Reading 225/45R17 91V from left to right
Five facts in eleven characters, in three different unit systems, which is why it looks like a part number rather than a specification.
| Part | Meaning | Unit |
|---|---|---|
| 225 | Section width, sidewall bulge to sidewall bulge | Millimetres |
| 45 | Aspect ratio -- sidewall height as a percentage of section width | Per cent, so it has no unit of its own |
| R | Radial construction | -- |
| 17 | Rim diameter -- the wheel it mounts on | Inches |
| 91 | Load index, an index number rather than a weight | Look-up table |
| V | Speed rating | Look-up table |
The aspect ratio being a percentage is the part that surprises people. A 45 on a 225 tire is a sidewall of 101.25 mm; the same 45 on a 275 tire is 123.75 mm. So two tires with the same aspect ratio do not have the same sidewall, and changing the width while keeping the ratio changes the overall diameter.
Working the example through: 225 x 0.45 gives a 101.25 mm sidewall. The wheel is 17 inches, which is 431.8 mm. There is a sidewall at the top and another at the bottom, so the overall diameter is 431.8 + 202.5 = 634.3 mm, which is 24.97 inches.
Load index is the number people skip
Load index is not a weight. It is an index into a table, and the relationship is not linear -- 91 is 1,356 lb per tire, 92 is 1,389, and the steps get larger as the numbers rise. The vehicle placard inside the driver's door jamb states the minimum load index for the car, and a replacement should meet or exceed it.
It gets skipped because the number is small, undramatic and sits at the end. But the consequence of under-rating is not subtle: a tire carrying more than it is built for runs hotter, and heat is what destroys tires. It matters most on vehicles that carry load -- vans, trucks, anything towing -- and on wide aftermarket sizes chosen for appearance, where a low-profile tire in an attractive size may simply not be rated for the vehicle it is going on.
LT-marked light truck tires complicate this further, because their load indices are quoted differently for single and dual-wheel fitments. If the sidewall carries two index numbers separated by a slash, the second is the dual-wheel figure.
Speed ratings, and what they are actually testing
A V rating means the tire passed a laboratory test at 149 mph under controlled load, pressure and temperature. It is not permission, it is not a warranty at that speed on a public road, and it does not survive under-inflation or damage. Its practical use is as a proxy for construction: higher-rated tires generally have stiffer sidewalls and a firmer ride, and a vehicle whose suspension was tuned around a W-rated tire will feel different on an H.
The letters are not alphabetical -- H sits between U and V, a historical accident from when H was the top of the scale. That is why the reference table exists rather than expecting anyone to remember the order.
The DOT date code
The last four digits of the DOT string are the manufacture date: two digits for the week, two for the year. 3419 is the 34th week of 2019, so late August. The full DOT code is often moulded in full on one side of the tire only, so if you cannot find the date, look at the inboard face.
Age matters independently of tread. Rubber oxidises and the compound hardens whether or not the tire turns, so a tire with full tread that has sat on a trailer or a spare wheel for a decade is not a good tire. Ten years is the common recommendation for replacement regardless of wear, six years is the point at which annual inspection is worth doing, and a "new" tire that has been in a warehouse for three years is not really new.
What this page does not do
It decodes one size. It does not tell you whether a size fits your vehicle, and that is a genuinely separate question: clearance against the strut, the liner and the suspension arms at full lock and full compression depends on the wheel offset and the specific car, not on the tire code. It also does not compare two sizes -- for overall diameter difference, speedometer error and revolutions per mile between a current and a proposed size, the tire size comparison calculator does that side by side.
Two more things the code does not carry: the actual tread width, which is narrower than the section width and is not published in the size, and the recommended rim width range, which is a separate specification. A 225 tire has a range of legal rim widths, and mounting it at the edge of that range changes the effective section width and the sidewall shape.
Questions people ask
Is 225 the tread width?
No. It is the section width, measured across the widest part of the inflated tire from sidewall to sidewall, on a specified reference rim. The tread that touches the road is narrower -- typically 15 to 25 mm less. It also changes with rim width: the same tire on a wider wheel measures wider and has a flatter sidewall profile.
What does the R mean, and what if mine says ZR?
R is radial construction, which is what essentially every road tire has been for decades. ZR is a legacy marking that once meant a tire rated above 149 mph, from before W and Y letters existed. On a modern tire the letter after the load index is the actual rating; a ZR marking alongside it is historical.
Can I fit a higher load index than the car needs?
Yes, and it is the safe direction to err. A higher index costs nothing in safety terms and often means a stiffer carcass. The trade is ride quality -- a heavier-duty tire on a light car rides harder. Going the other way, below the placard figure, is the one to avoid.
The sidewall has a second set of numbers in inches.
Some tires carry both metric and flotation sizing, and light truck and off-road tires are often sold in the inch format instead: 33x12.50R15 means 33 inches tall, 12.5 inches wide, on a 15 inch rim. The first number in that format is the overall diameter directly, which makes it easier to read and is why off-road buyers prefer it.
How old is too old for a tire with plenty of tread?
Ten years is the widely published limit from manufacturers and several national road authorities, regardless of tread depth. From about six years the sidewalls are worth inspecting each year for fine cracking. Tires stored outdoors, on a vehicle that sits, or in hot climates age faster, and a spare that has never touched the road ages at the same rate as the ones that have.