I've got these two tire sizes: 27 x 8.5 R12 16 PR, H42 x 15.0 R21 26 PR. Can someone tell me what each of these numbers and letters mean?

Also, if the diameter of the rim of the wheel is not included there, is there a way to estimate it from those numbers?

  • $\begingroup$ Related $\endgroup$
    – Pondlife
    Jul 25, 2021 at 19:21
  • $\begingroup$ I've always wondered why this is called "landing gear", as the first use is for takeoff, and for some aircraft they are even dropped after takeoff. $\endgroup$
    – mins
    Jul 25, 2021 at 19:44
  • 2
    $\begingroup$ @mins That might be an interesting question for English Language & Usage Stack Exchange. In German it its Fahrwerk (literally driving gear), so the term is unrelated to landing. On the other hand, the French train d'atterrissage is also related to landing... $\endgroup$
    – Bianfable
    Jul 26, 2021 at 13:32

1 Answer 1


$ \begin{array}{|c|c|c|c|c|c|} \hline \textsf {(H) M x N} & \textsf{(R)} & \textsf{D} & \textsf{(PR)} \\ \hline \textsf {27 x 8.5} & \textsf {R} & \textsf {12} & \textsf {16} \; \textsf {PR} \\ \hline \textsf {H42 x 15.0} & \textsf {R} & \textsf {21} & \textsf {26} \; \textsf {PR}\\ \hline \end{array} $

  • M: Nominal overall diameter
  • H: The same with higher percent of deflection
  • N: Nominal section width
  • R: Radial type
  • D: Rim size
  • PR: Ply rating code (static load that can be applied to the tire).

All dimensions in inches

Reference: Michelin and Dunlop.
Bonus: Tire manufacturing process


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