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305/35R21 in inches

A 305/35R21 tire is 29.4″ tall and 12.0″ wide, and fits a 21″ wheel. In inches it is written 29.4×12.0R21.

Diameter29.41″
Width12.01″
Sidewall4.20″
305/35R21 drawn to scale: 29.41 inch diameter, 4.20 inch sidewall, 21 inch wheel, 12.01 inch tread width21″ wheel4.20″29.41″ overall12.01″ wideSide view
Drawn to scale. Nominal dimensions calculated from the size code.
Manufacturer data

How tall 305/35R21 tires really are

Heights published by Michelin for the one tire in this size. Their published revolutions per mile work out to 28.5″ under load (below).

The one tire listed, the Michelin Pilot Alpin 5, measures 29.41″: 0.00″ taller than the size code's 29.41″.

Height under load28.5″

Published revs per mile (707) mean the tire rolls like a 28.5″ tire on the vehicle. Use this for speedometer calibration.

1 tire in 305/35R21

Sort by the spec you care about.

LoadRevs/miSource
Michelin Pilot Alpin 5WinterXL · 109V29.4136.710.52,271707Spec page

Is 305/35R21 a 29-inch tire?

≈29″

Close. The size code works out to 29.41″, and the 1 tire here publishes 29.41″, 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.

SizeDiameter ″vs 305/35R21Difference
335/30R2128.91-0.49″-1.7%
225/45R2128.97-0.43″-1.5%
255/40R2129.03-0.37″-1.3%
295/35R2129.13-0.28″-0.9%
345/30R2129.15-0.26″-0.9%
235/45R2129.33-0.08″-0.3%
265/40R2129.35-0.06″-0.2%
275/40R2129.66+0.26″+0.9%
245/45R2129.68+0.28″+0.9%
315/35R2129.68+0.28″+0.9%

How to read 305/35R21

How to read any tire size

305/35R21 109V as printed on the tire sidewall, with each part numbered305/35R21 109V123456
How 305/35R21 109V is printed on the sidewall. It's the most common code among the 1 tire here; 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

235Aspect ratio

A percentage, not a measurement: the sidewall is 35% as tall as the tire is wide, which is 106.8 mm or 4.20 inches.

3RConstruction

Radial construction.

421Wheel diameter

Mounts on a 21-inch wheel. Approved wheel widths run 10.0″ to 12.0″.

5109Load index

Each tire carries up to 2,271 lb (1,030 kg) at its maximum pressure.

6VSpeed rating

Rated for up to 149 mph sustained.

XLExtra load

Printed after the size.

SpecificationInchesMetric
Overall diameter29.41747 mm
Section width12.01305 mm
Sidewall height4.20106.8 mm
Circumference92.42,346 mm
Revolutions per mile (calculated, unloaded)685.9426 per km
Revolutions per mile (published by manufacturers)707439 per km
Approved wheel width10.0–12.0254–305 mm

Speedometer error after switching to 305/35R21

Your actual speed– mph

Questions about 305/35R21

What is 305/35R21 in inches?

29.41 inches tall, 12.01 inches wide, on a 21-inch wheel. Written in inches, that is roughly 29.4×12.0R21.

What wheel width do I need for 305/35R21?

Manufacturers approve 10.0–12.0″ wide wheels for this size.

How many revolutions per mile does a 305/35R21 make?

Published figures run 707 revolutions per mile. The calculated figure is 685.9, but manufacturers measure the tire under load, where it rolls on a smaller radius.

How much taller is 305/35R21 than 265/40R21?

0.06 inches taller on paper (29.41″ vs 29.35″), which raises the axle about 0.03″. Your speedometer will read about 0.2% slow.