Calculator 02 / Gear Ratio & Tires
Gear Ratio & Tire Calculator
Find engine RPM at a given road speed for your gear ratio and tire size, or how much a tire size change throws off your speedometer.
How this calculator works
The 336 constant in the RPM formula comes from converting tire revolutions per mile into RPM at a given road speed, assuming no tire slippage. It's the same constant used across most off-road and gearing calculators. For speedometer error: most factory speedometers are calibrated to the stock tire's rolling diameter, so fitting larger tires makes your actual speed higher than what's displayed (and smaller tires make it read fast) — useful to know before trusting cruise control or a speed-dependent inspection.
Why does tire diameter matter more than just the tire's labeled size?
Two tires with different tread patterns or aspect ratios can have the same nominal size label but different actual overall diameters — measure or look up the actual diameter for an accurate result.
What transmission ratio should I use for a general "cruising RPM" check?
Use 1.00 if your top gear is a direct 1:1 (common on many manual and automatic transmissions), or check your transmission's spec sheet if it has an overdrive ratio below 1.00.
Estimates only, and assume zero tire slippage. Confirm your vehicle's actual axle ratio and tire diameter before making gearing decisions.