Tire Size Calculator

This tool decodes the cryptic string stamped on your tyre sidewall and turns it into the numbers that actually matter: sidewall height, overall diameter, circumference, and revolutions per mile. The code — something like P225/45R17 or 195/55 R16 — is a compact specification of the tyre's width, profile, construction, and the wheel it fits. Knowing how to read it is genuinely useful whether you are buying replacements in an Indian tyre shop, plus-sizing to a sportier wheel, comparing two quotes, or working out why your speedometer reads differently after fitting a new set.

Decoding the tyre size code

Take 195/55 R16 as the worked example. The 195 is the section width in millimetres — the distance across the widest part of the tyre. The 55 is the aspect ratio: the sidewall height expressed as a percentage of the width, not an absolute measurement. The R means radial construction (the near-universal modern standard), and the 16 is the wheel rim diameter in inches. A leading letter such as P (passenger) or LT (light truck) sometimes appears before the width to indicate the service type. After the size you will often see a load index and a speed rating, e.g. 87V, which describe maximum load and maximum sustained speed.

The sidewall and diameter maths

The key calculation is sidewall height. Because the aspect ratio is a percentage of width: sidewall (mm) = width × (aspect ratio ÷ 100). For 195/55 R16 that is 195 × 0.55 = 107.25 mm per side. Convert to inches by dividing by 25.4 (107.25 ÷ 25.4 ≈ 4.22 in). The overall tyre diameter is the rim plus two sidewalls (top and bottom): diameter = rim + 2 × sidewall. So 16 + (2 × 4.22) ≈ 24.44 in. From the diameter, circumference is π × diameter, and revolutions per mile is 63360 ÷ circumference (since one mile is 63,360 inches). These last two figures are what your speedometer and odometer ultimately depend on.

How tyre size affects your speedometer

Your speedometer is calibrated to the factory tyre's revolutions per mile. Fit a tyre with a larger overall diameter and each revolution covers more ground, so the wheel turns fewer times per mile — the speedometer then reads slower than your true speed, and the odometer undercounts distance. A smaller diameter does the reverse: the speedometer reads faster than reality. As a rule of thumb, every 3% change in overall diameter shifts the reading by roughly 3% (about 3 km/h at 100 km/h). This is why a like-for-like overall diameter matters when you change sizes, and why plus-sizing deliberately keeps diameter constant.

Worked example: comparing two sizes

Suppose your car came with 195/55 R16 (≈ 24.44 in overall) and you are tempted by 205/45 R17. The new sidewall is 205 × 0.45 = 92.25 mm ≈ 3.63 in, giving an overall diameter of 17 + (2 × 3.63) ≈ 24.26 in. The two are within about 0.8% of each other — close enough that the speedometer error is negligible and the wider, lower-profile tyre would mostly change feel rather than accuracy. Run both through the calculator and the revolutions-per-mile figures confirm it at a glance.

Tips

• Your factory-recommended size is on a sticker in the driver's door jamb, inside the fuel-filler flap, or in the owner's manual — trust it over the size currently fitted, which may have been changed.
• When comparing options, aim to keep overall diameter within roughly 3% of stock to preserve speedometer accuracy and gearing.
• Wider tyres add grip and a planted feel but typically cost more and slightly reduce fuel economy; lower profiles sharpen steering but transmit more road shock — a real consideration on rough Indian roads.

Common mistakes

• Reading the aspect ratio as millimetres. The 55 in 195/55 is a percentage, not a 55 mm sidewall.
• Adding only one sidewall. Overall diameter needs two sidewalls plus the rim, not one.
• Ignoring load index and speed rating. A correctly sized tyre with too low a load or speed rating is unsafe — match or exceed the originals.
• Going too wide for the wheel well. Wider tyres can rub on suspension at full steering lock or compression; check fitment with a tyre shop before committing.