315/30R21 in inches

A 315/30R21 tire is 28.4″ tall and 12.4″ wide, and fits a 21″ wheel. In inches it is written 28.4×12.4R21.

Diameter28.44″

Width12.40″

Sidewall3.72″

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

Manufacturer data

How tall 315/30R21 tires really are

Heights published by Michelin and Toyo for all 13 tires in this size. Their published revolutions per mile work out to 27.6″ under load (below).

Real 315/30R21 tires measure 28.46–28.50″, against 28.44″ from the size code. Most run taller than the size code (11 of 13).

Height under load27.6″

Published revs per mile (730–731) mean the tire rolls like a 27.6″ tire on the vehicle. Use this for speedometer calibration.

Weight spread30.4–34.8 lb

The lightest (Michelin Pilot Alpin 5, XL) is 4.4 lb lighter per corner than the heaviest (Toyo Proxes Sport 2, XL).

13 tires in 315/30R21

Sort by the spec you care about.

	Load					Revs/mi	Source
Michelin Pilot Alpin 5Winter	XL · 105V	28.46	30.4	9.5	2,039	731	Spec page
Michelin Pilot Sport Cup 2 RPerformance	XL · 105(Y)	28.50	30.4	5.5	2,039	731	Spec page
Michelin Pilot Sport Cup 2 RPerformance	XL · 105(Y)	28.50	30.4	5.5	2,039	731	Spec page
Michelin Pilot Sport Cup 2Performance	XL · 105(Y)	28.50	30.9	7.2	2,039	731	Spec page
Michelin Pilot Sport Cup 2 ConnectPerformance	XL · 105(Y)	28.50	31.6	7.2	2,039	731	Spec page
Michelin Pilot Sport Cup 2 ConnectPerformance	XL · 105(Y)	28.50	31.8	7.2	2,039	731	Spec page
Michelin Pilot Sport 4 SPerformance	XL · 105(Y)	28.50	32.7	9.0	2,039	731	Spec page
Michelin Pilot Sport 4 SPerformance	XL · 105(Y)	28.50	32.9	9.5	2,039	731	Spec page
Michelin Pilot Alpin 5Winter	XL · 105V	28.46	33.1	9.5	2,039	731	Spec page
Michelin Pilot Sport 4 SPerformance	XL · 105(Y)	28.50	33.8	9.0	2,039	731	Spec page
Michelin Pilot Sport 4Performance	XL · 105(Y)	28.50	34.1	9.5	2,039	731	Spec page
Michelin Pilot Sport All-Season 4Performance	XL · 105V	28.50	34.1	9.5	2,039	731	Spec page
Toyo Proxes Sport 2Performance	XL · 105(Y)	28.50	34.8	9.2	2,039	730	Spec page

Is 315/30R21 a 28-inch tire?

≈28″

Close. The size code works out to 28.44″, and the 13 tires here publish 28.46–28.50″, 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 315/30R21	Difference
295/30R21	27.97	-0.47″	-1.7%
355/25R21	27.99	-0.45″	-1.6%
255/35R21	28.03	-0.41″	-1.5%
305/30R21	28.20	-0.24″	-0.8%
265/35R21	28.30	-0.14″	-0.5%
275/35R21	28.58	+0.14″	+0.5%
325/30R21	28.68	+0.24″	+0.8%
245/40R21	28.72	+0.28″	+1.0%
285/35R21	28.85	+0.41″	+1.5%
335/30R21	28.91	+0.47″	+1.7%

How to read 315/30R21

How to read any tire size

How 315/30R21 105Y is printed on the sidewall. It's the most common code among the 13 tires here (10 of them); tires in this size come in other load and speed ratings too.

1315Section width

315 mm across the widest point, or 12.40 inches. Every 315 size

230Aspect ratio

A percentage, not a measurement: the sidewall is 30% as tall as the tire is wide, which is 94.5 mm or 3.72 inches.

3RConstruction

Radial construction.

421Wheel diameter

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

5105Load index

Each tire carries up to 2,039 lb (925 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	28.44	722 mm
Section width	12.40	315 mm
Sidewall height	3.72	94.5 mm
Circumference	89.3	2,269 mm
Revolutions per mile (calculated, unloaded)	709.1	441 per km
Revolutions per mile (published by manufacturers)	730–731	454 per km
Approved wheel width	10.5–11.5	267–292 mm

Speedometer error after switching to 315/30R21

Your actual speed– mph

Questions about 315/30R21

What is 315/30R21 in inches?

28.44 inches tall, 12.40 inches wide, on a 21-inch wheel. Written in inches, that is roughly 28.4×12.4R21.

How much does a 315/30R21 tire weigh?

Between 30.4 lb and 34.8 lb for the 13 tires listed here. The lightest is the Michelin Pilot Alpin 5 (XL); the heaviest is the Toyo Proxes Sport 2 (XL).

What wheel width do I need for 315/30R21?

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

How many revolutions per mile does a 315/30R21 make?

Published figures run 730–731 revolutions per mile. The calculated figure is 709.1, but manufacturers measure the tire under load, where it rolls on a smaller radius.

How much taller is 315/30R21 than 265/35R21?

0.14 inches taller on paper (28.44″ vs 28.30″), which raises the axle about 0.07″. Your speedometer will read about 0.5% slow.