Tire Ratio Calculator
The Tire Ratio Calculator decodes tire size designations and calculates the aspect ratio, sidewall height, overall diameter, and circumference. Compare tire sizes to check speedometer accuracy and ensure proper fitment for your vehicle.
🕐 Recent Calculations
What is Tire Aspect Ratio?
Tire aspect ratio is the ratio of the tire's sidewall height to its section width, expressed as a percentage. In the tire size 225/45R17, '45' is the aspect ratio — the sidewall height is 45% of the 225mm width, equaling 101.25mm. Lower aspect ratios indicate a shorter sidewall (sportier handling), while higher ratios mean a taller sidewall (more comfort).
Tire aspect ratios typically range from 25 (ultra-low profile racing tires) to 80 (off-road and SUV tires). Standard passenger car tires use 50-65. The aspect ratio, combined with wheel diameter, determines the tire's overall diameter and rolling circumference, which affect speedometer calibration.
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%.
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 aspect ratio number mean on a tire? ▼
The aspect ratio is the second number in a tire size (e.g., 45 in 225/45R17). It means the sidewall height is 45% of the tire's width. Lower numbers = shorter sidewall, sportier; higher numbers = taller sidewall, more comfortable.
How does tire aspect ratio affect ride quality? ▼
Higher aspect ratios (55-70) provide more rubber between the road and rim, absorbing bumps better. Lower aspect ratios (35-45) are stiffer, providing sharper handling but transmitting more road imperfections to the cabin.
What aspect ratio is best for performance? ▼
Sport/performance tires typically use 35-45 aspect ratio for maximum grip and responsiveness. Track cars may go as low as 25-30. The tradeoff is harsher ride and increased risk of rim damage from potholes.
Can I change tire aspect ratio without changing wheel size? ▼
Yes, but the overall diameter should stay within 3% of original. Going from 225/55R17 to 225/50R17 lowers the car by about 11mm and makes the speedometer read slightly fast.
How much speedometer error is acceptable? ▼
Most jurisdictions allow 3-5% speedometer variance. Keep tire overall diameter within 3% of the factory size. At 3% oversized, your speedometer shows 60 mph when you're actually going 61.8 mph.