Tire Size Decoder
Decode a metric tire size like 225/45R17.
Overview
The Tire Size Decoder breaks a metric tire size like 225/45R17 into its component parts: section width in millimetres, aspect ratio, construction type and rim diameter in inches. The tool also returns the overall tire diameter, the calculated sidewall height and the rolling circumference, so you can compare two sizes for fit or speedometer drift.
It is built for car owners cross-shopping replacement tires, off-road enthusiasts up-sizing for ground clearance and shop staff explaining the markings to customers. Long-tail queries it covers include "what does 225 45 R17 mean", "tire size aspect ratio explained", "calculate tire diameter from size" and "tire speed rating letter chart".
How it works
The standard tire-size format is <width>/<aspect>R<rim> followed optionally by load index and speed rating. Section width is the tire's width in millimetres from sidewall to sidewall. Aspect ratio is the sidewall height as a percentage of the section width. "R" indicates radial construction. Rim diameter is the wheel size in inches.
From those three numbers the decoder computes sidewall height in mm (width × aspect / 100), overall diameter in mm (rim × 25.4 + 2 × sidewall) and rolling circumference (π × overall diameter). Speed rating letters map to a maximum speed in km/h and mph using the international tire industry table.
Examples
225/45R17 → width 225 mm, sidewall 101 mm, rim 17", overall 632 mm (24.9")
205/55R16 → width 205 mm, sidewall 113 mm, rim 16", overall 632 mm (24.9")
245/35R19 → width 245 mm, sidewall 86 mm, rim 19", overall 654 mm (25.7")
LT265/70R17 → light-truck size, width 265 mm, sidewall 186 mm, overall 803 mm
FAQ
What does the speed rating letter mean?
The letter at the end (e.g. H, V, W, Y) indicates the maximum sustained speed the tire is rated for. H is 210 km/h, V is 240 km/h, W is 270 km/h, Y is 300 km/h. Most modern street tires are H or higher.
Why do two different sizes have the same diameter?
Manufacturers sometimes offer "plus-sizing" packages where a smaller sidewall is paired with a larger rim to keep overall diameter the same — preserving speedometer accuracy while changing the look. 205/55R16 and 225/45R17 are roughly diameter-equivalent.
Will a wider tire fit my wheel?
It depends on rim width tolerance, fender clearance and suspension geometry. Each rim has a recommended tire width range; outside that range you risk poor handling or rubbing.
What's the difference between P, LT and ST?
P is for passenger cars (most common), LT is for light trucks (heavier load), and ST is for trailers (specific to trailer use and not allowed on a vehicle). The letter prefix is optional on P-metric sizes.
Does the load index matter?
Yes. The number before the speed rating maps to a maximum load in kg per tire. Replacing a higher-rated tire with a lower-rated one is unsafe even if the diameter matches.