Larger diameter means more distance per revolution
Circumference grows directly with diameter. If tire diameter rises 3%, one wheel revolution covers about 3% more road distance. At the same engine RPM and unchanged mechanical ratios, theoretical road speed also rises by about 3%. The reverse is true for a smaller tire.
Engine RPM at a fixed road speed
Because a larger tire needs fewer wheel revolutions to cover the same distance, engine RPM falls at the same true road speed if all transmission and final-drive ratios stay unchanged. A 3% increase in rolling diameter produces roughly a 3% decrease in required wheel RPM, and therefore a similar decrease in engine RPM in a locked mechanical driveline.
A simple comparison
Consider a nominal 25.0-inch tire and a 26.0-inch replacement. The replacement is 4% larger in diameter. If the original setup turns 2,500 engine rpm at a given true road speed, the geometric expectation with the larger tire is about 2,404 rpm, assuming the transmission, axle ratio and driveline state are unchanged.
Speedometer effect
A vehicle calibrated for the original tire assumes a certain distance per wheel revolution. A larger tire can make actual speed higher than the same indicated speed, while a smaller tire can make actual speed lower. The simple percentage relationship is useful for planning, but modern vehicles may combine wheel-speed sensors, GPS, transmission data or software corrections.
Nominal size versus rolling circumference
The 225/45R17 formula produces a nominal diameter from section width, aspect ratio and rim diameter. A real tire's rolling circumference also depends on construction, measuring rim, tread depth, inflation pressure, load and temperature. If calibration matters, a measured loaded rollout is more representative than nominal geometry.
Why width alone tells you very little
A wider tire is not automatically taller. For example, increasing section width while reducing aspect ratio can keep overall diameter nearly unchanged. Always calculate the complete size rather than judging by the first number in a metric tire designation.
Fitment is more than diameter
Two tires with nearly identical diameter can still differ in section width, load index, speed rating, required wheel width and clearance. Suspension, bodywork, brake hardware and manufacturer-approved sizes matter separately from gearing math. A small speedometer difference is not proof that a tire is suitable.
When to use each GIIRR tool
Use the Tire Diameter Calculator to convert a metric size into nominal geometry, the Speedometer Error Calculator to compare original and replacement sizes, and the RPM-to-Speed or Speed-to-RPM calculators when you also know transmission and final-drive ratios. Keeping those questions separate makes it easier to see which assumption is responsible for each result.
GIIRR is a calculation aid, not a substitute for manufacturer fitment limits, service data or professional engineering review.