Tire Size Calculator
A "225/45R17" looks cryptic, but it fully defines the physical tire your car rolls on. The two numbers that actually change your ride, ground clearance, and speedometer accuracy are the aspect ratio and wheel diameter — not the width. Enter a tire size below to get its true overall diameter, sidewall height, circumference, and revolutions per mile, then compare it against your stock size to see exactly how far off your speedo will read.
New / Target Tire
Stock / Reference Tire (for comparison)
From Tire Code to Physical Size
The overall diameter is what your car actually rides on, and it comes from three numbers:
Diameter (in) = (Width × Aspect% ÷ 100 ÷ 25.4 × 2) + Wheel
The sidewall is the width times the aspect ratio, converted from millimeters to inches, doubled (top and bottom), then added to the wheel diameter. Circumference is π × diameter, and revolutions per mile is 63,360 (inches in a mile) divided by that circumference. A taller tire simply rotates fewer times per mile — which is the root of every speedometer and odometer error.
Worked Example: 225/45R17 vs 205/55R16
A 225/45R17 has a sidewall of 225 × 0.45 = 101.25 mm (3.99 in), so its diameter is 3.99 × 2 + 17 = 24.97 in. The stock 205/55R16 has a 112.75 mm sidewall (4.44 in) for a diameter of 4.44 × 2 + 16 = 24.88 in. The "+1" wheel upgrade actually ends up just 0.09 in larger — well within ±1%, so the speedometer barely moves and there is no clearance issue. This is exactly why enthusiasts do "plus sizing" carefully: bump the wheel and drop the aspect ratio to keep the diameter nearly constant.
Why Diameter Discipline Matters
Beyond the speedo, a diameter change alters final-drive gearing (acceleration and fuel economy), ABS and traction-control calibration (which expect a known wheel speed), and physical clearance. As a rule of thumb, keep the new overall diameter within ±3% of stock. This calculator shows the exact percentage so you can confirm a fitment before buying rubber you can't return.
Frequently Asked Questions
How do I read a tire size like 225/45R17?
Width in mm / aspect ratio % / wheel diameter in inches (R = radial). The calculator turns it into physical diameter, sidewall, circumference, and revs per mile.
What does the aspect ratio control?
Lower aspect = shorter sidewall = stiffer, more responsive ride; higher = more cushion. It also shrinks overall diameter unless wheel size compensates.
How does tire size affect my speedometer?
Speedo assumes a fixed circumference. A larger tire travels farther per revolution, so indicated speed reads low. Error % = % change in overall diameter.
Is a bigger tire better for fuel economy?
Usually not — added mass and drag typically outweigh any gearing benefit. Net effect is neutral-to-negative MPG.
Can I change tire size without rubbing?
Each side loses half the diameter increase in clearance. Stay within ±3% of stock diameter to avoid fender/suspension contact.