How to Read a Tire Sidewall: Load Index and Speed Rating

Where the numbers are

On a tire sidewall, the size code is followed immediately by two more characters, in that order: a two- or three-digit load index, then a single letter, the speed rating. On a tire marked 225/45R17 91V, "91" is the load index and "V" is the speed rating. Both describe limits tested and assigned by the tire's manufacturer for that specific tire, not the vehicle.

Load index: what the number stands for

The load index is a code, not a weight. It maps to a maximum load a single tire can carry at its maximum rated inflation pressure, using a published index table. Per BFGoodrich's guide, "Typically, the load indexes of the tires used on passenger cars and light trucks range from 70 to 110." A higher number always means more carrying capacity, but the scale is not evenly spaced -- the jump from one index to the next does not always represent the same weight increment, which is why a table, not a formula, is the only reliable way to read it.

This page does not reproduce the full index-to-pounds table published in the ETRTO/TRA/ISO 4000-1 standards documents -- those are commercial publications this project does not have republication rights for. It does cite eight illustrative points from Pirelli's own published load index chart, each described there as "the maximum amount of weight that a tire can bear when inflated to its maximum pressure":

Load index	Max load (lbs)
75	852
82	1,047
88	1,235
91	1,356
95	1,521
100	1,764
104	1,984
109	2,271

These eight points span the common passenger/light-truck range but are not the complete table -- every index between and beyond these values has its own assigned figure. Always read the exact index off the tire being bought and check it, not an interpolated guess between two of the rows above. 49 CFR 571.139 (FMVSS No. 139), the federal standard for new radial tires, defines "maximum load rating" as one of the terms molded onto the sidewall alongside the load index, without itself publishing the index-to-pounds conversion table.

Speed rating: what the letter stands for

The speed rating letter corresponds to a maximum speed the tire is tested to sustain under the standard's test conditions. Per BFGoodrich's guide, "S is rated to 112 mph, T to 118, H to 130, V to 149, W to 168, and Y to 186." A replacement tire's speed rating should match or exceed the original fitment's rating; fitting a lower-rated tire lowers the vehicle's tested maximum for that corner. 49 CFR 571.139 section S6.2 specifies the regulation's own High Speed Performance test procedure at fixed test speeds of 140, 150 and 160 km/h -- a sustained-speed durability test, not a lookup table of every speed-rating letter. The full letter-to-mph table is, like the load index table, part of the licensed ETRTO/TRA standards and is not reproduced here.

Two different numbers, two different checks

The load index and the speed rating are independent of each other -- a tire can be upgraded on one axis without the other changing. Both should be checked against the vehicle's door placard (or owner's manual) first, since that is the manufacturer's requirement for the vehicle, not just whatever was on the tire being replaced. UTQG grading (treadwear, traction, temperature), covered under 49 CFR 575.104, is a separate rating system from both of these and is not covered on this page.

Sources and limits

Full letter/number tables (ETRTO, TRA, SAE) are commercial standards documents this project does not have a republication license for, so no complete table is reproduced here; only the regulatory definitions, the manufacturer-quoted illustrative BFGoodrich figures and the eight Pirelli load-index data points above are cited. Sidewall markings should always be confirmed against the specific tire being bought, not estimated from a general table. Full limits are covered on the site disclaimer page.