Find engine RPM at any speed - or find the speed at a given RPM - using your transmission gear, final drive, and tire diameter.
Don't know your tire diameter? Use the Tire Size Calculator first.
| Speed (mph) | Engine RPM |
|---|
Enter every gear (or load a transmission preset above - it fills these too) and your redline to see top speed in each gear and the RPM you land at after every upshift.
Uses the final drive and tire diameter from the inputs above. Leave unused gears blank. Top speed in the last gear is gearing-limited speed - real top speed is usually drag-limited first.
Your engine's RPM is determined by how fast the wheels are spinning, multiplied by all the gear reductions between the crankshaft and the tire contact patch. The formula uses two ratios: the transmission's ratio for the selected gear, and the final drive (differential) ratio.
Where Total Ratio = Transmission Gear Ratio × Final Drive Ratio. The unit-conversion constant is about 336.135; 336 is the familiar rounded version.
Calculated RPM uses nominal tire diameter. Loaded rolling circumference varies with tire model, pressure, load, speed, and wear, and transmission or converter slip can also change observed RPM.
A numerically higher final drive ratio (e.g., 4.10 vs 3.23) multiplies torque to the wheels more aggressively, improving acceleration but raising RPM at cruise speed and hurting fuel economy. Swapping final drive gears is one of the most effective ways to tune the power delivery character of a car.
A taller tire effectively lengthens your gear ratios - your RPM will drop at any given speed, which can make the car feel lazier off the line. A shorter tire has the opposite effect. Always recalculate after a tire size change.
There's no universally "best" final drive - it's a trade between acceleration and cruise comfort. A numerically higher final drive (say 3.73 → 4.10) keeps the engine higher in its powerband out of slow corners and tightens the spacing between gears, which is what you want on a tight track or in autocross where you rarely use top gear. The penalty is higher RPM and worse fuel economy on the highway, plus a lower gearing-limited top speed. A numerically lower final drive (or simply relying on a tall overdrive gear) does the reverse: relaxed, quiet, economical cruising at the cost of lazier in-gear acceleration. Builders who want both often leave the diff alone and tune tire diameter instead, since a slightly shorter tire mimics a higher final drive without opening the differential.
Take a car in 6th gear (ratio 1.000) with a 3.73 final drive on a 25.7-inch tire, cruising at 70 mph. First the total ratio, then the RPM:
With a 4.10 final drive, the same nominal calculation rises to about 3,755 RPM. A 26.2-inch nominal tire with the original 3.73 ratio calculates near 3,349 RPM. These are geometric comparisons, not a promise of observed road RPM or fuel economy.
RPM = (speed in mph × transmission ratio × final-drive ratio × 336.14) ÷ tire diameter in inches. The conventional 336 value is rounded; real RPM also depends on loaded rolling circumference and slip.
A higher final drive (e.g. 4.10 vs 3.23) multiplies torque to the wheels more aggressively, improving acceleration - at the cost of higher cruise RPM and worse fuel economy. It is one of the most effective ways to change a car's power delivery character.
Total drive ratio is the transmission gear ratio multiplied by the final drive (differential) ratio. It represents the complete reduction between the crankshaft and the wheels in the selected gear. For example, a 2.106 third gear with a 3.73 final drive gives a total ratio of 7.855 to 1, meaning the engine turns 7.855 times for every wheel revolution.
Tire diameter sits in the denominator of the RPM formula, so a larger tire lowers engine RPM at any given speed and a smaller tire raises it. A taller tire effectively lengthens every gear, dropping cruise RPM and fuel use but dulling acceleration; a shorter tire shortens the gearing for quicker response at the expense of higher highway RPM. Recalculate any time you change tire size.
For track and autocross use, a numerically higher (shorter) final drive keeps the engine in its powerband through tight corners and shortens the gaps between shifts, trading top speed you rarely reach for sharper acceleration. For highway and touring, a numerically lower (taller) final drive or a tall overdrive gear lowers cruise RPM for quieter, more economical running. Many builders compromise by keeping stock gearing and adjusting tire diameter instead.
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