A tire size reads something like 225/45R17 94V. The size is the first part. The 94 is the load index and the V is the speed rating, and together they define what the tire is certified to do.
Load index — a code, not a weight
The number maps to a maximum load per tire through a published table. It is not linear and it cannot be guessed: each step up is a few percent more capacity.
Common passenger car values run from the mid-80s to around 100. A 91 carries roughly 1,356 lb per tire; a 94 roughly 1,477 lb; a 100 roughly 1,764 lb.
Four tires at 94 give a theoretical 5,900 lb of tire capacity, which must exceed the vehicle's gross weight rating with margin. Never fit below the factory load index — the number on the door placard is chosen with that margin in mind, and dropping one step removes a slice of it.
Going above is harmless from a load standpoint and usually means a stiffer sidewall, which changes ride comfort more than most people expect.
Where load index actually bites
Trucks, vans and SUVs, where the loaded weight approaches the rating. This is where an under-rated tire fails.
XL or Reinforced tires carry a higher index in the same nominal size, and they require higher inflation pressure to reach it. Fitting XL tires and inflating to the standard-load pressure gives you a stiffer ride and none of the extra capacity.
Load range letters on light truck tires (C, D, E) are a parallel system describing the same idea through ply rating and pressure. An LT tire at load range E needs its rated pressure to carry its rated load, and running it at car pressures is how sidewalls fail.
Speed rating — sustained, not peak
The letter is the speed the tire is certified to sustain under load without heat failure. It is a test condition, not a suggestion.
Common ratings, from lower to higher: S 112 mph, T 118, H 130, V 149, W 168, Y 186.
The relevance in ordinary driving is not top speed. Higher-rated tires generally have stiffer construction, more heat resistance, and a softer, faster-wearing compound. That is why a V-rated tire feels sharper and lasts fewer miles than an equivalent T.
Why not simply drop the speed rating
Two reasons, and the first is not the obvious one.
Handling calibration. Stability control and ABS are tuned against the tire the car was developed on. A significantly softer-sidewalled tire changes the responses those systems are compensating for.
Insurance and liability. Fitting below the manufacturer's specified rating is, in several jurisdictions, grounds for an insurer to argue about a claim, and in some it is an inspection failure.
The one accepted exception is winter tires, which are commonly one or two ratings lower than the summer fitment. The convention is that the vehicle's speed is then limited accordingly, and many manufacturers require a sticker in the cabin stating it.
What to do at purchase
Read the door placard, not the tire currently on the car — the previous owner may have fitted something else. It lists the original size, load index, speed rating and pressures.
Then match or exceed both codes. Where a shop offers a cheaper tire "in the same size", check both characters after the size; that is usually where the difference is, and it is not a difference worth the saving.
Mixing ratings across an axle
Both codes have to match across an axle at minimum, and matching all four is better. A car with two V-rated tires on the front and two H-rated on the rear behaves differently in a way that is hardest to predict at exactly the moment it matters — a lane change at speed on a wet road.
Where only two tires are being replaced, the new pair goes on the rear regardless of which wheels drive. The reasoning is counter-intuitive and it is consistent across every manufacturer recommendation: a car that loses grip at the front understeers and slows; one that loses grip at the rear rotates, and most drivers cannot correct it.
The date code, while you are reading the sidewall
The last four digits of the DOT code are the week and year of manufacture — 3624 is the thirty-sixth week of 2024. Rubber ages whether it is used or not, and several manufacturers recommend replacement at six to ten years regardless of tread depth.
This matters when buying: a tire that has sat in a warehouse for three years is sold as new and is three years into its life. On a spare or a rarely-driven vehicle, age rather than wear is what ends the tire, and cracking at the base of the tread blocks is the sign to look for.