What is the sidewall height of 225/45R17?

A bigger tire with less sidewall than a smaller one

195/65R15 is a considerably smaller tire in every visible respect, and its sidewall is 4.99 in against this one's 3.99 in — a full inch more. The aspect ratio is a percentage of the section width, so 65% of a narrow 195 outruns 45% of a wider 225 comfortably. Comparing aspect ratios across different widths tells you nothing on its own, which is the single most useful thing to know about the middle number.

The same 225, up and down the ladder

Hold the width at 225 mm and vary the aspect ratio and rim, and the sidewall moves across a wide range:

• 225/40R18 — 3.54 in
• 225/45R17 — 3.99 in
• 225/50R17 — 4.43 in
• 225/55R17 — 4.87 in
• 225/60R16 — 5.31 in
• 225/65R17 — 5.76 in
• 225/75R15 — 6.64 in

Nearly twice the sidewall from bottom to top of that list, with the width identical throughout. Each five points of aspect ratio is worth a little under half an inch on a 225 mm section — a fixed amount, because the base never changes.

What it costs in overall height

Because the sidewall appears at both ends of the tire, 225/45R17 comes out at 24.97 in overall while 225/50R17 — one step up the list — reaches 25.86 in. Under half an inch of extra sidewall produced almost nine-tenths of an inch of extra tire, and 3.55% of speedometer difference along with it. Sidewall is the lever with the most mechanical advantage in the whole code.

101.3 mm, against 101.6 for four inches

Close enough that "about four inches of sidewall" is a fair description and 0.3 mm out. Whether that matters depends on what the figure is for: as a rough sense of how much casing sits between rim and road, four inches is fine; as an input to a diameter calculation it is not, because the sidewall is counted at both ends and small errors double.

Compare this size against a taller-sidewall alternative, or see aspect ratio for the full relationship.