What does 40x15.50R24 mean?

It is a flotation size, quoted in inches. 40 is the tire's overall diameter, 15.5 is its section width, R means radial construction and 24 is the rim diameter it mounts on. The first number is the whole tire, not the tread width, which is the most common misreading of this format.

How tall is a 40x15.50R24 tire?

40 inches (1016 mm) in nominal overall diameter. That is the diameter the size code implies; a specific tire's measured diameter depends on its mould and the rim width it is mounted on, and the manufacturer's specification sheet is the authority for that tire.

How many revolutions per mile does a 40x15.50R24 turn?

504 revolutions per mile, from the nominal circumference of 125.7 inches. A loaded tire deflects and turns slightly more often than that, so a manufacturer's published revolutions figure usually runs a few percent higher.

40x15.50R24 tire size

40 inches tall, 15.5 inches wide, on a 24 inch rim. Sidewall 8 in (203.2 mm) per side, 504 revolutions per mile.

This is geometry, not fitment approval. It says what a size measures, never whether it fits a vehicle.

Overall diameter: 40 in
Sidewall: 203.2 mm
Revs / mile: 504

What each part of 40x15.50R24 means

40: Overall diameter in inches, printed on the tire. This is the whole tire, not the tread width. Misreading it as a width is the commonest mistake with this format.
15.5: Section width in inches (393.7 mm).
R: Radial construction. The R here is construction, not the speed symbol R. The speed symbol sits after the size, in the service description.
24: Rim diameter in inches. The bead seat diameter of the wheel this tire mounts on.
no prefix: Euro-metric (no prefix). An unprefixed size is Euro-metric, not P-metric with the P left off. The prefix decides which standards body's load table rates the tire. What the prefixes mean

Dimensions 40x15.50R24 implies

Dimensions for 40x15.50R24 in imperial and metric units
Dimension	Imperial	Metric	Where it comes from
Section width	15.5 in	393.7 mm	Printed on the tire in inches.
Sidewall height (one side)	8 in	203.2 mm	Computed. Bead seat to tread, one side only.
Overall diameter	40 in	1016 mm	Printed on the tire. Not computed.
Rim diameter	24 in	609.6 mm	Printed on the tire. The bead seat diameter of the wheel it mounts on.
Circumference	125.66 in	3191.9 mm	Computed as pi times the nominal overall diameter.
Revolutions	504.2 per mile	313.3 per km	Computed from the nominal circumference. A loaded tire turns slightly more often.

These are the nominal dimensions the size code implies. A real tire's measured dimensions depend on its mould, its construction and the rim width it is mounted on, so a manufacturer's specification sheet for a specific tire is the authority for that tire.

Sizes closest to 40x15.50R24 on the same 24 inch rim

Ranked by how little they change the overall diameter. The speedometer column assumes the instrument read correctly on 40x15.50R24 and that nothing was recalibrated.

Sizes closest to 40x15.50R24 on the same 24 inch rim
Size	Overall diameter	Difference	Speedometer at an indicated 60 mph
38x15.50R24	38 in	-5.00%	57 mph actual (-3.0)
42x15.50R24	42 in	+5.00%	63 mph actual (+3.0)
37x13.50R24	37 in	-7.50%	55.5 mph actual (-4.5)
37x14.50R24	37 in	-7.50%	55.5 mph actual (-4.5)
315/50R24	36.4 in	-9.00%	54.6 mph actual (-5.4)
LT315/50R24	36.4 in	-9.00%	54.6 mph actual (-5.4)

A positive difference means the replacement is taller, so the vehicle travels further per revolution and the speedometer reads LOW. Many vehicles leave the factory reading slightly high on purpose, so treat this as a change from whatever the instrument did before, not as an absolute claim about it.

Same 394 mm width, different aspect ratio

What changing only the aspect ratio does, holding the width and the rim.

Questions this size raises

What does 40x15.50R24 mean?: It is a flotation size, quoted in inches. 40 is the tire's overall diameter, 15.5 is its section width, R means radial construction and 24 is the rim diameter it mounts on. The first number is the whole tire, not the tread width, which is the most common misreading of this format.
How tall is a 40x15.50R24 tire?: 40 inches (1016 mm) in nominal overall diameter. That is the diameter the size code implies; a specific tire's measured diameter depends on its mould and the rim width it is mounted on, and the manufacturer's specification sheet is the authority for that tire.
What is the sidewall height of a 40x15.50R24?: 8 inches (203.2 mm) per side.
How many revolutions per mile does a 40x15.50R24 turn?: 504 revolutions per mile, from the nominal circumference of 125.7 inches. A loaded tire deflects and turns slightly more often than that, so a manufacturer's published revolutions figure usually runs a few percent higher.

The rest of the sidewall

The size is one line of a sidewall. What follows it decides whether the tire is legal on the vehicle at all.

Load index table - the number after the size, and the mass it stands for. Must meet or exceed the vehicle placard.
Speed symbol table - the letter after the load index, including the ZR, W, Y and (Y) cases.
DOT date code - how old the tire is, and why a three digit code is not the same as a four digit one.
Every other marking - UTQG grades, load range letters, M+S against the three peak mountain snowflake.
Compare 40x15.50R24 against another size - diameter, clearance and speedometer error side by side.

Where these figures come from

Computed on this page, not looked up. Each figure and the arithmetic behind it:

Overall diameter Printed on the tire: 40 in. Not derived.
Sidewall height (overall diameter - rim diameter) / 2 = (40 - 24) / 2 = 8 in
Aspect ratio (not printed on this size) sidewall / section width x 100 = 8 / 15.5 x 100 = 51.6
Circumference pi x overall diameter = pi x 40 = 125.66 in
Revolutions per mile 63360 / circumference = 63360 / 125.66 = 504.2

Published values here were read from 2 independent sources:

Published tire size sitemap - SimpleTire, sitemap dated 13 August 2026 (read 2026-08-27)
Which sizes a national retailer publishes a catalog page for
Published tire size listing sitemap - Discount Tire, sitemap read 27 August 2026 (read 2026-08-27)
Which sizes a second national retailer publishes a catalog page for

These are independent publishers, not independent measurements. Each one reproduces a table originally set by a standards body, so agreement between them shows a value was transcribed correctly, not that the underlying table is right. Where a figure is safety relevant, check it against the tire manufacturer's own data before relying on it.

Before you fit anything

SidewallRef calculates the dimensions a size code implies and reproduces the published load index and speed symbol tables. It does not know your vehicle.

Load index and speed rating must meet or exceed what the vehicle placard specifies. The placard is in the driver's door jamb on most vehicles and is the authority for your vehicle, not this site.

Clearance at full lock and full suspension travel, wheel width and offset, brake and suspension component clearance, axle load capacity, inflation pressure and any legal requirement where you drive are the owner's and the fitter's responsibility.

Nothing here approves a fitment. Confirm any change with a qualified tire fitter and with the vehicle manufacturer's own specification before driving on it.