Top Speed Calculator: Find Your Speed Fast

How fast will a car actually go? The answer is whichever comes first: the gearing limit (redline in top gear) or the drag limit (the speed where engine power can no longer overcome air resistance). This top speed calculator checks both. Enter your gearing to find the theoretical maximum at redline, enter horsepower and weight to find the drag-limited speed, and compare the two to see which one actually caps your car. A third mode covers measured runs from distance and time.

Theoretical estimates only. Real top speed is also limited by the rev limiter, tire speed ratings, drivetrain losses, weather, and road conditions. Never attempt to verify top speed on public roads; use a closed course or a dynamometer. Not engineering advice.

Why top speed has two different limits

Think of it as two ceilings. The gearing ceiling is pure mechanics: at redline in your tallest gear, the wheels turn at a fixed RPM, and the tire circumference converts that into road speed. Nothing about power matters here, only ratios and tire size. The drag ceiling is physics: air resistance grows with the square of speed, so the power needed grows with the cube of speed. Doubling your speed needs roughly eight times the power. Your car's actual top speed is whichever ceiling is lower, which is why a 400 hp sports car can be gearing-limited well below what its power could theoretically push.

The gearing formula (and where 336 comes from)

The classic gearhead formula is: mph = (engine RPM x tire diameter in inches) / (overall gear ratio x 336), where the overall ratio is the gear ratio multiplied by the final drive ratio. The 336 is not magic; it is the constants of the full equation (60 minutes per hour, pi for tire circumference, 63,360 inches per mile) reduced to a single rounded number. This calculator uses the unreduced form with the exact constants, which is why its answers differ from the 336 shortcut by a fraction of a percent. Tire diameter comes from your tire size: diameter = wheel diameter + 2 x (width x aspect ratio / 100) / 25.4. For a 245/40R18 that works out to 25.72 inches.

The horsepower formula done properly

Some online tools estimate top speed with a linear shortcut like (horsepower x 375) / (weight x drag coefficient). That is not how air works. The honest equation is P = 0.5 x air density x CdA x v^3 + rolling resistance x weight x v, and this calculator solves it numerically for v. The cubic term is the whole story: it is why adding 50 hp at 120 mph buys you far less extra speed than adding 50 hp at 60 mph. Rolling resistance (typical coefficient 0.012 for car tires) is included, which the shortcut tools skip entirely.

Worked example: a 400 hp sports car

Take a sports car with a 6,500 RPM redline, 245/40R18 tires (25.72 in diameter), a 3.73 final drive, and gear ratios 3.80, 2.31, 1.69, 1.32, 1.00, 0.80. In top gear the overall ratio is 0.80 x 3.73 = 2.984, so the gearing ceiling is 6,500 x 25.72 / (2.984 x 336) = 166.7 mph (268.2 km/h). The per-gear table shows the rest: 1st tops out at 35.1 mph, 2nd at 57.7, 3rd at 78.9, 4th at 101.0, 5th at 133.3.

Now the drag side: 400 hp, 3,300 lb, sports car drag area of 7 ft2. Solving the cubic gives a drag-limited speed of 199.5 mph. Since 166.7 is lower, this car is gearing-limited: it will hit the rev limiter in 6th before drag stops it. Both numbers were produced by the calculator above from exactly these inputs.

Reading the per-gear table

The table answers the question "how fast in each gear at redline," which is useful far beyond top speed: it tells you your shift points, whether a gearing change will fix a too-tall 1st gear, and what RPM you will cruise at on the highway. If you change tire size, every row moves: taller tires raise speed at a given RPM (and make the speedometer read low), shorter tires do the opposite. The effective-ratio rule of thumb: new ratio = old ratio x new tire diameter / old tire diameter.

Common mistakes

The biggest is using the nominal tire diameter from the sidewall math without checking: a worn or underinflated tire runs smaller than the formula says, and manufacturers' sizes are nominal. The second is forgetting the final drive ratio or using only the transmission gear; the overall ratio is gear x final drive, and leaving one out can double your answer. The third is torque converter slip in automatics without lockup, which costs a few percent versus the locked-drivetrain math. The fourth is comparing the gearing number against reality and wondering why the car will not get there: drag, the rev limiter, and the tire speed rating all get a vote.

Frequently asked questions

What limits a car's top speed, gearing or power?

Whichever is lower. Compute the gearing ceiling (redline in top gear) and the drag ceiling (power vs air resistance); the smaller number is the real limit. Most stock sports cars are gearing-limited; most underpowered cars are drag-limited.

Where does the 336 in the gearing formula come from?

It is the reduced form of the full equation: engine RPM x 60 minutes x pi x tire diameter, divided by gear ratio x 63,360 inches per mile. This calculator uses the unreduced constants directly, which is slightly more accurate.

How do I find my tire diameter from the sidewall?

Diameter in inches = wheel diameter + 2 x (width in mm x aspect ratio / 100) / 25.4. A 245/40R18 works out to 25.72 inches. You can also measure the mounted, inflated tire directly for the most accurate number.

Why does doubling power not double top speed?

Because aerodynamic drag grows with the square of speed, the power required grows with the cube. Roughly eight times the power is needed to double the speed, which is why top speed gains get expensive fast.

What is a good drag area (CdA) for my car?

Typical values in square feet: sports car about 7, sedan about 8.5, SUV about 12, pickup about 14, motorcycle with rider about 6. Use the presets, or enter a measured CdA if you have one.

Does the calculator account for hills or wind?

No. It assumes flat ground and still air. A headwind or grade lowers the drag-limited speed; a tailwind raises it.

Can I use this for motorcycles?

Yes. Use the motorcycle drag-area preset and enter the bike plus rider weight. The gearing math is identical.

What about electric cars?

The gearing mode works if you know the motor redline, the single gear ratio, and the final drive (many EVs are single-speed). The power mode works the same as for any car. Note that many EVs are software-limited below both ceilings.

Why is my speedometer off after changing tire size?

The speedometer assumes the stock tire diameter. Actual speed = indicated speed x new diameter / old diameter. Taller tires make the speedo read low.