn to scale. Nominal dimensions calculated from the size code.
Manufacturer data

How tall 305/25R21 tires really are

Heights published by Michelin for the one tire in this size. Their published revolutions per mile work out to 26.2″ under load (below).

The one tire listed, the Michelin Pilot Sport 4 S, measures 27.00″: 0.00″ shorter than the size code's 27.00″.

Height under load26.2″

Published revs per mile (771) mean the tire rolls like a 26.2″ tire on the vehicle. Use this for speedometer calibration.

1 tire in 305/25R21

Sort by the spec you care about.

LoadRevs/miSource
Michelin Pilot Sport 4 SPerformanceXL · 98(Y)27.0029.69.51,653771Spec page

Is 305/25R21 a 27-inch tire?

≈27″

Yes. The size code works out to 27.00″, and the 1 tire here publishes 27.00″, so people call it a 27.

All 27-inch tire sizes, on every wheel size.

Compare with nearby sizes

Differences in overall diameter, calculated from each size code.

SizeDiameter ″vs 305/25R21Difference
275/25R2126.41-0.59″-2.2%
245/30R2126.79-0.22″-0.8%
295/25R2126.81-0.20″-0.7%
255/30R2127.02+0.02″+0.1%
255/35R2027.03+0.02″+0.1%
265/30R2127.26+0.26″+0.9%
325/25R2127.40+0.39″+1.5%
275/30R2127.50+0.49″+1.8%
285/30R2127.73+0.73″+2.7%
245/35R2127.75+0.75″+2.8%

How to read 305/25R21

How to read any tire size

305/25R21 98Y as printed on the tire sidewall, with each part numbered305/25R21 98Y123456
How 305/25R21 98Y 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

225Aspect ratio

A percentage, not a measurement: the sidewall is 25% as tall as the tire is wide, which is 76.3 mm or 3.00 inches.

3RConstruction

Radial construction.

421Wheel diameter

Mounts on a 21-inch wheel. Approved wheel widths run 10.5″ to 11.5″.

598Load index

Each tire carries up to 1,653 lb (750 kg) at its maximum pressure.

6YSpeed rating

Rated for up to 186 mph sustained.

XLExtra load

Printed after the size.

SpecificationInchesMetric
Overall diameter27.00686 mm
Section width12.01305 mm
Sidewall height3.0076.3 mm
Circumference84.82,155 mm
Revolutions per mile (calculated, unloaded)746.9464 per km
Revolutions per mile (published by manufacturers)771479 per km
Approved wheel width10.5–11.5267–292 mm

Speedometer error after switching to 305/25R21

Your actual speed– mph

Questions about 305/25R21

What is 305/25R21 in inches?

27.00 inches tall, 12.01 inches wide, on a 21-inch wheel. Written in inches, that is roughly 27.0×12.0R21.

What wheel width do I need for 305/25R21?

Manufacturers approve 10.5–11.5″ wide wheels for this size.

How many revolutions per mile does a 305/25R21 make?

Published figures run 771 revolutions per mile. The calculated figure is 746.9, but manufacturers measure the tire under load, where it rolls on a smaller radius.

How much taller is 305/25R21 than 295/25R21?

0.20 inches taller on paper (27.00″ vs 26.81″), which raises the axle about 0.10″. Your speedometer will read about 0.7% slow.