TireFitLab

Size validation

Size validation

Decode markings, compare sizes, and understand diameter, load and speed changes.

FAQ

Size validation
Decode markings, compare sizes, and understand diameter, load and speed changes.
What should I verify before using this information?
Use TireFitLab values as a sizing reference, then verify the vehicle handbook, tire placard, rim compatibility, load rating, and physical clearance before fitting.
How to read a tire size How to read a tire size: decode every number and letter in P205/55R16 91V — width, aspect ratio, rim diameter, load index, speed rating. Tire type guide — P, LT, metric, radial & flotation What do the letters in a tire size mean? P, LT, metric, radial (R) and flotation tire types explained. Tyre sidewall markings decoder Complete guide to every marking on a tyre sidewall: size string, construction code, load index, speed rating, DOT date, M+S, 3PMSF snowflake, UTQG, max pressure, country of manufacture, and brand-specific symbols. Tire load index chart Use the tire load index chart to convert ratings 60–130 into kg and lb, then verify the vehicle placard and axle-load requirements. Tire speed rating chart Use the tire speed rating chart to convert symbols from L to Y/ZR into km/h and mph for replacement tyre checks. Tire section width guide What does the first number in a tire size mean? Tire section widths from 165 to 305 mm explained with compatible sizes and rim widths. Tire aspect ratio guide What does the aspect ratio (series) number mean in a tire size? 35, 40, 45, 50, 55, 60, 65, 70 explained with sidewall heights and popular sizes. Plus sizing guide What is plus sizing? How to go from plus 0 to plus one or plus two with a larger rim and lower-profile tire. Plus-sizing fitment study Original TireFitLab study of plus/minus tire-sizing candidates computed from the same engine as the live calculator. Speedometer error guide Why changing tire diameter causes speedometer error, how to calculate it, EU/US legal limits, and how to check if your new tires will pass. Tyre size conversion guide How to convert tyre sizes between metric (225/45R18), P-metric (P225/60R16), LT (LT265/70R17), flotation (31×10.50R15LT), and classic inch-based (6.00-16) formats. Includes conversion formulas, worked examples, and when each format is used. Tyre upsizing and downsizing guide What happens when you fit wider tyres, lower-profile tyres, or a larger rim: effects on wet/dry grip, ride comfort, fuel consumption, speedometer accuracy, wheel arch clearance, and tyre load capacity. Low-profile tyres: handling, comfort, damage risk, and aspect ratios explained Low-profile tyre guide: what aspect ratio means and how it defines a low-profile tyre, the handling advantages of a shorter sidewall (less flex, faster steering response, wider contact patch), the ride comfort penalties, the increased vulnerability to pothole damage and kerb damage, TPMS importance on low-profile fitments, and winter suitability of low-profile tyres. Tyre load capacity guide How tyre load capacity works: the load index table, how inflation pressure affects load carrying ability, single vs dual fitment ratings, and why overloading is dangerous. Tyre speed rating and use guide What happens when you exceed a tyre speed rating: heat buildup, structural failure risk, legal status in EU/UK/USA, how ratings are tested, and which rating your vehicle requires. What happens when you exceed a tyre speed rating Guide to tyre speed rating exceedance: what the speed rating means (the maximum sustained speed the tyre can handle at its rated load without heat-related structural failure), how the rating is tested (ISO 28580 or FMVSS 119 sustained test at rated speed for 10 minutes), what happens when the speed rating is exceeded (heat buildup accelerates with speed squared — at 20% above the rated speed the tyre generates approximately 44% more heat per unit time), whether a brief exceedance is dangerous (transient exceedance for overtaking is very different from sustained exceedance at motorway speed), the legal position in Germany (§36 StVZO — the tyre must be approved for the vehicle's design maximum speed), UK (Road Traffic Act 1988 — unsuitable tyre), and how to read the speed symbol on the sidewall. Tyre ply rating and load range: what LT tyre load ranges mean and how to choose the right one Guide to tyre ply rating and load range: what ply rating originally meant (number of cotton carcass plies) and why modern tyres use load range letters instead (C/D/E/F/G), how each load range corresponds to a maximum inflation pressure (C = 50 psi / 3.45 bar, D = 65 psi, E = 80 psi / 5.5 bar, F = 95 psi, G = 110 psi), how to calculate maximum load capacity for a given load range and size, LT tyre vs P-metric tyre load capacity comparison (LT carries more at same size — LT225/75R16 E vs P225/75R16 example), when to choose a higher load range (towing, payload, frequent full-load use), the trade-off between load range and ride quality (stiffer sidewall = harsher ride on normal roads), and how to read the load range from the tyre sidewall. How tyre aspect ratio affects handling, ride comfort, and noise Guide to tyre aspect ratio and its effects: what aspect ratio means (sidewall height divided by section width, expressed as a percentage — 205/55R16 has a 55% aspect ratio, meaning the sidewall is 55% of 205 mm = 113 mm tall), how lower aspect ratio (35–45 series) increases tyre lateral stiffness, sharpens steering response, and reduces cornering flex, why lower aspect ratio tyres transmit more road shock to the cabin (less sidewall to absorb bumps), how road noise increases with lower profile (less rubber to damp tread noise), the trade-off between handling performance and daily comfort (55–65 series is the sweet spot for most road use), how aspect ratio interacts with rim diameter when plus-sizing, and why OEM aspect ratios are chosen for specific load/speed/comfort targets. How tyre section width affects grip, fuel economy, noise, and aquaplaning Guide to tyre section width effects: how the section width number (185, 205, 225, 255, 275 mm) determines contact patch width and area, why wider tyres develop more dry cornering grip (larger lateral contact area = higher lateral force at same slip angle), why wider tyres aquaplane earlier than narrower ones (snow-plow vs knife-through-water — a narrow tyre parts water with higher contact pressure, a wide tyre tries to push water sideways), how rolling resistance increases with section width (more rubber mass, larger contact patch flexing per revolution), why road noise increases with section width (more tread blocks in contact, more air column noise), when to choose a narrower section width than OEM (fuel economy priority, wet/snow performance priority, lower tyre cost), and the safe range of section widths for a given rim width (EU ETRTO guidelines).

More tools

Seasonal check

Planning a long summer drive?

Use the budget and running-cost tools before a trip, especially if the current tyres are worn or the replacement size changes diameter.

Estimate tyre budget
Last reviewed: 2026-06-28
What changed
  • Added pillar pages for the existing guide bundle taxonomy.