What matters
- The 245/40R18 is about 0.74 inch taller than the 225/45R17.
- Its nominal rolling diameter is about 2.98% larger, so an indicated 60 mph is roughly 61.8 mph.
- Similar diameter does not prove that the wider tire and wheel will clear the suspension or fender.
What each tire-size number means
In 225/45R17, 225 is the nominal section width in millimeters, 45 is the sidewall height expressed as a percentage of that width, and 17 is the wheel diameter in inches. NHTSA uses the same definitions in its tire-marking guidance. The dimensions are nominal: the mounted measurement can change with the tire model, measuring rim, pressure, and load.
Diameter = 17 + (2 × 3.986) = 24.97 in
Circumference = 24.97 × π = 78.45 in
Diameter = 18 + (2 × 3.858) = 25.72 in
Circumference = 25.72 × π = 80.79 in
The speedometer effect
The new nominal circumference is approximately 2.98% larger. Each revolution carries the car farther, so the speedometer—if calibrated for the smaller tire—reads lower than the actual road speed. Multiply the indicated speed by the new-to-stock diameter ratio.
At an indicated 30 mph, actual speed is about 30.9 mph. At 70 mph, it is about 72.1 mph. Vehicle speedometers may already contain factory error, so this is the change caused by nominal tire dimensions, not a GPS calibration certificate.
What the diameter calculation does not prove
A tire can be close in diameter and still rub. The 245 is nominally 20 mm wider, and its location depends on wheel width and offset. Brake clearance, suspension shape, alignment, ride height, tire construction, and fender geometry all matter. Use the wheel offset calculator after the diameter check, then confirm with the wheel and tire manufacturers and a physical clearance measurement.
Sources and methodology
Calculations are shown so the assumptions can be checked. Safety-critical fitment, fueling, and calibration decisions must be verified against current manufacturer documentation and the professional responsible for the work.