305/30R21 in inches

A 305/30R21 tire is 28.2″ tall and 12.0″ wide, and fits a 21″ wheel. In inches it is written 28.2×12.0R21.

Diameter28.20″

Width12.01″

Sidewall3.60″

Drawn to scale. Nominal dimensions calculated from the size code.

Manufacturer data

How tall 305/30R21 tires really are

Heights published by Michelin for all 6 tires in this size. Their published revolutions per mile work out to 27.4″ under load (below).

Real 305/30R21 tires measure 28.20–28.23″, against 28.20″ from the size code. They run both taller and shorter than the size code.

Height under load27.4″

Published revs per mile (737) mean the tire rolls like a 27.4″ tire on the vehicle. Use this for speedometer calibration.

Weight spread27.2–31.7 lb

The lightest (Michelin Pilot Sport 4 S, XL) is 4.5 lb lighter per corner than the heaviest (Michelin Pilot Sport 4 S, XL).

6 tires in 305/30R21

Sort by the spec you care about.

	Load					Revs/mi	Source
Michelin Pilot Sport 4 SPerformance	XL · 104(Y)	28.20	27.2	9.5	1,984	737	Spec page
Michelin Pilot Alpin 5Winter	XL · 104W	28.23	29.6	9.5	1,984	737	Spec page
Michelin Pilot Sport Cup 2 ConnectPerformance	XL · 104(Y)	28.20	30.4	7.2	1,984	737	Spec page
Michelin Pilot Alpin 5Winter	XL · 104V	28.23	30.6	9.5	1,984	737	Spec page
Michelin Pilot Alpin 5Winter	XL · 104V	28.23	31.5	9.5	1,984	737	Spec page
Michelin Pilot Sport 4 SPerformance	XL · 104(Y)	28.20	31.7	9.0	1,984	737	Spec page

Is 305/30R21 a 28-inch tire?

≈28″

Yes. The size code works out to 28.20″, and the 6 tires here publish 28.20–28.23″, so people call it a 28.

All 28-inch tire sizes, on every wheel size.

Compare with nearby sizes

Differences in overall diameter, calculated from each size code.

Size	Diameter ″	vs 305/30R21	Difference
285/30R21	27.73	-0.47″	-1.7%
245/35R21	27.75	-0.45″	-1.6%
345/25R21	27.79	-0.41″	-1.5%
295/30R21	27.97	-0.24″	-0.8%
355/25R21	27.99	-0.22″	-0.8%
255/35R21	28.03	-0.18″	-0.6%
265/35R21	28.30	+0.10″	+0.3%
315/30R21	28.44	+0.24″	+0.8%
275/35R21	28.58	+0.37″	+1.3%
325/30R21	28.68	+0.47″	+1.7%

How to read 305/30R21

How to read any tire size

How 305/30R21 104Y is printed on the sidewall. It's the most common code among the 6 tires here (3 of them); 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

230Aspect ratio

A percentage, not a measurement: the sidewall is 30% as tall as the tire is wide, which is 91.5 mm or 3.60 inches.

3RConstruction

Radial construction.

421Wheel diameter

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

5104Load index

Each tire carries up to 1,984 lb (900 kg) at its maximum pressure.

6YSpeed rating

Rated for up to 186 mph sustained.

XLExtra load

Printed after the size.

Specification	Inches	Metric
Overall diameter	28.20	716 mm
Section width	12.01	305 mm
Sidewall height	3.60	91.5 mm
Circumference	88.6	2,251 mm
Revolutions per mile (calculated, unloaded)	715.1	444 per km
Revolutions per mile (published by manufacturers)	737	458 per km
Approved wheel width	10.5–11.5	267–292 mm

Speedometer error after switching to 305/30R21

Your actual speed– mph

Questions about 305/30R21

What is 305/30R21 in inches?

28.20 inches tall, 12.01 inches wide, on a 21-inch wheel. Written in inches, that is roughly 28.2×12.0R21.

How much does a 305/30R21 tire weigh?

Between 27.2 lb and 31.7 lb for the 6 tires listed here. The lightest is the Michelin Pilot Sport 4 S (XL); the heaviest is the Michelin Pilot Sport 4 S (XL).

What wheel width do I need for 305/30R21?

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

How many revolutions per mile does a 305/30R21 make?

Published figures run 737 revolutions per mile. The calculated figure is 715.1, but manufacturers measure the tire under load, where it rolls on a smaller radius.

How much taller is 305/30R21 than 255/35R21?

0.18 inches taller on paper (28.20″ vs 28.03″), which raises the axle about 0.09″. Your speedometer will read about 0.6% slow.