Enter a tire size, final-drive ratio and gearbox ratios to see RPM at 30, 45, 60, 70 and 80 mph, and how a tire-size change shifts effective gearing.
The default gearbox figures are an example 6-speed, not a specific transmission. Replace them with the ratios from your owner’s manual or a factory service manual. Leave a box empty to omit that gear. For a single-gear check, fill only that ratio.
| Current tire diameter | – |
|---|---|
| New tire diameter | – |
| Effective final drive (new tire) | – |
| Change | – |
RPM uses 1056/π (about 336.13) and the size-code diameter. Totals are engine RPM in each gear with the torque converter locked or a manual clutch engaged. Automatic slip on launch will read higher. The address-bar link saves this setup.
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RPM = mph × gear ratio × final drive × 1056 ÷ π ÷ tire diameter (inches). A 225/45R17 is 24.97 inches in overall diameter from the size code. Effective final drive after a tire change = stock final drive × (old diameter ÷ new diameter).
See the matching speedometer error with the speedometer error calculator, or compare two sizes in full with the tire size calculator.
Taller tires lower cruise RPM and can make the vehicle feel longer-geared. Shorter tires do the opposite. That is useful on a highway-heavy commuter and sometimes unwelcome on a truck that just added several inches of tire height — a common reason to consider a different ring-and-pinion. This page does not tell you what axle ratio to buy; it shows the math so you can talk to a driveline shop.
Engine RPM ≈ speed (mph) × gear ratio × final-drive ratio × 1056 ÷ π ÷ tire diameter in inches. The familiar “336” constant is 1056 ÷ π rounded. A 225/45R17 tire is about 24.97 inches in overall diameter from the size code.
RPM at a given speed drops, the same way a numerically lower (longer) final drive would. Throttle response and grade climbing can feel lazier, and your speedometer will also read low unless it is recalibrated.
It is the final drive that would give the same RPM as your new tire, with the old tire. Effective final drive = stock final drive × (old diameter ÷ new diameter). A 3.73 axle with tires 3% taller behaves like about a 3.62.
It depends on how far you moved and how you use the vehicle. A small plus-size on a car often does not need a new axle ratio. A large jump on a truck (for example several inches of tire height) is a common reason to consider a shorter, numerically higher ring-and-pinion. Ask a driveline shop.
Yes, if you enter the actual gear ratios (including any overdrive and, if you want stall effects, note that a torque converter can let engine RPM rise above the calculated wheel-driven figure). For lock-up cruising, the formula is a good estimate. For hard launches, expect higher recorded RPM than the table shows.
Diameter is computed from the size code and varies slightly by brand, load and wear. This calculator uses 1056/π (about 336.13) instead of rounding to 336. The difference is a few RPM, much smaller than tire variation.