Engine RPM Calculator
Calculate engine rpm from road speed, gearing and tire size.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Estimates engine speed from road speed, gear ratio, final drive and nominal tire dimensions. It assumes no tire growth or slip; actual loaded rolling circumference, gear selection and transmission behavior can change rpm.
Worked example
calculated with the default values below
Road speed 60 mph · Gear ratio 1 :1 · Final drive ratio 3.73 :1 · Tire width 225 mm · Aspect ratio 45 % · Wheel diameter 17 in Engine speed 3,012 rpm · Tire diameter 24.97 in · Tire circumference 78.45 in · Speed at 2,000 rpm in this gear 39.8 mph
General background references
Disclaimer
These calculators give estimates for general information only. Real fuel consumption, performance and range vary with driving style, load, terrain, temperature and vehicle condition. Do not rely on these results for towing limits, payload or braking decisions — use the rating plate of your vehicle.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
RPM = v × G × F × 1056 ÷ (π × (D + 2 × w × a ÷ 100 ÷ 25.4))
- v
- Vehicle speed (mph)
- G
- Gear ratio
- F
- Final drive ratio
- w
- Tire width (mm)
- a
- Aspect ratio (%)
- D
- Rim diameter (in)
Tire diameter (in) = rim diameter + 2 × (width × aspect ratio ÷ 100) ÷ 25.4. Engine rpm = mph × gear ratio × final drive × 1,056 ÷ tire circumference (in). Assumes no tire slip or growth.
Frequently asked questions
Why does a taller tire lower engine rpm at the same speed?
Tire diameter sets the circumference the wheel rolls per turn, so a bigger tire covers more ground per engine revolution. The model builds diameter from rim size plus twice the sidewall, where sidewall equals width times aspect ratio over 100, then divides circumference into road speed times the gear and final-drive totals times 1,056. At 225/45R17 the tire is about 25 inches, giving roughly 3,010 rpm at 60 mph in a 1.0 to 3.73 gearing.