Tire Ratio Calculator
Calculate tire aspect ratios, compare overall diameters between tire sizes, and determine speedometer error when changing tires. Enter tire size codes (like 225/45R17) to decode dimensions, find equivalent sizes, and understand how tire changes affect your vehicle's performance and accuracy.
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What is Tire Aspect Ratio?
The tire aspect ratio is the sidewall height expressed as a percentage of the tire's section width. In a 225/45R17 tire, the aspect ratio is 45 — meaning the sidewall height is 45% of the 225mm width, equaling 101.25mm. Combined with the wheel diameter (17 inches), this determines the tire's overall diameter, which directly affects speedometer accuracy, gear ratios, ground clearance, and ride quality.
When changing tire sizes, maintaining a similar overall diameter is critical. A tire that is too tall makes the speedometer read slower than actual speed (dangerous), while a tire that is too short makes it read faster. Most manufacturers recommend staying within 3% of the original tire diameter. This calculator compares any two tire sizes and shows the exact diameter difference and speedometer error.
使用される計算公式・方程式
この計算ツールは5つの主要な公式を使用しています：
1 Sidewall Height ▼
225mm width × 45/100 = 101.25mm sidewall height.
2 Overall Tire Diameter ▼
225/45R17: Diameter = (2 × 101.25) + (17 × 25.4) = 202.5 + 431.8 = 634.3mm = 24.97 inches.
3 Speedometer Error ▼
Going from 634mm to 660mm: Error = (660/634 - 1) × 100 = +4.1%. Speedometer reads slow by 4.1%.
Explore all calculation options on the 比率計算ツール home page.
比率計算ツールの使い方
この比率計算ツールは、以下の3ステップで簡単にご利用いただけます：
数値を入力
入力欄に既知の比率の値を入力します。求めたい未知数の入力欄は空欄のままにしておきます。
モードを選択
比率モード（解く、簡素化、スケーリング）を選択します。各モードで異なる計算式が適用されます。
結果を確認
計算するボタンを押します。結果画面に答えと、視覚的な比率バー、円グラフ、詳細なステップバイステップの解決プロセスが表示されます。
実例問題と段階的な解説
本比率計算ツールを使って、以下の3つの例題をステップバイステップで解決するプロセスです：
入力 1 Decode tire size 245/40R18
入力 2 Compare 225/45R17 vs 235/40R18
入力 3 Find aspect ratio for target diameter
よくある質問 (FAQ)
What does the tire aspect ratio number mean? ▼
The aspect ratio is the sidewall height as a percentage of the tire's width. In a 225/45R17, the aspect ratio 45 means the sidewall is 45% of 225mm = 101.25mm tall. Lower aspect ratios (30-40) are 'low profile' with short sidewalls; higher ratios (55-75) have taller sidewalls.
How do I calculate overall tire diameter? ▼
Overall Diameter = (2 × Sidewall Height) + Wheel Diameter. Sidewall = Width × (Aspect Ratio / 100). For 225/45R17: Sidewall = 225 × 0.45 = 101.25mm. Diameter = (2 × 101.25) + (17 × 25.4mm) = 202.5 + 431.8 = 634.3mm (24.97 inches).
How does tire size affect speedometer accuracy? ▼
Speedometer Error % = ((New Diameter − Original Diameter) / Original Diameter) × 100. A tire 3% larger than stock makes the speedometer read 3% slow (showing 60 MPH when actually traveling 61.8 MPH). A tire 3% smaller reads 3% fast.
What is plus-sizing for wheels and tires? ▼
Plus-sizing increases wheel diameter while decreasing tire aspect ratio to maintain approximately the same overall diameter. Plus-1 means 1 inch larger wheel (e.g., 16 to 17 inches), plus-2 means 2 inches larger. Lower-profile tires improve handling but reduce ride comfort.
How much tire size change is safe? ▼
Most manufacturers and tire professionals recommend staying within ±3% of the original tire's overall diameter. Beyond 3%, speedometer error, ABS calibration, traction control, and stability systems may be affected enough to impact safety.
What does the R in a tire size mean? ▼
R indicates radial construction — the most common tire type today. The plies run radially (perpendicular to the direction of travel). Other codes include B (bias-belted) and D (diagonal/bias-ply), which are rare on modern passenger vehicles.
How do I find equivalent tire sizes? ▼
To find equivalents, maintain approximately the same overall diameter. If changing from 205/55R16 (632mm diameter) to 17-inch wheels, calculate: 632mm − (17 × 25.4 = 431.8mm) = 200.2mm available for sidewalls. Per side = 100.1mm. Aspect ratio = 100.1/205 × 100 ≈ 49. Closest standard: 205/50R17 (637mm).
Does tire width affect fuel economy? ▼
Yes. Wider tires have more contact area, which increases rolling resistance and reduces fuel economy by approximately 1-2% per 10mm of additional width. However, the effect varies with tire compound, tread pattern, and inflation pressure.
What tire aspect ratio gives the best ride comfort? ▼
Higher aspect ratios (55-70) provide more sidewall flex, absorbing bumps and providing a smoother ride. Lower aspect ratios (30-45) transmit more road imperfections to the cabin but improve steering precision. For comfort-focused vehicles, stay at 55+ aspect ratio.
How do I read an off-road tire size like 33×12.5R15? ▼
This format is simpler: 33 is the overall diameter in inches, 12.5 is the width in inches, and 15 is the wheel diameter in inches. The aspect ratio is implicit: Sidewall = (33 − 15) / 2 = 9 inches. Aspect ratio equivalent ≈ 9/12.5 × 100 = 72%.