315/40R21 in inches

A 315/40R21 tire is 30.9″ tall and 12.4″ wide, and fits a 21″ wheel. In inches it is written 30.9×12.4R21.

Diameter30.92″

Width12.40″

Sidewall4.96″

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

Manufacturer data

How tall 315/40R21 tires really are

Heights published by Michelin for all 7 tires in this size. Their published revolutions per mile work out to 30.0″ under load (below).

Real 315/40R21 tires measure 30.90–31.00″, against 30.92″ from the size code. They run both taller and shorter than the size code.

Height under load30.0″

Published revs per mile (673) mean the tire rolls like a 30.0″ tire on the vehicle. Use this for speedometer calibration.

Weight spread33.7–39.7 lb

The lightest (Michelin Pilot Sport 4 SUV, XL) is 6.0 lb lighter per corner than the heaviest (Michelin Latitude Sport 3, SL).

7 tires in 315/40R21

Sort by the spec you care about.

	Load					Revs/mi	Source
Michelin Pilot Sport 4 SUVPerformance	XL · 115(Y)	31.00	33.7	9.0	2,679	673	Spec page
Michelin Pilot Sport All-Season 4Performance	XL · 115V	30.90	35.3	9.0	2,679	673	Spec page
Michelin Pilot Alpin 5Winter	XL · 115V	30.91	36.3	9.5	2,680	673	Spec page
Michelin Primacy Tour A/STouring	SL · 111H	30.90	36.5	8.5	2,403	673	Spec page
Michelin Pilot Sport 4 SUVPerformance	XL · 115Y	30.90	37.6	9.5	2,679	673	Spec page
Michelin Latitude Sport 3Performance	SL · 111Y	30.90	39.7	8.5	2,403	673	Spec page
Michelin Latitude Sport 3Performance	SL · 111Y	30.90	39.7	8.5	2,403	673	Spec page

Is 315/40R21 a 31-inch tire?

≈31″

Yes. The size code works out to 30.92″, and the 7 tires here publish 30.90–31.00″, 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.

Size	Diameter ″	vs 315/40R21	Difference
235/50R21	30.25	-0.67″	-2.2%
295/40R21	30.29	-0.63″	-2.0%
265/45R21	30.39	-0.53″	-1.7%
305/40R21	30.61	-0.31″	-1.0%
275/45R21	30.74	-0.18″	-0.6%
275/50R20	30.83	-0.09″	-0.3%
325/35R22	30.96	+0.04″	+0.1%
285/40R22	30.98	+0.06″	+0.2%
255/50R21	31.04	+0.12″	+0.4%
285/45R21	31.10	+0.18″	+0.6%

How to read 315/40R21

How to read any tire size

How 315/40R21 111Y is printed on the sidewall. It's the most common code among the 7 tires here (2 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

240Aspect ratio

A percentage, not a measurement: the sidewall is 40% as tall as the tire is wide, which is 126.0 mm or 4.96 inches.

3RConstruction

Radial construction.

421Wheel diameter

Mounts on a 21-inch wheel. Approved wheel widths run 10.5″ to 12.5″.

5111Load index

Each tire carries up to 2,403 lb (1,090 kg) at its maximum pressure.

6YSpeed rating

Rated for up to 186 mph sustained.

Specification	Inches	Metric
Overall diameter	30.92	785 mm
Section width	12.40	315 mm
Sidewall height	4.96	126.0 mm
Circumference	97.1	2,467 mm
Revolutions per mile (calculated, unloaded)	652.2	405 per km
Revolutions per mile (published by manufacturers)	673	418 per km
Approved wheel width	10.5–12.5	267–318 mm

Speedometer error after switching to 315/40R21

Your actual speed– mph

Questions about 315/40R21

What is 315/40R21 in inches?

30.92 inches tall, 12.40 inches wide, on a 21-inch wheel. Written in inches, that is roughly 30.9×12.4R21.

How much does a 315/40R21 tire weigh?

Between 33.7 lb and 39.7 lb for the 7 tires listed here. The lightest is the Michelin Pilot Sport 4 SUV (XL); the heaviest is the Michelin Latitude Sport 3 (SL).

What wheel width do I need for 315/40R21?

Manufacturers approve 10.5–12.5″ wide wheels for this size.

How many revolutions per mile does a 315/40R21 make?

Published figures run 673 revolutions per mile. The calculated figure is 652.2, but manufacturers measure the tire under load, where it rolls on a smaller radius.

How much taller is 315/40R21 than 275/45R21?

0.18 inches taller on paper (30.92″ vs 30.74″), which raises the axle about 0.09″. Your speedometer will read about 0.6% slow.