Top Speed Calculator - From Horsepower, Weight and Drag

    Find the speed where air drag uses up every horsepower: enter power, weight, drag coefficient and frontal area, and see the power each speed takes on the way up.

    Parameters

    Enter data for calculations

    As in your spec sheet

    30 to 2,000 hp (22 to 1,500 kW)

    Share of power reaching the wheels

    For weight and frontal area

    1,100 to 11,000 lb (500 to 5,000 kg)

    From 0.15 to 0.80

    10.8 to 43 sq ft (1.0 to 4.0 m2)

    Form progress0 / 7 fields

    💡 Fill in all required fields to unlock the calculate button

    Top speed from horsepower, weight and drag

    At some speed the air pushes back as hard as the engine can push forward, and the car stops gaining. That speed is what this top speed calculator finds. It takes the rated power in hp, kW or PS, the weight, the drag coefficient (Cd), the frontal area and the drivetrain efficiency, and solves the point where the power at the wheels equals the power needed against air drag and rolling resistance. Next to the answer you get the power each speed from 30 mph up actually takes, and how much a 10% change in power, drag area or weight would move the result.

    Quick start: 200 hp, an automatic, 3,400 lb, Cd 0.28 and 23 sq ft of frontal area give 155.1 mph (250 km/h). At 65 mph the same car needs only 21.3 hp from the engine, about 11% of what it has.

    Seven inputs and where to find them

    1. Engine power and unit - rated power from the spec sheet, in hp, kW or PS (metric horsepower, common in European data). From 30 to 2,000 hp.
    2. Units - US (lb, sq ft) or metric (kg, m2) for the next two fields.
    3. Weight - curb weight plus the people on board. It barely changes the top speed, but it sets the rolling resistance.
    4. Drag coefficient (Cd) - from 0.15 to 0.80, published by many manufacturers and in road tests.
    5. Frontal area - the silhouette seen from straight ahead. When no figure is published, width x height is an upper bound, because the body is rounded and the gap under it does not count.
    6. Drivetrain efficiency - newer manual 96%, older manual 91%, automatic 89%, or your own value from 50 to 100% (a dyno sheet with both crank and wheel figures gives it directly).

    Seven vehicles run through the calculator

    Typical inputs for each body style, not any one model. Change a number to match your own vehicle and the result follows.

    An older small hatchback

    Inputs: 90 hp, older manual, 2,200 lb, Cd 0.35, 20 sq ft
    Result: 116.3 mph. At 100 mph the engine already gives 66% of its power.

    A compact with a newer manual

    Inputs: 150 hp, newer manual, 2,800 lb, Cd 0.32, 22 sq ft
    Result: 140.7 mph, with 144 hp at the wheels.

    A slippery midsize sedan

    Inputs: 200 hp, automatic, 3,400 lb, Cd 0.28, 23 sq ft
    Result: 155.1 mph. Air drag is 91% of the resistance at that speed.

    A midsize SUV

    Inputs: 300 hp, automatic, 4,500 lb, Cd 0.35, 28 sq ft
    Result: 155.0 mph: 100 hp more than the sedan buys no extra speed against a larger, blunter front.

    A full-size pickup

    Inputs: 400 hp, automatic, 5,200 lb, Cd 0.45, 33 sq ft
    Result: 149.3 mph. A roof load adding 10% to the frontal area (36.3 sq ft) takes it to 144.7 mph.

    A sports coupe

    Inputs: 450 hp, automatic, 3,800 lb, Cd 0.34, 21 sq ft
    Result: 198.9 mph, at 8.4 lb per rated hp.

    1,000 horsepower

    Inputs: 1,000 hp, automatic, 3,600 lb, Cd 0.36, 21 sq ft
    Result: 257.1 mph. More than twice the coupe's power adds 58 mph, because drag power grows with the cube of speed.

    How fast the power bill grows

    The sedan from the quick start, speed by speed. Engine power is the wheel power divided by 0.89. Going from 55 to 100 mph, less than double the speed, takes about four times the power.

    Speed At the wheels From the engine Share of 200 hp
    55 mph13.2 hp14.8 hp7%
    65 mph18.9 hp21.3 hp11%
    75 mph26.4 hp29.6 hp15%
    100 mph54.0 hp60.7 hp30%
    120 mph87.6 hp98.5 hp49%
    140 mph133.6 hp150.2 hp75%
    155.1 mph (top)178.0 hp200.0 hp100%

    Ten percent here or there

    Each change applied alone to the same sedan. Power and drag area trade almost one for one; weight hardly registers, because at top speed nine tenths of the resistance is air.

    Change Top speed Difference
    None (200 hp, Cd x A 6.44 sq ft)155.1 mph-
    10% more power160.5 mph+5.3 mph
    10% less drag area160.5 mph+5.4 mph
    10% more drag area150.4 mph-4.7 mph
    10% more weight154.6 mph-0.5 mph

    Top speed questions with numbers attached

    How much horsepower does it take to go 200 mph?
    It depends on the drag area far more than on weight. The coupe above, at Cd x A of 7.14 sq ft, gets to 198.9 mph on 450 hp. The pickup, with twice the drag area, would need far more: at 140 mph it already uses 333 of its 400 hp.
    Why does doubling the power not double the top speed?
    Air drag rises with the square of speed, and the power to overcome it is drag times speed, so it rises with the cube. Twice the power gives roughly the cube root of 2, about 1.26 times the speed, a little more once rolling resistance is counted.
    My car's official top speed is lower than the result. Why?
    Often an electronic speed limiter, or gearing that runs out of rpm before drag catches up. Check the second with the Transmission Gear Ratio Calculator: if the redline speed in the top gear is below this result, the gearing sets the limit. Wind and a grade also lower the real figure. At altitude the thinner air cuts drag but also the power of an engine without a turbo, which this model does not cover.
    Where do the drivetrain percentages come from?
    From hub-dynamometer measurements of more than 600 cars by the Swedish test institute Rototest, analyzed by Zybtsev and Voroshylov: about 91% for older and up to 96% for newer manual gearboxes, about 89% for automatics. If you have a dyno sheet for your own car, enter its wheel-to-crank ratio as your own value.
    Does a lighter car have a higher top speed?
    Only a little. Weight enters through rolling resistance, which at top speed is about a tenth of the total for the sedan above: 10% more weight costs it 0.5 mph, while 10% more drag area costs 4.7 mph. Weight matters far more for acceleration and climbing than for the last few mph.
    Can I enter wheel horsepower from a dyno run?
    Yes. Enter the wheel figure as the power and choose your own efficiency of 100%, so nothing is taken off twice. If the sheet shows both a crank and a wheel number, divide wheel by crank for the efficiency instead and keep the rated power.

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