Metric Tire Size Guide
How to read every number in a metric tire code, convert to inches step-by-step, understand aspect ratios, load indexes, speed ratings, and find the metric equivalent of any inch-based tire size.
What does 225/55R17 actually mean?
225 = section width in mm. 55 = sidewall height as % of width. R = radial. 17 = wheel diameter in inches. The calculator converts this into total diameter, sidewall height, and circumference instantly.
Is a 265/70R17 the same as a 33-inch tire?
Close but not exact — a 265/70R17 measures about 31.6 inches overall. A true 33-inch equivalent in metric is closer to 285/75R16 or 275/70R18.
P-metric vs Euro-metric — does the prefix matter?
Yes. A P225/65R17 tire has slightly lower load capacity than a 225/65R17 (Euro-metric) of the same size. Use whichever the door placard specifies for your vehicle.
How much does aspect ratio change diameter?
A lot — going from 55 to 65 aspect ratio on a 225mm wide tire adds about 22.5mm to each sidewall, or ~45mm (1.8 inches) to overall diameter.
Can I plus-size and keep diameter the same?
Yes. Increase wheel diameter by 1 inch, then lower the aspect ratio to compensate. The metric tire calculator shows the exact diameter before and after.
Speedometer off after a size change?
A taller tire makes the speedometer read slow. Even a 2% diameter increase causes a real difference at highway speeds. Always check the diameter delta first.
Skip the manual math
Metric tire size calculator
Enter any metric tire size and instantly see diameter, width, sidewall height, circumference, and speedometer comparison.
On this page
Metric tire code anatomy
Every number and letter in a metric tire code has a precise meaning. Understanding each position is the foundation of reading, comparing, and converting tire sizes correctly.
Example tire code:
1PService type prefix
Identifies the tire's construction standard. P = Passenger car (P-metric). No letter = Euro-metric. LT = Light Truck. ST = Special Trailer.
Example: P225/65R17 or 225/65R17 (Euro-metric)
2225Section width (mm)
The tire's width from sidewall to sidewall at the widest point, measured in millimeters. This is NOT the tread contact width — it is the full cross-section width.
Example: 225 mm ≈ 8.86 inches
365Aspect ratio
Sidewall height expressed as a percentage of the section width. 65 means the sidewall is 65% of 225mm = 146.25mm tall. This is the most misunderstood number in tire sizing.
Example: 225 × 0.65 = 146.25mm sidewall height
4RConstruction type
R = Radial (standard on all modern tires). B = Bias-ply (rare, vintage and specialty). D = Diagonal bias. Virtually every modern passenger and light truck tire is radial.
Example: All modern tires: R
517Rim diameter (inches)
The wheel diameter the tire is designed to fit — measured in inches. This must match your wheel exactly. Metric tire sizes mix mm and inches; rim diameter is always in inches.
Example: 17" wheel = 17 × 25.4 = 431.8mm
691Load index
A numerical code for max load capacity per tire. 91 = 1,356 lbs (615 kg). Higher numbers = more capacity. Always match or exceed the OEM load index on replacement tires.
Example: 91 → 615 kg per tire → 2,460 kg total (4 tires)
7HSpeed rating
Maximum safe operating speed when the tire is at full load and correct pressure. H = 130 mph (210 km/h). Speed ratings are not top speed — they are safety limits.
Example: H = 130 mph / V = 149 mph / W = 168 mph
Step-by-step: converting metric tire size to inches
The metric system mixes millimeters for width, a unitless percentage for aspect ratio, and inches for rim diameter. Here is how to work through the full conversion manually — using 225/65R17 as the example.
1Convert section width to inches
There are exactly 25.4 mm per inch.
2Calculate single sidewall height in mm
The aspect ratio is a percentage of the width, not an absolute measurement.
3Convert sidewall height to inches
4Calculate total tire diameter
Multiply sidewall by 2 because there is sidewall above AND below the rim.
5Calculate tire circumference
Circumference determines how far the vehicle travels per wheel revolution — directly affecting speedometer calibration.
Result: 225/65R17 = approximately 28.5 inches overall diameter, 8.86 inches wide, 5.76 inch sidewall.
Use the metric tire calculator to get this result for any size instantly — including circumference, revolutions per mile, and comparison against a second size.
P-metric vs Euro-metric vs LT-metric vs ST
The prefix (or its absence) before the tire size tells you which load standard was applied when the tire was rated. This changes how much the tire can legally carry — even if the dimensions are identical.
P-metric
P225/65R17 91H
When to use: Use when the door placard specifies P-metric, or for typical daily-driver passenger car replacements.
Euro-metric
225/65R17 102H
When to use: Use when the placard does not show a P prefix, or for European vehicle replacements. Can replace P-metric of the same size on most vehicles.
LT-metric
LT265/70R17 121/118S
When to use: Required on trucks that tow or haul near their limits. Do not swap from LT to P-metric on a heavy-use truck without verifying load capacity.
ST-metric
ST205/75R15 101L
When to use: Trailers only. Using ST tires on a vehicle axle is unsafe.
Never replace LT-metric tires with P-metric or Euro-metric tires on a truck that regularly tows or hauls near its rated limits. The load capacity difference is significant — LT-metric tires run at higher inflation and carry far more weight per tire.
Aspect ratio guide
The aspect ratio is the most misunderstood part of a metric tire code. It is not a measurement — it is a ratio. A tire with aspect ratio 70 has a sidewall height equal to 70% of the tire's section width. Change the width and the sidewall height changes too, even with the same aspect ratio number.
| Aspect Ratio | Sidewall Profile | Ride Quality | Handling | Common On | Off-Road Notes |
|---|---|---|---|---|---|
| 80–85 | Very tall | Excellent cushioning | Soft, vague | Heavy trucks, light commercial, older economy cars | Good for airing down |
| 70–75 | Tall | Good cushioning | Comfortable daily driver | Trucks, SUVs, CUVs, minivans | Solid all-round off-road option |
| 60–65 | Medium | Balanced | Good stability at speed | Most mainstream sedans, family SUVs, hatchbacks | Acceptable for mild off-road |
| 50–55 | Low | Firmer | Responsive, sporty | Sport sedans, entry sports cars, street trucks | Not suitable |
| 35–45 | Very low | Harsh on rough roads | Very responsive, precise | Sports cars, performance vehicles, large-wheel packages | No |
Sidewall height formula: Sidewall (mm) = Width (mm) × (Aspect Ratio ÷ 100). A 265/70R17 sidewall = 265 × 0.70 = 185.5mm = 7.3". A 265/55R17 sidewall = 265 × 0.55 = 145.75mm = 5.74". Going from ratio 70 to 55 on a 265mm tire reduces sidewall by 39.75mm (1.57") per side — that is 3.13" off total diameter.
Tire load index chart (key range)
The load index is a two or three-digit code on the sidewall that encodes max single-tire load capacity. Always match or exceed the OEM load index when selecting replacement tires. Never go below the placard specification.
| Load Index | Max Load (lbs) | Max Load (kg) | Typical Vehicle |
|---|---|---|---|
| 71 | 761 lbs | 345 kg | Small economy car |
| 75 | 853 lbs | 387 kg | Compact car |
| 80 | 992 lbs | 450 kg | Mid-size sedan |
| 85 | 1135 lbs | 515 kg | Full-size sedan |
| 88 | 1235 lbs | 560 kg | SUV / large sedan |
| 91 | 1356 lbs | 615 kg | Common SUV / CUV |
| 94 | 1477 lbs | 670 kg | Light truck / larger SUV |
| 97 | 1609 lbs | 730 kg | Half-ton truck (lighter loads) |
| 100 | 1764 lbs | 800 kg | Common truck / larger CUV |
| 104 | 1984 lbs | 900 kg | Half-ton truck |
| 110 | 2337 lbs | 1060 kg | Heavy half-ton / 3/4-ton truck |
| 118 | 2910 lbs | 1320 kg | LT truck single wheel |
| 121 | 3197 lbs | 1450 kg | LT truck dual rear wheel |
Never install tires with a load index lower than the manufacturer's minimum requirement. The door-jamb placard shows the minimum load index for your vehicle's weight class. This is not optional — it is a safety specification.
Tire speed rating chart
Speed rating is the letter after the load index number (e.g., the "H" in 91H). It defines the maximum safe operating speed for the tire under full load and correct inflation. It is a construction and heat-management specification — not just a top speed advisory.
| Speed Rating | Max Speed (mph) | Max Speed (km/h) | Typical Use |
|---|---|---|---|
| Q | 100 mph | 160 km/h | Studded winter tires, some light truck tires |
| S | 112 mph | 180 km/h | Passenger cars, light trucks, standard highway tires |
| T | 118 mph | 190 km/h | Family sedans, vans, some light trucks |
| H | 130 mph | 210 km/h | Sport sedans, most passenger tires |
| V | 149 mph | 240 km/h | Sport cars, performance sedans |
| W | 168 mph | 270 km/h | High-performance and sports cars |
| Y | 186 mph | 300 km/h | Exotic and ultra-performance vehicles |
| (Y) | 186+ mph | 300+ km/h | Racing-derived tires, no stated limit |
H before V — the odd exception:The H speed rating (130 mph) falls between T (118 mph) and V (149 mph) in the speed sequence, but historically it was introduced before the T and V ratings were standardized. This is why H is "out of alphabetical order" — it is not a mistake.
Metric to inch equivalents table
A quick-reference table for the most common metric ↔ inch tire conversions — especially for the 30–35 inch off-road and truck range where both naming conventions are used interchangeably.
| Metric Size | Approx. Inch Notation | Total Diameter | Common Use |
|---|---|---|---|
| 235/75R15 | 29 × 9.3 × 15 | 28.9" | Older SUVs, Jeep Cherokee stock |
| 245/75R16 | 30.5 × 9.6 × 16 | 30.5" | Tacoma, older Jeep, Ranger stock |
| 265/70R17 | 31.6 × 10.4 × 17 | 31.6" | Common F-150, Silverado base size |
| 275/65R18 | 32.1 × 10.8 × 18 | 32.1" | F-150, Silverado 18" packages |
| 285/70R17 | 32.7 × 11.2 × 17 | 32.7" | Close to 33" — RAM Rebel adjacent |
| LT275/70R18 | 33.2 × 10.8 × 18 | 33.2" | RAM Rebel / Warlock OEM, near-33" |
| 285/75R16 | 32.8 × 11.2 × 16 | 32.8" | Common Jeep / truck 33" territory |
| LT295/70R18 | 34.3 × 11.6 × 18 | 34.3" | Between 33" and 35" on 18" wheels |
| LT315/70R17 | 34.4 × 12.4 × 17 | 34.4" | Near 35" on 17" wheels |
| 35×12.5R17LT | 35 × 12.5 × 17 | 35.0" | Classic off-road 35" flotation size |
| 305/65R18 | 33.6 × 12.0 × 18 | 33.6" | Wide near-34" for 18" wheels |
| LT315/75R16 | 35 × 12.4 × 16 | 35.0" | True metric 35" on 16" wheels |
All diameters are calculated values. Actual mounted size varies slightly by brand, mold, and inflation pressure. Use the metric tire calculator to verify the exact diameter for any specific size before purchasing.
Plus sizing with metric tires
Plus sizing means going up in wheel diameter while lowering the aspect ratio to maintain near-stock overall tire diameter. The speedometer stays accurate, brake clearance increases, and the wheel-and-tire package looks different without affecting gearing or ADAS calibration.
| Starting Size | Plus One (+1 inch wheel) | Plus Two (+2 inch wheel) | Notes |
|---|---|---|---|
| 205/65R15 (24.5") | 215/55R16 (24.3") | 215/45R17 (24.0") | Classic economy/small sedan progression. Diameter stays tightly matched. |
| 225/65R17 (28.5") | 235/55R18 (28.2") | 245/45R19 (27.7") | Mainstream CUV/SUV progression. +2 step starts to reduce sidewall noticeably. |
| 265/65R17 (30.6") | 265/60R18 (30.5") | 275/55R20 (31.9") | Truck/SUV progression. The +2 jump to 20" changes sidewall feel considerably. |
| 275/65R18 (32.1") | 275/60R20 (32.4") | 275/50R22 (31.9") | Premium truck appearance path. +2 step to 22" wheels shortens sidewall significantly. |
Plus-sizing golden rule: Keep total diameter within ±3% of stock. This protects speedometer accuracy, maintains transmission shift calibration, and preserves ADAS system assumptions (lane-keep, adaptive cruise). Use the calculator to confirm the exact percentage before ordering.
Common metric tire sizing mistakes
Most metric tire errors come from misreading the aspect ratio or ignoring the load and speed specifications. These are the six that cause the most problems.
Thinking the aspect ratio is the sidewall height in mm
The most common metric tire misconception. 225/65R17 does NOT have a 65mm sidewall. The sidewall is 65% of 225mm = 146.25mm. The aspect ratio is always a percentage of width, never an absolute dimension.
Comparing widths without accounting for aspect ratio
A 235/55R17 and a 215/65R17 have different widths but end up at nearly the same overall diameter (26.3" vs 27.9"). You cannot choose a tire by width alone without checking how the aspect ratio changes total diameter.
Assuming P-metric and Euro-metric are identical
A P225/65R17 and a 225/65R17 (Euro-metric) look the same but have different load ratings. The P-metric has a built-in load reduction factor — it carries about 9% less load than the same-numbered Euro-metric tire.
Using a higher load index than needed on a passenger car
A higher load index is safe, but it often means a heavier, stiffer tire that rides worse and costs more. Match or exceed the door placard — you do not need to dramatically exceed it on most passenger vehicles.
Ignoring speed rating when replacing tires
Replacing a V-rated tire with an H-rated tire might seem trivial, but the speed rating must match or exceed the OEM specification. On high-speed vehicles, downgrading speed rating is a safety issue, not a cosmetic one.
Assuming the same diameter means the same size
A 225/70R16 and a 245/60R17 are both approximately 28" in diameter, but they have different widths, different sidewall heights, and different wheel diameter requirements. Diameter alone does not mean "same size".
Verify any metric size before buying
Compare two metric tire sizes side by side
Enter current and replacement metric sizes — see diameter, sidewall, circumference, and speedometer difference instantly.
Load index, speed rating, and diameter values in this guide are based on standardized industry formulas and reference data. Actual mounted tire dimensions vary by manufacturer and mold. Always verify load index and speed rating against your vehicle's door-jamb placard before purchasing replacement tires. TireCalculatorHub is not liable for any issues arising from use of information on this page.
Frequently asked questions
The metric tire questions asked most — answered with the depth that short sidewall explainers skip.