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Tire Size Comparison

Will that plus-size fit? Check diameter, sidewall, and speedo error.

Swapping tire sizes affects your speedometer accuracy, ABS, traction control, and actual acceleration. Within ±3% overall diameter change, most modern cars handle the swap fine. This calculator shows the exact difference so you know before you buy.

Overall diameter change
+2.98%
Old OD
634.3 mm
New OD
653.2 mm
Sidewall change
-3.3 mm
Speedo error at 100
+3.0%
Actual @ 100 indicated
103.0 km/h

Within 3% — safe plus-size, minimal speedo/TCS impact.

How to Read a Tire Size

The three numbers on every tire contain all the geometry you need. Once you understand them, comparing sizes is straightforward.

  • First number (225): tread width in millimetres
  • Second number (45): aspect ratio — sidewall height as a percentage of width. 225/45 = 101.25mm sidewall
  • Third number (17): rim diameter in inches
  • Overall diameter = (rim × 25.4) + (2 × width × aspect/100)
  • Example: 225/45R17 OD = (17 × 25.4) + (2 × 225 × 0.45) = 431.8 + 202.5 = 634.3mm

Plus Sizing: How It Works

Plus sizing (e.g., +1 or +2) means increasing the rim diameter by 1 or 2 inches while reducing the aspect ratio to maintain approximately the same overall diameter. The benefit: a wider contact patch, better lateral grip, and room for larger brake rotors.

  • +0 sizing: same rim, wider or different profile tire — bolt-on look change
  • +1 sizing: rim up 1", profile down — slightly larger footprint, sportier look
  • +2 sizing: rim up 2", profile down further — aggressive stance, requires clearance check
  • Target OD: within ±3% of stock for safe operation
  • Always check wheel arch clearance and possible fender rubbing at full lock

Speedometer Error: What It Means in Practice

A larger tire travels farther per revolution than the stock tire. The wheel speed sensor (which drives your speedometer) counts the same number of pulses, so it thinks you're going slower than you actually are.

  • +2% OD: at 100 km/h indicated, actual speed is 102 km/h
  • +5% OD: at 100 km/h indicated, actual speed is 105 km/h
  • This affects all speed-based systems: speedometer, odometer, ABS threshold, cruise control
  • Speedometer recalibration: available via a PCM tune or aftermarket module (~$50-200)
  • Smaller tires have the opposite effect — actual speed is less than indicated

What Can Go Wrong With Oversized Tires

Beyond the ±3% OD rule, problems compound quickly. Physical and electronic issues both arise.

  • Fender rubbing at full steering lock or over bumps — especially on lowered cars
  • ABS/ESC fault codes on some vehicles due to unexpected wheel speed readings
  • Inaccurate odometer affects oil change intervals and lease mileage calculations
  • Reduced acceleration and higher effective gear ratio (feels like a taller final drive)
  • For lifted trucks with oversized tires: regearing is usually required to restore performance

How to Compare Tire Sizes

Three steps to safely evaluate a tire size swap.

  1. 1
    Enter your stock tire size
    Find your current tire size on the sidewall or in your door jamb sticker. Enter width, aspect ratio, and rim diameter in the 'Old' fields.
  2. 2
    Enter the new size you're considering
    Enter the candidate tire's width, aspect ratio, and rim diameter in the 'New' fields.
  3. 3
    Check the diameter difference
    If the overall diameter change is within ±3%, you're in the safe zone. Note the speedometer error — at an indicated 100 km/h, your actual speed will be 100 × (new OD ÷ old OD).

FAQ

What's the safe diameter difference for plus-sizing?
Within ±3% is the industry rule of thumb. It keeps speedometer, ABS, and ESC behavior predictable. Over 5% starts causing noticeable issues.
Will bigger tires mess up my speedometer?
Yes — a larger tire rolls farther per revolution, so your speedo under-reads actual speed. A 2% bigger tire reads 2% low; at an indicated 100 you're actually doing ~102 km/h.
Does this affect ABS and traction control?
Only if the size change exceeds what the ECU expects (usually ±3-5%). Modern cars are surprisingly tolerant, but large changes can trigger fault codes or degraded system response.
How do I read a tire size like 225/45R17?
225 = tread width in mm. 45 = aspect ratio (sidewall height as % of width, so 225×0.45 = 101.25mm sidewall). R = radial construction. 17 = rim diameter in inches.
Does wheel size (rim diameter) matter for the comparison?
Yes — a larger rim with a lower profile tire can have nearly the same overall diameter as a smaller rim with a taller tire. That's the whole point of plus-sizing: keep OD stable while fitting bigger brakes.

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