What does a tire size like 235/55R17 mean?

Width × aspect ratio × rim diameter. 235 = section width in mm (235mm wide). 55 = aspect ratio as a percent (sidewall is 55% of width = 129.25mm). R = radial construction. 17 = rim diameter in inches. Combined, this tire has a sidewall height of 5.09 inches and overall diameter of 27.2 inches.

How is sidewall height calculated?

Sidewall = section_width × aspect_ratio / 100 (in mm). Divide by 25.4 for inches. For 235/55R17: 235 × 0.55 = 129.25mm sidewall = 5.09 inches. A lower aspect ratio (e.g. 40 vs 55) gives a shorter sidewall — sharper handling but rougher ride. Higher aspect ratio = taller sidewall = softer ride but more flex.

How is overall diameter calculated?

Overall diameter = (2 × sidewall_height) + rim_diameter. The two sidewalls (top + bottom of the wheel) plus the rim itself. For 235/55R17: (2 × 5.09) + 17 = 27.18 inches overall diameter. Used for revs/mile + speedometer error calculations.

What's the formula for revolutions per mile?

Revs/mile = 63,360 / (π × diameter_inches). 63,360 = inches per mile (5,280 ft × 12). The tire's circumference is π × diameter. For 235/55R17 (27.18" diameter), revs/mile = 63,360 / 85.40 = 742 revs/mile. Higher revs/mile = smaller tire; lower = larger tire.

How does tire size affect speedometer accuracy?

Speedometers count revolutions × rev-to-speed conversion factor calibrated for original tire size. A LARGER tire travels further per revolution, so the speedo reads LOWER than actual speed. Example: 5% larger diameter = speedo reads 57 mph at actual 60 mph (3 mph low). A SMALLER tire is the opposite — speedo reads HIGH. The calculator surfaces both.

What's the maximum size change before speedometer recalibration?

General rule: ±3% diameter change is the safe threshold for minor speedometer drift without recalibration. Beyond ±3%, ABS/TPMS/transmission-shift-point logic may misbehave. Beyond ±5%, full recalibration is required (and may not be possible on older vehicles without ECU reflash). Plus-sizing (larger wheel, lower aspect ratio) typically keeps overall diameter within ±1% to avoid this.

What's 'plus sizing'?

Going up in rim diameter while keeping the same overall tire diameter. Common plus sizes: 17→18 (P+1), 17→19 (P+2). Example: stock 215/55R17 (26.31") to plus-1 225/50R18 (26.86") to plus-2 225/45R19 (26.97"). Maintains speedometer accuracy + ABS calibration. Pros: better cornering grip, wider stance, sportier look. Cons: rougher ride, more rim damage risk from potholes, more expensive tires.

Will the new tire fit my car?

Beyond just diameter, check (1) section width vs wheel-well clearance — wider tires can rub against fender/strut at full steering lock; (2) load index + speed rating — must meet or exceed your vehicle's placard requirements (these are on the door jamb sticker); (3) load-bearing capacity for trucks/SUVs. This calculator is geometry-only; physical fitment requires checking your vehicle's manual or asking the tire shop.

Why do different tire brands have slightly different diameters at the same listed size?

Manufacturing tolerances. TRA/ETRTO standards allow ±2% variation in advertised dimensions. Brands like Bridgestone tend to run slightly larger than spec; Michelin slightly smaller. The advertised size is a nominal target; actual diameter can vary 0.3-0.5 inches between brands at the same listed size.

What's the difference between metric and P-metric sizing?

P-metric (P prefix): 'Passenger' designation, US standard since 1970s. Used for cars, light trucks, SUVs. Example: P215/55R17. Metric (no prefix): European standard. Example: 215/55R17. The dimensions are identical; the 'P' just signals passenger-tire load tables apply. Light-truck tires use 'LT' prefix (e.g. LT265/70R17) — higher load capacity, different sidewall specs.

What about ZR / W / Y speed ratings?

Speed ratings indicate maximum sustained speed: H=130mph, V=149mph, W=168mph, Y=186mph, ZR=149+mph (any letter above V). The letter is part of the tire size: 235/55ZR17 means the tire is rated above V. For street vehicles, you should match or exceed your vehicle's placard speed rating — lower rating can void insurance + cause handling issues at highway speeds.

Will the calculator work for motorcycle tires?

Yes for the geometry — motorcycle tires use the same P-metric or metric sizing convention. Just enter your width/aspect/rim. The fitment + load + speed considerations are vastly different from car tires though; consult motorcycle-specific resources for actual fitment decisions.

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Tire Size Calculator — Sidewall + Diameter + Speedometer Error

Drop your tire size (width × aspect × rim) and reference — get sidewall height, overall diameter, revolutions per mile, and speedometer error vs reference. Built on TRA + ETRTO P-metric sizing standards.

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Reviewed by CalcBold Editorial · Sources: Tire and Rim Association (TRA) Yearbook 2026 + ETRTO standards + common P-metric sizing conventionsLast verified May 15, 2026Methodology

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Frequently Asked Questions

The most common questions we get about this calculator — each answer is kept under 60 words so you can scan.

What does a tire size like 235/55R17 mean?
Width × aspect ratio × rim diameter. 235 = section width in mm (235mm wide). 55 = aspect ratio as a percent (sidewall is 55% of width = 129.25mm). R = radial construction. 17 = rim diameter in inches. Combined, this tire has a sidewall height of 5.09 inches and overall diameter of 27.2 inches.
How is sidewall height calculated?
Sidewall = section_width × aspect_ratio / 100 (in mm). Divide by 25.4 for inches. For 235/55R17: 235 × 0.55 = 129.25mm sidewall = 5.09 inches. A lower aspect ratio (e.g. 40 vs 55) gives a shorter sidewall — sharper handling but rougher ride. Higher aspect ratio = taller sidewall = softer ride but more flex.
How is overall diameter calculated?
Overall diameter = (2 × sidewall_height) + rim_diameter. The two sidewalls (top + bottom of the wheel) plus the rim itself. For 235/55R17: (2 × 5.09) + 17 = 27.18 inches overall diameter. Used for revs/mile + speedometer error calculations.
What's the formula for revolutions per mile?
Revs/mile = 63,360 / (π × diameter_inches). 63,360 = inches per mile (5,280 ft × 12). The tire's circumference is π × diameter. For 235/55R17 (27.18" diameter), revs/mile = 63,360 / 85.40 = 742 revs/mile. Higher revs/mile = smaller tire; lower = larger tire.
How does tire size affect speedometer accuracy?
Speedometers count revolutions × rev-to-speed conversion factor calibrated for original tire size. A LARGER tire travels further per revolution, so the speedo reads LOWER than actual speed. Example: 5% larger diameter = speedo reads 57 mph at actual 60 mph (3 mph low). A SMALLER tire is the opposite — speedo reads HIGH. The calculator surfaces both.
What's the maximum size change before speedometer recalibration?
General rule: ±3% diameter change is the safe threshold for minor speedometer drift without recalibration. Beyond ±3%, ABS/TPMS/transmission-shift-point logic may misbehave. Beyond ±5%, full recalibration is required (and may not be possible on older vehicles without ECU reflash). Plus-sizing (larger wheel, lower aspect ratio) typically keeps overall diameter within ±1% to avoid this.
What's 'plus sizing'?
Going up in rim diameter while keeping the same overall tire diameter. Common plus sizes: 17→18 (P+1), 17→19 (P+2). Example: stock 215/55R17 (26.31") to plus-1 225/50R18 (26.86") to plus-2 225/45R19 (26.97"). Maintains speedometer accuracy + ABS calibration. Pros: better cornering grip, wider stance, sportier look. Cons: rougher ride, more rim damage risk from potholes, more expensive tires.
Will the new tire fit my car?
Beyond just diameter, check (1) section width vs wheel-well clearance — wider tires can rub against fender/strut at full steering lock; (2) load index + speed rating — must meet or exceed your vehicle's placard requirements (these are on the door jamb sticker); (3) load-bearing capacity for trucks/SUVs. This calculator is geometry-only; physical fitment requires checking your vehicle's manual or asking the tire shop.
Why do different tire brands have slightly different diameters at the same listed size?
Manufacturing tolerances. TRA/ETRTO standards allow ±2% variation in advertised dimensions. Brands like Bridgestone tend to run slightly larger than spec; Michelin slightly smaller. The advertised size is a nominal target; actual diameter can vary 0.3-0.5 inches between brands at the same listed size.
What's the difference between metric and P-metric sizing?
P-metric (P prefix): 'Passenger' designation, US standard since 1970s. Used for cars, light trucks, SUVs. Example: P215/55R17. Metric (no prefix): European standard. Example: 215/55R17. The dimensions are identical; the 'P' just signals passenger-tire load tables apply. Light-truck tires use 'LT' prefix (e.g. LT265/70R17) — higher load capacity, different sidewall specs.
What about ZR / W / Y speed ratings?
Speed ratings indicate maximum sustained speed: H=130mph, V=149mph, W=168mph, Y=186mph, ZR=149+mph (any letter above V). The letter is part of the tire size: 235/55ZR17 means the tire is rated above V. For street vehicles, you should match or exceed your vehicle's placard speed rating — lower rating can void insurance + cause handling issues at highway speeds.
Will the calculator work for motorcycle tires?
Yes for the geometry — motorcycle tires use the same P-metric or metric sizing convention. Just enter your width/aspect/rim. The fitment + load + speed considerations are vastly different from car tires though; consult motorcycle-specific resources for actual fitment decisions.

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