Convert tire sizes to overall diameter, calculate speedometer errors, and determine true speed after tire or gear changes. Essential tools for lifted trucks, modified cars, and accurate odometer readings.
Convert metric tire sizes (275/60R15) to actual measurements in inches. Get section width, sidewall height, overall diameter, and circumference.
Enter tire size in format: Width/Aspect-Ratio-R-Rim-Diameter (example: 275/60R15)
How to read tire sizes: In 275/60R15, "275" is section width in mm, "60" is aspect ratio (sidewall height as % of width), "R" means radial construction, "15" is rim diameter in inches.
Calculate your true speed after changing tire sizes or gear ratios. Essential for accurate speed readings and avoiding speeding tickets.
Example: If your speedometer reads 60 MPH but you've installed larger tires, your actual speed is higher. This calculator shows your true speed.
When you change tire sizes, your speedometer and odometer become inaccurate because they're calibrated for the original tire diameter. Larger tires make your speedometer read slower than actual speed, while smaller tires make it read faster. This affects not only your speed awareness but also your odometer mileage and fuel economy calculations.
Your vehicle's speedometer calculates speed based on how fast the wheels are rotating. When you install larger diameter tires, each revolution covers more ground, so you're traveling faster than the speedometer indicates. For example, if you install tires that are 10% larger in diameter, your actual speed will be 10% higher than what your speedometer shows.
Lifted Trucks: Installing larger tires on lifted trucks is common but can result in significant speedometer error. A typical lift with 35" tires replacing 31" tires creates about 13% speedometer error.
Plus-Sizing Wheels: When upgrading to larger diameter wheels, you typically use lower profile tires to maintain similar overall diameter. If done correctly, speedometer accuracy is maintained.
Performance Cars: Performance enthusiasts often install wider tires without changing overall diameter to maintain speedometer accuracy while gaining traction.
Classic Cars: Restorers may need to install modern tire sizes that differ from original equipment, requiring speedometer gear changes or electronic correction.
An inaccurate speedometer can lead to speeding tickets if you're traveling faster than you think. In some jurisdictions, an incorrect speedometer can fail vehicle inspection. Additionally, your odometer will record incorrect mileage, affecting resale value, warranty claims, and lease agreements.
275/60R15: Classic muscle car tire, 28" diameter, good for 3.73-4.10 gears
285/75R16: Popular truck tire, 32.8" diameter, common on 4x4 pickups
35x12.50R17: Aggressive off-road tire, 35" diameter, requires lift kit
305/35R20: Performance car tire, 28.4" diameter, wide contact patch
33x12.50R15: Classic off-road size, 33" diameter, Jeep favorite
Popular tire sizes and their actual measurements:
| Tire Size | Overall Diameter | Section Width | Common Use |
|---|---|---|---|
| 265/70R17 | 31.6" | 10.4" | Stock truck/SUV |
| 275/60R15 | 28.0" | 10.8" | Muscle car |
| 285/75R16 | 32.8" | 11.2" | Light truck |
| 295/50R15 | 26.6" | 11.6" | Drag racing |
| 305/35R20 | 28.4" | 12.0" | Performance car |
| 33x12.50R15 | 33.0" | 12.5" | Off-road/Jeep |
| 35x12.50R17 | 35.0" | 12.5" | Lifted truck |