Explainer

A tyre sidewall carries a compact description of what the tyre is and what it may safely do. Most drivers read the size and ignore the rest.

The size string

A typical marking runs something like 265/70R17. The first number is section width in millimetres. The second is aspect ratio — sidewall height as a percentage of width, so a lower number means a shorter sidewall. The letter denotes construction, R for radial. The final number is wheel diameter in inches.

An LT prefix marks a light truck tyre, built for heavier loads and higher inflation pressures than a passenger tyre of the same dimensions.

Load index and speed rating

After the size come a number and a letter — the load index and speed rating.

The load index corresponds to a maximum load per tyre in a published table. It is not the weight in pounds; it is a code.

The speed rating is a letter denoting a maximum sustained speed. Aggressive light-truck tyres frequently carry lower speed ratings than the highway tyres a truck was delivered on, which is the issue behind the Ram software recall — the truck does not know what you fitted.

Load range

Light truck tyres also carry a load range letter — C, D, E and so on — indicating the pressure and load the carcass is built for. A higher load range means more capacity and a stiffer ride, and it requires higher inflation to achieve its rating.

Fitting load range E and inflating to the pressure you used on a lighter tyre gives you the ride penalty without the capacity.

The date code

The DOT code ends in four digits: week and year of manufacture. 3224 means the 32nd week of 2024.

This matters because rubber ages regardless of tread depth. A tyre with excellent tread that has sat for a decade is not a good tyre, and spare tyres in particular are frequently far older than the vehicle’s owner realises.

Maximum pressure is not recommended pressure

The pressure moulded into the sidewall is the maximum the tyre is rated for, not the pressure it should run at.

The correct operating pressure comes from the vehicle’s door sticker, adjusted for load — and if the tyres are not the original specification, that adjustment is a real calculation rather than a guess.

This article was researched and drafted with the assistance of AI tools and reviewed, fact-checked and edited by Leonidas Clark before publication. Figures vary by vehicle and configuration; confirm against your own door sticker and owner’s manual.