What does speed rating R mean?

Two Rs on the same sidewall, meaning different things

In LT235/85R16 120R the letter R appears twice. The first one, buried in the size, denotes radial construction. The second one, at the end of the service description, is the speed symbol and means 170 km/h. They are unrelated markings that happen to share a character, and misreading one for the other is the most common mistake made with this particular rating.

Position is what distinguishes them: construction letters sit between the aspect ratio and the rim diameter, speed symbols sit after the load index.

Where a 106 mph rating is appropriate

R is the standard marking on heavy-duty light-truck tires — the load-range D and E casings fitted to three-quarter-ton and one-ton pickups, and to many all-terrain and mud-terrain patterns in large sizes. A stiff, heavy, deep-treaded casing generates a great deal of heat at speed, and the rating reflects what it was tested to sustain rather than any limitation of the vehicle.

There is a further wrinkle: some heavy light-truck tires carry no speed symbol at all, and older ones may carry a maximum speed printed in words elsewhere on the sidewall instead. An absent symbol is not the same as a low one, and it cannot be inferred.

Its neighbours on the chart

Q is 160 km/h and S is 180. Through this section the series moves in flat 10 km/h steps, so R is not special in its spacing — only in its letter. Below it sit the markings used on winter tires and temporary spares; above it, the ratings for passenger cars.

Reading it correctly

A service description like 120R decodes to a load index of 120 and a speed symbol of R, which is 1,400 kg at up to 170 km/h. Put one through the decoder to see both halves separated, or check construction codes for what the other R in the size actually denotes and what replaces it on non-radial tires.