255/40R21 in inches

A 255/40R21 tire is 29.0″ tall and 10.0″ wide, and fits a 21″ wheel. In inches it is written 29.0×10.0R21.

Diameter29.03″

Width10.04″

Sidewall4.02″

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

Manufacturer data

How tall 255/40R21 tires really are

Heights published by Michelin and Toyo for all 16 tires in this size. Their published revolutions per mile work out to 28.1–28.2″ under load (below).

Real 255/40R21 tires measure 29.00–29.02″, against 29.03″ from the size code. Most run shorter than the size code (13 of 16).

Height under load28.1–28.2″

Published revs per mile (716–717) mean the tire rolls like a 28.1–28.2″ tire on the vehicle. Use this for speedometer calibration.

Weight spread28.5–32.5 lb

The lightest (Michelin Pilot Sport All-Season 4, XL) is 4.0 lb lighter per corner than the heaviest (Michelin X-Ice Snow, XL).

16 tires in 255/40R21

Sort by the spec you care about.

	Load					Revs/mi	Source
Michelin Pilot Sport All-Season 4Performance	XL · 102Y	29.00	28.5	10.0	1,874	717	Spec page
Michelin Pilot Sport 4 SPerformance	XL · 102(Y)	29.00	28.7	9.0	1,874	717	Spec page
Michelin Pilot Sport 4 SUVPerformance	XL · 102Y	29.00	29.2	8.5	1,874	717	Spec page
Michelin Pilot Sport 4 SUVPerformance	XL · 102Y	29.00	29.4	8.5	1,874	717	Spec page
Michelin Pilot Sport 4 SPerformance	XL · 102(Y)	29.00	29.5	9.0	1,874	717	Spec page
Michelin CrossClimate2All-season	XL · 102V	29.00	30.1	10.0	1,874	717	Spec page
Michelin Primacy Tour A/STouring	XL · 102W	29.00	30.2	9.0	1,874	717	Spec page
Michelin Pilot Sport 5Performance	XL · 102Y	29.00	30.4	9.0	1,874	717	Spec page
Toyo Proxes Sport 2Performance	XL · 102Y	29.00	30.4	9.2	1,874	716	Spec page
Michelin Pilot Sport EVPerformance	XL · 102Y	29.00	30.9	8.5	1,874	717	Spec page
Michelin Pilot Sport 4 SUVPerformance	XL · 102Y	29.00	31.1	9.5	1,874	717	Spec page
Michelin Pilot Sport 4 SUVPerformance	XL · 102Y	29.00	31.1	9.5	1,874	717	Spec page
Michelin Primacy Tour A/STouring	XL · 102W	29.00	31.1	8.5	1,874	717	Spec page
Toyo Celsius SportPerformance	XL · 102Y	29.02	31.6	9.6	1,874	716	Spec page
Michelin Pilot Alpin 5Winter	XL · 102V	29.02	32.4	9.5	1,874	717	Spec page
Michelin X-Ice SnowWinter	XL · 102H	29.02	32.5	10.5	1,874	717	Spec page

Is 255/40R21 a 29-inch tire?

≈29″

Yes. The size code works out to 29.03″, and the 16 tires here publish 29.00–29.02″, so people call it a 29.

All 29-inch tire sizes, on every wheel size.

Compare with nearby sizes

Differences in overall diameter, calculated from each size code.

Size	Diameter ″	vs 255/40R21	Difference
325/30R21	28.68	-0.35″	-1.2%
245/40R21	28.72	-0.31″	-1.1%
285/35R21	28.85	-0.18″	-0.6%
335/30R21	28.91	-0.12″	-0.4%
225/45R21	28.97	-0.06″	-0.2%
295/35R21	29.13	+0.10″	+0.3%
345/30R21	29.15	+0.12″	+0.4%
235/45R21	29.33	+0.30″	+1.0%
265/40R21	29.35	+0.31″	+1.1%
305/35R21	29.41	+0.37″	+1.3%

How to read 255/40R21

How to read any tire size

How 255/40R21 102Y is printed on the sidewall. It's the most common code among the 16 tires here (9 of them); tires in this size come in other load and speed ratings too.

1255Section width

255 mm across the widest point, or 10.04 inches. Every 255 size

240Aspect ratio

A percentage, not a measurement: the sidewall is 40% as tall as the tire is wide, which is 102.0 mm or 4.02 inches.

3RConstruction

Radial construction.

421Wheel diameter

Mounts on a 21-inch wheel. Approved wheel widths run 8.5″ to 10.0″.

5102Load index

Each tire carries up to 1,874 lb (850 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	29.03	737 mm
Section width	10.04	255 mm
Sidewall height	4.02	102.0 mm
Circumference	91.2	2,317 mm
Revolutions per mile (calculated, unloaded)	694.7	432 per km
Revolutions per mile (published by manufacturers)	716–717	445–446 per km
Approved wheel width	8.5–10.0	216–254 mm

Speedometer error after switching to 255/40R21

Your actual speed– mph

Questions about 255/40R21

What is 255/40R21 in inches?

29.03 inches tall, 10.04 inches wide, on a 21-inch wheel. Written in inches, that is roughly 29.0×10.0R21.

How much does a 255/40R21 tire weigh?

Between 28.5 lb and 32.5 lb for the 16 tires listed here. The lightest is the Michelin Pilot Sport All-Season 4 (XL); the heaviest is the Michelin X-Ice Snow (XL).

What wheel width do I need for 255/40R21?

Manufacturers approve 8.5–10.0″ wide wheels for this size.

How many revolutions per mile does a 255/40R21 make?

Published figures run 716–717 revolutions per mile. The calculated figure is 694.7, but manufacturers measure the tire under load, where it rolls on a smaller radius.

How much taller is 255/40R21 than 225/45R21?

0.06 inches taller on paper (29.03″ vs 28.97″), which raises the axle about 0.03″. Your speedometer will read about 0.2% slow.