Tire Size Meaning: Every Number and Letter Explained

Tire size meaning is mostly hidden in plain sight: the string on your sidewall is a specification, and every character in it is doing a job. This guide takes P225/65R17 98H apart one field at a time, shows the arithmetic each field feeds, and gives the reference tables for the two fields you cannot derive — the load index and the speed symbol.

Nothing here is vehicle-specific. The size code describes the tire, not the car it belongs on. For what your vehicle calls for, the authority is the Tire and Loading Information label on the driver's side door edge or the owner's manual, both of which NHTSA points to.

The whole code at a glance

Field	Example	What it is	Unit
Service type	P	What class of vehicle the tire is built for	letter code
Section width	225	Width across the widest part of the tire	millimetres
Aspect ratio	65	Sidewall height as a percentage of section width	percent
Construction	R	Internal construction, radial here	letter code
Rim diameter	17	Wheel diameter the tire mounts on	inches
Load index	98	Maximum load per tire, via a published table	index number
Speed symbol	H	Rated speed, via a published table	letter code

The mixed units are not an error, they are history. The width and sidewall are metric, the rim is imperial, and the two halves of the code were standardised at different times in different places. Every calculation below converts at exactly 25.4 mm per inch.

Field one: the service type letter

The letter or letters before the width tell you what the tire was designed to carry. Tire Rack's sidewall size page sets these out:

| P | P-metric: designed for vehicles used mainly as passenger vehicles. | | (none) | Euro-metric size (no service-type letter). | | LT (leading) | Light Truck-metric size, for vehicles that carry heavy cargo or tow large trailers. | | LT (trailing) | Flotation light-truck size, written as diameter x width R rim, e.g. 33x12.50R17LT. | | C (trailing) | Commercial tire for vans or delivery trucks (Euro-metric). | | T | Temporary spare, for short-term use until the flat is repaired or replaced. | | ST | Special Trailer Service size, for trailers only. | | HL | Higher load capacity than an XL tire of the same dimensions; the size also ends with XL. |

The distinction that catches people out most often is the leading LT against the trailing LT. A leading LT, as in LT265/70R17, marks a light-truck metric size. A trailing LT, as in 33x12.50R17LT, marks a flotation size, which is written as overall diameter × section width R rim diameter, all in inches. The two formats describe the same kind of tire in completely different units.

A size with no letter at all is a Euro-metric size. P-metric and Euro-metric sizes with the same numbers are not interchangeable in load terms even though their dimensions match, because the two systems compute load capacity differently. That difference is in the service description, not the dimensions.

Field two and three: section width and aspect ratio

The three-digit number is the section width in millimetres: the distance across the widest part of the inflated tire, sidewall bulge to sidewall bulge. It is not the tread width, which is narrower, and it is not the width at the bead. A 225 is 225 mm across its widest point when mounted on its measuring rim at its measuring pressure.

The two-digit number after the slash is the aspect ratio, which Tire Rack's page describes as the height of the sidewall expressed as a percentage of the section width. This is the single most misread field in the whole code, because it is relative:

sidewall height = section width × aspect ratio ÷ 100

Size	Section width	Aspect ratio	Sidewall height
205/65R16	205 mm	65	133.3 mm
225/65R17	225 mm	65	146.3 mm
265/65R18	265 mm	65	172.3 mm
225/45R17	225 mm	45	101.3 mm
225/75R17	225 mm	75	168.8 mm

Three tires with an aspect ratio of 65 have sidewalls that differ by 39.0 mm. "Sixty-five series" is not a height. Equally, holding the width at 225 and moving the aspect ratio from 45 to 75 changes the sidewall by 67.5 mm, which is 135.0 mm of overall diameter, because the sidewall counts twice.

That doubling is the key to the whole code. Everything you do to the aspect ratio or the width happens at the top and the bottom of the wheel at once.

Field four: the construction letter

Code	Meaning
R	Radial construction.
D	Bias-ply (diagonal) construction.
RF	Self-supporting run-flat construction.

R for radial is what almost every modern passenger tire carries. You will also meet ZR, which appears on some high-performance sizes; it is a radial, and where a service description follows it, the speed symbol in that service description is the one that applies.

Field five: rim diameter

The number after the construction letter is the diameter of the wheel the tire is built to mount on, in inches. It is the one field with no tolerance to it at all: a 17-inch tire goes on a 17-inch wheel, and nothing else. Width has a range of acceptable rim widths; diameter does not.

Half-inch sizes exist, such as 16.5, and they are a different fitment from 16 or 17 despite how close the numbers look. This is the same trap as the bolt pattern near-miss covered in the bolt pattern guide: a number that is nearly right is a different specification, not a tolerance.

Putting the dimensions together

With width, aspect ratio and rim diameter you can compute every dimension of the tire:

overall diameter = rim × 25.4 + 2 × (width × aspect ÷ 100)

circumference = π × overall diameter    revolutions per mile = 1,609,344 ÷ circumference

For P225/65R17: sidewall = 225 × 0.65 = 146.25 mm, rim = 17 × 25.4 = 431.8 mm, overall diameter = 724.3 mm or 28.52 inches. The circumference is 2275 mm, and the tire turns 707.3 times in a mile.

The revolutions-per-mile figure is the one that connects the size code to your instrument cluster. A speedometer counts rotations and multiplies by the circumference it was calibrated for, so fitting a tire with a different rolling circumference moves the needle by exactly the ratio of the two diameters. The tire size calculator runs that comparison and prints the error at 30, 45, 60 and 75 mph.

These are nominal dimensions computed from the code. A mounted tire measures slightly differently depending on brand, rim width, pressure and load, which is why no serious figure here carries more than one decimal place.

The service description: load index and speed symbol

After the size comes the service description, which Tire Rack describes as the tire's load index and speed rating together: in 98H, the 98 is the load index and the H is the speed symbol. Unlike the dimensions, these two cannot be derived. They are index numbers that point into published tables.

Load index

The load index gives the maximum load the tire can carry, per tire, at its rated inflation. The table below is reproduced from Tire Rack's load index page, retrieved 2026-10-16. It covers index 70 to 126, which is the range that page prints; outside it, this site shows "not available" rather than extrapolating.

Index	lb	kg	Index	lb	kg	Index	lb	kg
70	739	335	89	1279	580	108	2205	1000
71	761	345	90	1323	600	109	2271	1030
72	783	355	91	1356	615	110	2337	1060
73	805	365	92	1389	630	111	2403	1090
74	827	375	93	1433	650	112	2469	1120
75	853	387	94	1477	670	113	2535	1150
76	882	400	95	1521	690	114	2601	1180
77	908	412	96	1565	710	115	2679	1215
78	937	425	97	1609	730	116	2756	1250
79	963	437	98	1653	750	117	2833	1285
80	992	450	99	1709	775	118	2910	1320
81	1019	462	100	1764	800	119	2998	1360
82	1047	475	101	1819	825	120	3086	1400
83	1074	487	102	1874	850	121	3197	1450
84	1102	500	103	1929	875	122	3307	1500
85	1135	515	104	1984	900	123	3417	1550
86	1168	530	105	2039	925	124	3527	1600
87	1201	545	106	2094	950	125	3638	1650
88	1235	560	107	2149	975	126	3748	1700

Two points matter more than the numbers. First, the index is per tire, so four tires at index 98 carry 4 × 1,653 lb between them — but the figure you must satisfy is the axle load on your vehicle's Tire and Loading Information label, not the gross weight of the vehicle divided by four. Second, the rating applies at the inflation pressure the tire is rated at. An underinflated tire does not carry its rated load, which is the reason NHTSA's tire page says to check the pressure of all tires, including the spare, at least once a month.

Load index is not load range. Load range is a separate letter class, common on light-truck tires, describing the construction and inflation class. A tire carries both, and only the load index converts directly to pounds.

Speed symbol

The speed symbol is a single letter giving the speed the tire is rated for. From Tire Rack's speed rating page, retrieved 2026-10-16:

Symbol	mph	km/h	Symbol	mph	km/h
L	75	120	T	118	190
M	81	130	U	124	200
N	87	140	H	130	210
P	93	150	V	149	240
Q	99	160	W	168	270
R	106	170	Y	186	300
S	112	180

The letters are not in alphabetical order by speed — H sits between U and V, a leftover from an older scheme — so the table is the only safe way to read them. Tire Rack's page also notes that a Y enclosed in parentheses in a service description, as in 285/35ZR19 (99Y), means the tire's top speed has been tested above 186 mph (300 km/h).

NHTSA's position is worth knowing: in a published interpretation the agency states that speed ratings are not required under the federal motor vehicle safety standards, and that manufacturers are permitted to label additional information on the sidewall including speed rating codes. The symbol is a manufacturer's rating, not a federal grade.

The markings that are not part of the size

Several other things are moulded on the sidewall and are often mistaken for part of the size code.

Marking	What it is
DOT Tire Identification Number	The TIN. NHTSA states the last four digits give the week and year the tire was made.
Treadwear, Traction, Temperature	The UTQG grades. NHTSA states these ratings are on the sidewall of every passenger vehicle tire sold in the United States.
Max load and max pressure	The tire's own maximum figures, not the pressure to run. The pressure to run is on the vehicle's label.
M+S, 3PMSF	Mud and snow marking, and the three-peak mountain snowflake for severe snow service.
XL or Extra Load	A higher-load version of the same dimensions. Tire Rack's page notes that HL marks a tire with higher load capacity than an XL tire of equivalent dimensions.

The one that causes real trouble is max pressure. It is the tire's ceiling, not a recommendation. The pressure your vehicle wants is on the Tire and Loading Information label on the driver's door edge, which is also where NHTSA says to find it.

Reading a size change, not just a size

Once the code is legible, the useful question becomes what happens when it changes. Three moves are possible, and they do different things:

• Width only, 225 to 235 at the same aspect ratio: the section gets wider and, because the sidewall is a percentage of width, the diameter grows too. 225/65R17 to 235/65R17 adds 13.0 mm of diameter, a +1.79% change.
• Aspect ratio only, 65 to 55 at the same width: the sidewall shrinks and the diameter drops by twice that. 225/65R17 to 225/55R17 is -45.0 mm, or -6.21%.
• Rim diameter with a compensating aspect ratio, often called plus-sizing: 225/65R17 to 245/55R18 changes the look and the sidewall while holding diameter close, here +0.33%.

The third pattern is the one worth understanding, because it shows what the code is for. The two sizes are different in almost every field and nearly identical in rolling diameter, which is what the speedometer, the ABS and the final drive care about. The fields that changed — width and sidewall height — are the ones that change clearance, ride and steering feel.

None of this tells you whether a size fits your car. Diameter is only one of the constraints; the others are fender and suspension clearance, the rim width the tire is approved for, and the load index against your axle load. Those belong to your vehicle, and the geometry on this page cannot see it.

Checking your own sidewall

Work left to right. Letters before the number give you the service type. Three digits are the width in millimetres. Two digits after the slash are the aspect ratio, a percentage of that width. A letter is the construction. The next number is the rim in inches. Then the service description: a number for load, a letter for speed. Anything further along the sidewall — DOT code, UTQG grades, max load, max pressure, M+S — is a different marking, not part of the size.

Then compare what is on the car against what the vehicle's label specifies. If those differ, the size on the tire is not authority for anything; the label is. Understanding tire size meaning is what lets you see that the two strings differ in a field that matters, instead of assuming that two codes which look alike are interchangeable.

Sources

• Tire Rack, How Do I Read My Tire Size On My Sidewall? — service type prefixes, section width, aspect ratio, construction, rim diameter and service description. Retrieved 2026-10-16.
• Tire Rack, What Is Load Index? — load index table, 70 to 126. Retrieved 2026-10-16.
• Tire Rack, How Do I Check A Tire's Speed Rating? — speed symbol table and the parenthetical Y note. Retrieved 2026-10-16.
• NHTSA, Tire Safety Ratings and Awareness — Tire and Loading Information label, monthly pressure checks, DOT TIN date code, UTQG grades. Retrieved 2026-10-16.
• All dimensional figures on this page are computed from the size codes shown, using the formulas on the methodology page.