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Tire Size Calculator

Compare two tire sizes side by side โ€” see diameter, circumference, sidewall height, and speedometer difference instantly

1
Original Tire
/ R

Width / Aspect R Rim (e.g., 225/45R17)

2
New Tire
/ R

Width / Aspect R Rim (e.g., 235/40R18)

About This Tool

Changing tire sizes affects more than just looks โ€” it changes your speedometer reading, odometer accuracy, ground clearance, and fuel economy. Before you buy new tires or wheels, you need to know exactly how the new size compares to your original. This calculator takes two tire sizes in standard format (like 225/45R17) and shows you everything: overall diameter, sidewall height, circumference, revolutions per mile, and most importantly โ€” the speedometer error. If your speedometer shows 60 mph, what's your actual speed with the new tires? Use it when upgrading wheels, switching to a plus-size setup, comparing all-terrain vs stock tires, or checking if a tire size from a junkyard wheel will work on your car.

How to Use

1. Enter your current (original) tire size in the left box โ€” format: 225/45R17 2. Enter the new tire size you're considering in the right box 3. Results update instantly showing both sizes side by side 4. Check the speedometer difference โ€” green means close enough, yellow is noticeable, red is significant 5. Review diameter difference to ensure the new size will fit without rubbing

Formula

Sidewall height = Width ร— (Aspect ratio / 100) Overall diameter = (2 ร— Sidewall height) + Rim diameter Circumference = ฯ€ ร— Diameter Revs per mile = 63,360 / Circumference (in inches) Speedometer error = ((New diameter / Old diameter) - 1) ร— 100% Actual speed = Indicated speed ร— (New diameter / Old diameter)

Frequently Asked Questions

How do I read a tire size like 225/45R17?
The format is Width/AspectRatio R RimDiameter. 225 is the width in millimeters. 45 is the aspect ratio (sidewall height as % of width). R means radial construction. 17 is the rim diameter in inches. So 225/45R17 is a 225mm wide tire with a sidewall that's 45% of 225mm (101mm), mounted on a 17-inch wheel.
How much tire size difference is acceptable?
Most manufacturers recommend staying within 3% of the original tire diameter. This keeps speedometer error minimal and ensures proper clearance. Going beyond 3% may cause rubbing, affect ABS/traction control calibration, and void warranties. Some vehicles have more room for larger tires than others.
Will bigger tires make my speedometer read slow or fast?
Bigger tires make your speedometer read SLOWER than actual speed. If you install tires 3% larger in diameter, when your speedometer shows 60 mph, you're actually going about 62 mph. Your odometer will also under-report miles driven. Smaller tires have the opposite effect.
What is plus sizing for wheels and tires?
Plus sizing means going to a larger wheel diameter while reducing the tire sidewall to keep overall diameter similar. Plus-1 means 1 inch larger wheels (e.g., 17โ†’18). Plus-2 is 2 inches larger. The benefit is improved handling from shorter sidewalls and a more aggressive look, while keeping speedometer accuracy.
Does tire size affect fuel economy?
Yes, but the effect is usually small. Larger tires are heavier and have more rolling resistance, which can reduce MPG by 1-2%. However, taller tires also mean fewer engine revolutions per mile, which can slightly improve highway fuel economy. The net effect depends on the specific tires and driving conditions.
Can I use a different width tire on my current wheels?
Within limits, yes. Each wheel has a range of acceptable tire widths. A 7-inch wide wheel typically accepts tires from 195mm to 225mm wide. Going too narrow or too wide affects handling, wear patterns, and can be unsafe. Check your wheel specs for the recommended tire width range.
How do I calculate revolutions per mile?
Revolutions per mile = 63,360 รท tire circumference in inches. The number 63,360 is the number of inches in a mile. A typical passenger car tire does about 800-900 revolutions per mile. Larger tires have fewer revolutions per mile, which is why your odometer under-counts with bigger tires.

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