What do the numbers and letters on my tire size mean?
Quick answer
In a marking such as 205/55 R16 91V, 205 is the tire width in millimeters, 55 is the sidewall-height-to-width ratio, R means radial construction, and 16 is the rim diameter in inches. The final pair, 91V, is the service description: a load index and a separate speed symbol. Treat this as a decoding example, not a replacement recommendation; check your vehicle’s approved specifications. [S1]
The question, the details, the next step.
I’m looking at a tire sidewall and see something like 205/55 R16 91V. What am I meant to read from that?
Read it in two groups: the size portion, 205/55 R16, and the service description, 91V. Width is given in millimeters, the aspect ratio relates sidewall height to width, R indicates radial construction, and the rim diameter is given in inches. The load index and speed symbol are separate parts of the service description. [S1]
For the illustrative marking 205/55 R16 91V, 205 is the width, 55 is the aspect ratio, R is the construction letter, and 16 is the rim diameter. The example explains the format; it does not say that this tire is suitable for a particular vehicle.
Does 55 mean the tire is 55 millimeters tall? And is R a size?
No. The 55 is an aspect ratio: it expresses the sidewall height relative to the tire’s width, rather than giving a sidewall height in millimeters. R identifies radial construction; it is not another dimension. The 16 is the rim diameter, expressed in inches. [S1]
A useful way to read the size group is: width / aspect ratio, construction, rim diameter. Keep the units and roles separate rather than treating every number in the marking as the same kind of measurement.
What do 91 and V tell me? Can I just match the first four parts when replacing a tire?
In this format, 91 is the load index and V is the speed symbol. They are distinct from the dimensions before them, and both belong to the service description. [S1] The supplied source does not provide the exact load or speed value represented by 91 or V, so don’t infer a specific capacity or speed from this guide alone.
Matching only the size portion does not establish that a replacement is suitable. Michelin says to use the vehicle manufacturer’s specifications to guide replacement selection. [S1] Check the marking against your vehicle’s approved specification rather than assuming the example applies.
Can you show me how to use that without accidentally treating the example as my car’s approved size?
Use a two-column note, and fill in the vehicle specification from an appropriate vehicle reference rather than assuming the example applies. For instance:
- Sidewall I’m reading: 205/55 R16 91V (illustrative example only)
- Size portion: 205 / 55 / R / 16
- Service description: 91 / V
- Vehicle’s approved specification: [copy the complete specification from the vehicle’s approved reference]
- Question to check: “Does this marking match the approved specification for my vehicle?”
Then compare the marking with the approved specification. If you cannot confirm the specification or whether the marking matches it, do not assume that the example identifies a suitable replacement. The example and source explain how markings are organized; they do not identify your vehicle’s approved tire.
A practical checklist
Copy the complete sidewall marking, including the service description.
Separate the width, aspect ratio, construction letter, and rim diameter.
Record the load index and speed symbol as a pair; don’t guess their exact values from this guide.
Find the complete approved tire specification for your vehicle.
Check the marking against the approved specification rather than assuming the example applies.
A few more useful answers
Is the tire sidewall marking by itself enough to choose a replacement?
No. The marking describes the tire, but it does not establish that it is approved for your particular vehicle. Use the vehicle manufacturer’s specifications to guide replacement selection. [S1]
Does the source explain the exact load or speed value for 91V?
No. It identifies the load index and speed symbol as separate parts of the service description, but the supplied extract does not give the exact value for either symbol. [S1]
Is 205/55 R16 91V a recommendation for my vehicle?
No. It is an illustrative marking used here to explain the format. Check your vehicle’s approved specification before selecting a replacement. [S1]
Sources
References used for this guide.
- [S1] Michelin USA: How to Read Tire Markings and Sidewall Codes michelinman.com · Checked