GarageCalc 
How to compare tire sizes
The single most important number when changing tire size is the overall diameter. It determines your speedometer accuracy, effective gear ratio, ground clearance and whether the tire fits in the wheel well. Width matters for grip and rubbing at full steering lock; sidewall height affects ride comfort and rim protection.
The 3 percent diameter threshold does not establish safe fitment on stock suspension. The diameter card above turns yellow past 3 percent and red past 5 percent.
How it's calculated
What a diameter change does
- Taller tire (positive change): speedometer reads slow, odometer under-counts, gearing gets taller, ride height rises.
- Shorter tire (negative change): speedometer reads fast, odometer over-counts, gearing gets shorter, ride height drops.
Swapping wheels at the same time? Check the offset math with the wheel offset calculator and confirm your new wheels match the hub with the bolt pattern calculator.
Guides to common tire changes
- 225 vs 215 tires: width, profile and fitment checks
- 17 vs 18-inch wheels: compare the complete tire
- Bigger tires: speedometer, odometer and MPG corrections
Frequently asked questions
How much bigger can I go on tire size?
This calculator uses 3 percent as a comparison threshold, not a safe-fitment guarantee. Even a smaller change can be unsuitable. Use vehicle-approved sizes and check actual tire dimensions, rim width, load and speed ratings, and clearances.
Why does my speedometer read wrong after changing tire size?
Your speedometer counts wheel rotations and assumes the factory tire diameter. A taller tire covers more ground per rotation, so the speedometer reads lower than your true speed. A shorter tire does the opposite. The table in this calculator shows your actual speed at each indicated speed.
Do bigger tires affect fuel economy and acceleration?
Yes, slightly. A larger diameter effectively raises your gearing, which reduces acceleration and can hurt or help highway fuel economy depending on the engine. Heavier tires also add rotating mass, which hurts both acceleration and braking. A 1 to 2 percent diameter change is rarely noticeable; larger jumps are.
What is a plus-one or plus-two tire fitment?
Plus sizing means moving to a wheel 1 or 2 inches larger in diameter while lowering the tire aspect ratio, so the overall diameter stays nearly the same. For example, 205/55R16 to 225/45R17 is a classic plus-one fitment: the diameter changes by less than 1 percent.