Vintage Tire Size Conversion: How to Find the Modern Equivalent

Old tyre sizes do not convert to modern ones by any rule. What they convert by is overall diameter: match the height of the original within about three per cent and the speedometer, the gearing and the arch clearance all stay right. This page explains the three sizing systems and gives the computed overall diameter of every modern size likely to fit a 13, 14, 15 or 16 inch rim.

The three systems, and how to tell them apart

Numeric, before 1964. Written with a decimal point, as in 6.70-15: section width in inches, then rim diameter. Nominal aspect ratio about 90 per cent, so the sidewall was nearly as tall as the tyre was wide.

Numeric, 1965 onward. The aspect ratio dropped to about 80 per cent, and the decimal point was dropped to tell the two generations apart — 670-15 rather than 6.70-15. That missing full stop is two tyres more than an inch apart in height.

Alpha-numeric, from around 1968. A letter, two digits and a rim size: G78-15. The two digits are the aspect ratio. The letter is a load and size class, and it is rim-specific — a C78-14 and a C78-15 are not the same width, so no letter-to-width rule works across rim sizes.

Metric, current. 205/75R15 is a 205 mm section width, sidewall height 75 per cent of that, radial, 15 inch rim.

Why this page does not print an old-size-to-new-size lookup

Because the lookups in circulation contradict one another. Across three widely-published charts the same alpha-numeric size gets three different modern equivalents, and one lists an overall diameter for a common modern size more than an inch from what it actually measures. They were copied rather than measured, and the errors travelled.

Arithmetic cannot rescue it either: applying the right aspect ratio means knowing which era a size came from, and the only thing distinguishing them on paper is a decimal point most charts have already lost. A table built on that would look authoritative and be unreliable — the worst combination available.

What to do instead

Work from overall diameter. If the original tyre is still on the car, measure it. If not, multiply the section width by the aspect ratio, double it for the two sidewalls and add the rim diameter. Then find a modern size below within about three per cent — the tolerance vintage tyre specialists work to.

Section width matters second. A modern tyre slightly wider than the original is usually acceptable and often unavoidable, since metric sizes are a coarser grid. Check the wider carcass clears the arch at full lock and full droop, not just standing still.

Overall diameter of modern tyre sizes

Every figure below is calculated from the size itself, so it is exact rather than quoted. Sizes run smallest diameter to largest, which is the order you want when matching a number you already have.

Method and precision

Overall diameter is the rim diameter plus twice the sidewall height, sidewall height being section width times aspect ratio — arithmetic on the size itself at the exact 25.4 mm inch, so these values carry no measurement error. They are NOMINAL: a fitted tyre's true diameter varies with rim width, inflation pressure and manufacturer, typically by a fraction of an inch. The three per cent tolerance is the figure vintage tyre specialists publish for staying close enough to the original. Sources: Published tyre-code conventions: 90 per cent nominal aspect ratio before 1964, 80 per cent from 1965, the decimal point dropped to tell the two apart. Coker Tire size guidance — replace on overall diameter, within about three per cent of the original. Exact unit definitions — no measurement involved.

Before you fit them

Old wheels and modern tyres are not automatically compatible. Check the rim width suits the section width, the load rating meets the vehicle weight, and on pre-war rims whether the wheel was designed for a tube. Judge age by date code, not tread depth — a vintage car covers little distance and the rubber ages regardless.

Tools are in automotive tools, torque equipment and socket sets.