📋 Enter Your Tire Size Code (e.g. 225/60R16)
mm
First number on sidewall
Enter width (e.g. 225).
%
Middle number (sidewall %)
Enter aspect ratio.
in
Last number (wheel inches)
Enter rim diameter.
📋 Original Tire (Stock)
mm
Required.Required.
in
Required.New Tire
mm
Required.Required.
in
Required.📋 Stock Size
mm
Required.Required.
in
Required.New Size
mm
Required.Required.
in
Required.mph
Speedometer reading to check (e.g. 60 mph)
Enter indicated speed.
📋 Decode Load Index & Speed Rating
Number after tire size (e.g. 225/60R16 91H)
Enter load index (70-126).
Letter after load index (e.g. 91H)
Select speed rating.
📋 Find Tire Manufacture Date from DOT Code
Week number (01-52) from last 4 DOT digits
Enter week (1-52).
Year digits (e.g. 19 = 2019, 23 = 2023)
Enter year digits.
ℹ️ Find the 4-digit DOT date code on your tire’s inner sidewall. Example: DOT code ending in 2419 = week 24 of 2019.
Result
—
⚠️ Disclaimer: Tire size calculations use standard ISO metric formulas. Actual mounted dimensions can vary 1-2% by manufacturer, load, and air pressure. Always verify fitment with a professional installer before purchasing. Speed and load ratings must meet or exceed OEM specifications for safety.
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Sources & Methodology
All tire size formulas use ISO 4000-1 and TRA standards. Load index and speed rating tables from TRA Yearbook 2026. DOT age calculations per NHTSA guidelines.
TRA — Tire & Rim Association Yearbook 2026
The Tire and Rim Association (TRA) publishes the authoritative US standard for tire dimensions, load tables, and speed rating specifications. All load index values (lbs per tire), speed rating speeds, and recommended rim width ranges used in this calculator are sourced from TRA Yearbook data.
ISO 4000-1 — Passenger Car Tyres and Rims Standard
International Organization for Standardization tire sizing methodology: Overall diameter = 2 x (Width mm x Aspect ratio / 100) + Rim diameter in mm (rim diameter x 25.4). Circumference = pi x Diameter. Revolutions per mile = 63,360 / circumference in inches. All formulas implemented exactly per ISO 4000-1.
NHTSA — Tire Safety Guidelines 2026
National Highway Traffic Safety Administration tire safety guidance including DOT date code interpretation, tire age recommendations (replace at 6 years, mandatory at 10 years), and AWD tire diameter mismatch warnings used in the comparison and AWD safety sections.
Methodology:
Sidewall height (mm) = Width × (Aspect / 100)
Overall diameter (mm) = (Rim diameter × 25.4) + (2 × Sidewall height)
Circumference (in) = π × (Overall diameter / 25.4)
Revolutions/mile = 63,360 / Circumference (in)
Speedometer error % = (New diameter − Original diameter) / Original diameter × 100
Actual speed = Indicated speed × (New diameter / Original diameter)
How to Read Tire Size Numbers — Complete Sidewall Decoder
Every tire has a standardized size code molded into the sidewall that tells you everything about its dimensions. Understanding how to read tire size numbers lets you verify fitment, find alternatives, and understand how a size change will affect your vehicle's speedometer, odometer, and handling.
Example: P225/60R16 95H
P = P-metric passenger tire (LT = light truck)
225 = section width in millimeters (widest point, sidewall to sidewall)
60 = aspect ratio — sidewall height is 60% of 225mm = 135mm sidewall
R = radial construction (B = bias-belt, D = diagonal/bias)
16 = rim diameter in inches (the wheel size)
95 = load index — each tire supports up to 1,521 lbs
H = speed rating — tire is rated for sustained speeds up to 130 mph
Overall diameter: (16 × 25.4) + (2 × 135) = 406.4 + 270 = 676.4mm (26.63 in)
225 = section width in millimeters (widest point, sidewall to sidewall)
60 = aspect ratio — sidewall height is 60% of 225mm = 135mm sidewall
R = radial construction (B = bias-belt, D = diagonal/bias)
16 = rim diameter in inches (the wheel size)
95 = load index — each tire supports up to 1,521 lbs
H = speed rating — tire is rated for sustained speeds up to 130 mph
Overall diameter: (16 × 25.4) + (2 × 135) = 406.4 + 270 = 676.4mm (26.63 in)
Tire Load Index Chart — What Each Number Means
The load index is a standardized code that tells you the maximum weight each individual tire can support at its maximum inflation pressure. Never install tires with a lower load index than your vehicle manufacturer specifies — doing so is a safety issue, not just a recommendation. The load index is particularly important when switching to wider tires or upgrading to lower-profile performance tires.
| Load Index | Max Load per Tire | Load Index | Max Load per Tire |
|---|---|---|---|
| 80 | 992 lbs (450 kg) | 95 | 1,521 lbs (690 kg) |
| 82 | 1,047 lbs (475 kg) | 97 | 1,609 lbs (730 kg) |
| 84 | 1,102 lbs (500 kg) | 100 | 1,764 lbs (800 kg) |
| 87 | 1,201 lbs (545 kg) | 103 | 1,929 lbs (875 kg) |
| 89 | 1,279 lbs (580 kg) | 106 | 2,094 lbs (950 kg) |
| 91 | 1,356 lbs (615 kg) | 109 | 2,271 lbs (1,030 kg) |
| 93 | 1,433 lbs (650 kg) | 112 | 2,469 lbs (1,120 kg) |
Tire Speed Rating Chart — From S to Y
| Rating | Max Speed | Typical Vehicle |
|---|---|---|
| S | 112 mph (180 km/h) | Family sedans, basic passenger cars |
| T | 118 mph (190 km/h) | Minivans, light trucks, economy cars |
| H | 130 mph (210 km/h) | Sport sedans, CUVs, popular OEM choice |
| V | 149 mph (240 km/h) | Sports cars, performance sedans |
| W | 168 mph (270 km/h) | Exotic and high-performance vehicles |
| Y | 186 mph (300 km/h) | Ultra-high-performance supercars |
Speedometer Error from Tire Size Changes
Your speedometer reads speed by counting wheel revolutions per unit of time, calibrated to your original tire's circumference. When you change tire size, the circumference changes, and so does the speed reading accuracy. A larger tire makes the speedometer read low (you're going faster than it shows). A smaller tire makes it read high (you're going slower than it shows). This also affects your odometer — over thousands of miles, odometer error compounds into measurable inaccuracies in your service intervals and warranty mileage tracking.
| Diameter Change | Speedo at 60 mph indicated | Effect | Safe? |
|---|---|---|---|
| +1% larger | Actually 60.6 mph | Reading 1% too low | Yes — within tolerance |
| +3% larger | Actually 61.8 mph | Reading 3% too low | Generally acceptable |
| +5% larger | Actually 63.0 mph | Reading 5% too low | Borderline — recalibration advised |
| -3% smaller | Actually 58.2 mph | Reading 3% too high | Generally acceptable |
| +10% larger | Actually 66 mph | Major error | No — affects ABS, traction control |
Plus Sizing — Going to Larger Wheels
Plus sizing means increasing wheel diameter while proportionally reducing tire aspect ratio to maintain the same overall tire diameter. The goal is to stay within 3% of the original tire's diameter to preserve speedometer accuracy and ABS calibration. Benefits include improved handling response, better appearance, and the ability to fit larger brake calipers. The trade-off is a firmer ride due to shorter sidewall and less ability to absorb pothole impacts.
| Starting Size | Overall Diameter | Plus 1 | Plus 2 |
|---|---|---|---|
| 205/55R16 | 24.88 in | 215/45R17 (24.62") | 225/40R18 (24.98") |
| 225/60R16 | 26.63 in | 235/50R17 (26.30") | 245/45R18 (26.67") |
| 235/65R17 | 28.04 in | 245/55R18 (27.98") | 255/50R19 (28.03") |
| 265/70R17 | 30.61 in | 275/60R18 (30.02") | 285/55R19 (30.27") |
⚠️ AWD Safety Warning: All-wheel drive vehicles require all four tires to be within 0.25 inches (6.35mm) of the same overall diameter. Running even one tire slightly different in size forces the AWD coupling to work continuously, generating heat and causing premature wear that can cost $1,000 to $5,000 to repair. Always replace AWD tires in sets of four, and verify sizes match exactly when buying replacement tires.
How to Find and Read the DOT Code for Tire Age
Every tire sold in the US has a DOT (Department of Transportation) code on the sidewall. The complete code begins with "DOT" and ends with a 4-digit date code: the first two digits are the week of manufacture (01-52) and the last two are the year. Example: a code ending in "4222" means the tire was made in the 42nd week of 2022 (October 2022). Tires over 6 years old should be inspected by a professional regardless of tread depth, as rubber degrades from the inside out. Tires over 10 years old should always be replaced regardless of appearance — internal structure degradation creates blowout risk that is invisible externally.
Frequently Asked Questions
225 = section width in mm (widest point). 60 = aspect ratio — sidewall is 60% of 225mm = 135mm tall. R = radial construction. 16 = rim diameter in inches. The code after the size (e.g., 95H) means load index 95 (1,521 lbs per tire) and speed rating H (130 mph max). Overall diameter = (16 x 25.4) + (2 x 135) = 676.4mm = 26.63 inches. Enter any size into Mode 1 (Decode) for instant full breakdown.
Speedometer accuracy depends on tire circumference — how far the tire travels per revolution. A larger tire = more distance per revolution = speedometer reads low (you're going faster than shown). Formula: Actual speed = Indicated speed x (New diameter / Original diameter). A 3% diameter increase means your 60 mph indicated is actually 61.8 mph. Use Mode 3 (Speedo Error) to calculate exactly for any two tire sizes at any speed.
Load index is the standardized code (70-126) indicating max weight per tire. Never go below your vehicle's OEM load index — it's a safety specification, not a preference. Common values: 87 = 1,201 lbs, 91 = 1,356 lbs, 97 = 1,609 lbs, 100 = 1,764 lbs. Load index 91H means each tire carries up to 1,356 lbs at up to 130 mph. Enter your load index into Mode 4 (Load & Speed Rating) to see the exact weight and speed limits.
Plus sizing = increasing wheel diameter (+1" or +2") while reducing tire aspect ratio to maintain the same overall tire diameter (within 3%). Example: 205/55R16 (24.88") goes Plus 1 to 215/45R17 (24.62"). Benefits: sharper handling, larger brake clearance, aggressive look. Trade-off: shorter sidewall = firmer ride, more pothole risk. Stay within 3% of original diameter to preserve speedometer accuracy and ABS calibration.
Very dangerous for the AWD system. Maximum allowable diameter difference between all four tires is 0.25 inches (6.35mm) — about 1% or less. Even one tire with different tread wear forces the AWD coupling to compensate continuously, generating heat and accelerating wear. AWD drivetrain repair costs $1,000 to $5,000+. Always replace AWD tires in complete sets of four, and match sizes exactly. Use Mode 2 (Compare) to check if your tires are within safe tolerance.
Find the DOT code on your tire's inner sidewall — it starts with "DOT" and ends with 4 digits. The first two are the week (01-52) and last two are the year. DOT code "2419" = week 24 of 2019 = June 2019. Replace tires over 6 years old regardless of tread, and always replace tires over 10 years old — the rubber degrades internally even if it looks fine. Enter your DOT digits into Mode 5 (Tire Age) above for instant age calculation.
Tires are safely interchangeable when the overall diameter is within 3% of your OEM size. For 225/60R16 (26.63"), acceptable range is 25.83" to 27.43". Common alternatives: 215/65R16 (26.54"), 235/55R16 (26.16"), 225/65R16 (27.52"). Always match or exceed the original load index and speed rating. Enter your original and candidate size into Mode 2 (Compare) to check diameter difference percentage and confirm safety.
Aspect ratio is the sidewall height expressed as a percentage of the tire's section width. A 225/60R16 has an aspect ratio of 60, meaning the sidewall is 60% of 225mm = 135mm tall. Lower aspect ratio (35, 40, 45) = shorter, stiffer sidewall = better handling, harsher ride, more pothole damage risk. Higher aspect ratio (65, 70, 75) = taller sidewall = softer ride, more impact absorption, better for rough roads. Most passenger car OEM tires are 50-65 for a comfort/handling balance.
Revolutions per mile = 63,360 in/mile / (pi x diameter in inches). For a 26.63" diameter tire: 63,360 / (3.14159 x 26.63) = 63,360 / 83.65 = 757.4 rev/mile. This value controls speedometer, odometer, ABS, and traction control calibration. A 1% diameter change = 1% change in rev/mile = 1% speedometer error. Enter any tire size into Mode 1 (Decode) to see exact revolutions per mile instantly.
P-metric (starting with P, e.g., P225/60R16) = designed for passenger vehicles. LT-metric (starting with LT) = designed for light trucks and SUVs requiring higher load capacity. An LT tire in the same metric size as a P tire will be heavier, stiffer, and has higher load capacity. Some trucks require LT tires — substituting P-metric may void warranty coverage. If your door jamb sticker shows an LT size, do not substitute P-metric without checking the vehicle manufacturer's fitment guide.
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