315/30R18 in inches

A 315/30R18 tire is 25.4″ tall and 12.4″ wide, and fits a 18″ wheel. In inches it is written 25.4×12.4R18.

Diameter25.44″

Width12.40″

Sidewall3.72″

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

Manufacturer data

How tall 315/30R18 tires really are

Heights published by BFGoodrich, Michelin and Toyo for all 7 tires in this size. Their published revolutions per mile work out to 24.7″ under load (below).

Real 315/30R18 tires measure 25.47–25.50″, against 25.44″ from the size code. Every one runs taller than the size code.

Height under load24.7″

Published revs per mile (816–817) mean the tire rolls like a 24.7″ tire on the vehicle. Use this for speedometer calibration.

Weight spread28.2–30.6 lb

The lightest (Toyo Proxes R, SL) is 2.4 lb lighter per corner than the heaviest (Toyo Proxes R888R, SL).

7 tires in 315/30R18

Sort by the spec you care about.

	Load					Revs/mi	Source
Toyo Proxes RPerformance	SL · 98W	25.50	28.2	3.7	1,653	816	Spec page
Toyo Proxes RRPerformance	SL	25.47	29.1	3.6	1,653	816	Spec page
Michelin Pilot Sport PS2Performance	SL · 98(Y)	25.50	29.3	8.5	1,653	817	Spec page
Toyo Proxes Sport RPerformance	SL · 98W	25.50	30.0	7.2	1,653	816	Spec page
BFGoodrich g-Force Rival SPerformance	SL · 91W	25.47	30.4	7.2	1,356	817	Spec page
BFGoodrich g-Force Rival+Performance	SL · 91W	25.47	30.4	7.2	1,521	817	Spec page
Toyo Proxes R888RPerformance	SL · 98Y	25.47	30.6	6.3	1,653	816	Spec page

Is 315/30R18 a 25-inch tire?

≈25″

Close. The size code works out to 25.44″, and the 7 tires here publish 25.47–25.50″, so people call it a 25.

Compare with nearby sizes

Differences in overall diameter, calculated from each size code.

Size	Diameter ″	vs 315/30R18	Difference
295/30R18	24.97	-0.47″	-1.9%
255/35R18	25.03	-0.41″	-1.6%
225/40R18	25.09	-0.35″	-1.4%
265/35R18	25.30	-0.14″	-0.5%
235/40R18	25.40	-0.04″	-0.2%
275/35R18	25.58	+0.14″	+0.5%
215/45R18	25.62	+0.18″	+0.7%
245/40R18	25.72	+0.28″	+1.1%
285/35R18	25.85	+0.41″	+1.6%
335/30R18	25.91	+0.47″	+1.9%

How to read 315/30R18

How to read any tire size

How P315/30R18 91W 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.

1PP-metric

Marks a P-metric (passenger) tire. Sizes with no letter are Euro-metric.

2315Section width

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

330Aspect 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.

4RConstruction

Radial construction.

518Wheel diameter

Mounts on a 18-inch wheel. Approved wheel widths run 10.5″ to 11.5″.

691Load index

Each tire carries up to 1,356 lb (615 kg) at its maximum pressure.

7WSpeed rating

Rated for up to 168 mph sustained.

Specification	Inches	Metric
Overall diameter	25.44	646 mm
Section width	12.40	315 mm
Sidewall height	3.72	94.5 mm
Circumference	79.9	2,030 mm
Revolutions per mile (calculated, unloaded)	792.7	493 per km
Revolutions per mile (published by manufacturers)	816–817	507–508 per km
Approved wheel width	10.5–11.5	267–292 mm

Speedometer error after switching to 315/30R18

Your actual speed– mph

Questions about 315/30R18

What is 315/30R18 in inches?

25.44 inches tall, 12.40 inches wide, on a 18-inch wheel. Written in inches, that is roughly 25.4×12.4R18.

How much does a 315/30R18 tire weigh?

Between 28.2 lb and 30.6 lb for the 7 tires listed here. The lightest is the Toyo Proxes R (SL); the heaviest is the Toyo Proxes R888R (SL).

What wheel width do I need for 315/30R18?

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

How many revolutions per mile does a 315/30R18 make?

Published figures run 816–817 revolutions per mile. The calculated figure is 792.7, but manufacturers measure the tire under load, where it rolls on a smaller radius.

How much taller is 315/30R18 than 235/40R18?

0.04 inches taller on paper (25.44″ vs 25.40″), which raises the axle about 0.02″. Your speedometer will read about 0.2% slow.