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.
🕐 Recent Calculations
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.
Formules et Équations utilisées
Ce calculateur s'appuie sur 5 équations fondamentales :
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 Calculateur de Ratio home page.
Comment utiliser ce calculateur
Pour utiliser ce Calculateur de Ratio, suivez ces 3 étapes :
Saisir les Valeurs
Saisissez les valeurs de ratio connues dans les champs. Laissez un champ vide — c'est la valeur manquante que le calculateur va chercher à résoudre.
Choisir le Mode
Sélectionnez le mode de ratio — Résoudre, Simplifier ou Mettre à l'échelle. Chaque mode applique des formules différentes à vos valeurs.
Obtenir les Résultats
Cliquez sur Calculer. L'écran de résultats affiche la réponse avec une barre de ratio interactive, un diagramme circulaire et le détail des calculs.
Exemples Pratiques et Solutions Étape par Étape
Voici 3 exemples de calculs résolus avec les étapes détaillées grâce à ce calculateur :
Saisie 1 Decode tire size 245/40R18
Saisie 2 Compare 225/45R17 vs 235/40R18
Saisie 3 Find aspect ratio for target diameter
Foire Aux Questions
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%.