RPM at Speed Calculator: Cruising RPM in Top Gear
How many rpm will my engine turn at 70 mph or 120 km/h? Enter your cruising speed, the gear you are in, the final drive ratio and the tire size to get the engine speed for that single gear. Need a table of top speed in every gear at the redline instead? Use the speed from RPM calculator.
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Formula
- Tire diameter:
D (mm) = rim (in) × 25.4 + 2 × width (mm) × aspect ÷ 100 - Wheel speed:
wheel rpm = speed (m/s) × 60 ÷ (π × D in metres) - Engine speed:
engine rpm = wheel rpm × gear ratio × final drive - Reverse (the Solve for switch):
speed = (rpm ÷ (gear × final drive)) × π × D ÷ 60
Worked example
225/45 R17 tires have D = 17 × 25.4 + 2 × 225 × 0.45 = 634.3 mm. At 70 mph (31.29 m/s) the wheels turn 942.2 rpm. In a 0.74 top gear with a 3.73 final drive the engine runs at 2600.7 rpm. Going the other way, 2500 rpm in the same gear is 67.29 mph.
Use this when
You know the speed and the gear and want to know how hard the engine is working: checking highway cruising rpm, comparing an overdrive gear, or seeing what a different final drive or tire does to revs.
Use another calculator instead when
- You want the speed in every gear at one rpm: use the speed from RPM calculator.
- You have ring and pinion tooth counts and need the axle ratio first: use the gear ratio calculator.
- You only want the tire diameter or speedometer error: use the tire size calculator.
Typical values
Final drive ratios of roughly 2.5 to 4.5 and top-gear ratios of roughly 0.6 to 1.0 are typical for road cars. Cruising at about 1,800 to 3,000 rpm is typical for petrol cars at highway speed.
Common mistakes
- Entering the first-gear ratio when you mean the gear you cruise in.
- Forgetting the final drive, which multiplies every gear.
- Mixing mph and km/h: set the speed unit to match the number you typed.
- Expecting exact agreement with the tachometer. The model uses nominal tire diameter and no slip, so a torque converter or worn tires will shift the result.
Frequently asked questions
How do I calculate engine RPM from speed?
Convert speed to wheel revolutions per minute using the tire circumference, then multiply by the gear ratio and the final drive ratio. At 100 km/h on 205/55 R16 tires the wheels turn 839.6 rpm, so in a 1.0 gear with a 3.5 final drive the engine runs at 2938.5 rpm.
How do I get tire diameter from 205/55 R16?
Diameter is the rim in millimetres plus twice the sidewall height. The sidewall is the width times the aspect ratio: 205 × 0.55 = 112.75 mm. So 16 × 25.4 + 2 × 112.75 = 631.9 mm.
How do I find speed from RPM?
Divide engine RPM by gear ratio and final drive to get wheel RPM, then multiply by the tire circumference. At 3000 rpm in a 0.8 gear with a 3.5 final drive that is 127.62 km/h (79.30 mph).
Why does my speedometer differ from this result?
This uses the nominal unloaded tire diameter and no slip. Tire wear, pressure, load and the transmission's real ratios all shift the number slightly, and speedometers often read a few percent high.
What RPM should my engine be at 70 mph?
It depends on the gear, final drive and tire. A 225/45 R17 tire with a 0.74 sixth gear and a 3.73 final drive turns the engine at 2600.7 rpm at 70 mph. Many modern cars cruise at roughly 1,800 to 3,000 rpm in top gear, but that range is typical, not a rule.
How do I calculate RPM at 120 km/h?
Find the wheel rpm from the tire circumference, then multiply by the gear and final drive ratios. A 205/55 R16 tire turns 1007.5 rpm at 120 km/h, so with a 0.8 gear and a 3.5 final drive the engine runs at 2820.9 rpm.
What happens to cruising RPM if I fit bigger wheels?
A taller tire covers more road per turn, so the engine turns slower at the same speed. Going from 225/45 R17 (634.3 mm) to 245/40 R18 (653.2 mm) drops the 70 mph figure from 2600.7 to 2525.5 rpm, about 3 percent lower.