0-60 & Power-to-Weight Calculator

What this calculator does

Give it horsepower, weight and a drivetrain and it estimates how long a car
takes to reach 60 mph, along with the quarter mile, trap speed, a drag-limited
top speed and power-to-weight both ways round. Look your car up instead and it
fills the numbers in for you.

It reports a range rather than a single figure, because a single figure would
be a lie. Two cars with the same horsepower and the same weight can be a second
apart to 60, and no calculator that prints one number to two decimal places is
telling you about that second.

How to work out power-to-weight by hand

Divide one by the other. In the United States the usual form is pounds per
horsepower: take the curb weight and divide it by the horsepower. A 3,500 lb car
with 300 hp is 11.7 lb/hp, and lower is quicker.

Most of the rest of the world uses the ratio the other way up, as horsepower
per ton or kilowatts per tonne, where higher is quicker. They are the same
number. The calculator shows all three.

Two details change the answer. Use curb weight, not gross vehicle weight
rating, which is the maximum the car is allowed to weigh with people and luggage
in it. And add roughly 180 lb if you want to match the way a magazine measures
it, because their figures include a driver.

What counts as a good ratio

Why power-to-weight does not give you 0-60

This is the part every other calculator skips, and it is the reason they
disagree with each other.

Start with the physics floor. If every horsepower reached the road for the
whole run, the time to 60 mph would be:

t = 0.0000608 × weight (lb) × 3600 / horsepower

That constant is not fitted to anything. It is the unit conversions
rearranged, and it is exact. For a 3,500 lb car with 300 hp it gives 2.55
seconds. The same car in the real world takes about 5.5. Something is eating
more than half the performance, and a calculator is only useful if it can say
what.

Part of it is the drivetrain, which loses 12 to 18 percent between the
crankshaft and the road. Part of it is the engine, which only makes peak power
in a narrow band and spends most of the run somewhere below it. Part of it is
gear changes, which cost about half a second in a manual and a tenth in a
dual-clutch. And a large part of it, larger than most people expect, is that the
tyres cannot use the power.

The grip ceiling

Below some speed a car is limited by traction, not by power. Push harder and
the tyres slip. Above that speed there is enough load on the tyres to take
everything the engine makes, and power becomes the constraint. Where the
crossover sits depends on how much power there is, and it moves in the direction
you would not guess.

An ordinary car spends a little under half the run to 60 grip-limited. A car
under 7 lb/hp spends nearly all of it that way. The more power a car has, the
less of its 0-60 is actually decided by power.

The first row of that table rests on a small sample, because the database is
weighted toward cars people look up rather than cars people own. The trend
across the other rows does not depend on it.

At the far end this stops being a tendency and becomes a wall. Sixteen cars in
our database are grip-limited for the entire run to 60, the Bugatti Chiron among
them. For those cars acceleration to 60 is set by the coefficient of friction and
nothing else. Give a Chiron another 150 hp and the 0-60 time does not move,
which is a result the model produces rather than an opinion we hold.

What else changes a real 0-60

Drivetrain, and not in the simple way. Under acceleration weight shifts
rearward. On a rear-drive car it shifts onto the driven wheels, so grip rises as
the car pulls harder. On a front-drive car it shifts off them, so grip falls
exactly when it is needed. That single difference is worth around 25 percent in
launch grip between two otherwise identical cars.

All-wheel drive is not automatically quicker. It costs about four points of
drivetrain efficiency through the transfer case, and it only earns that back
when grip is the binding constraint. A 150 hp all-wheel-drive car is slower to
60 than the same car in rear-drive, because it is paying for traction it never
runs out of.

Gearing decides how many times the driver has to stop accelerating. A car that
reaches 60 in second beats an otherwise identical car that needs third, which is
why manufacturers gear high-powered cars taller than the power alone would
suggest.

Then there is the surface. Published times are set by professional drivers on
prepared surfaces at optimal temperatures, usually after several attempts. The
calculator shows a second figure for a cold public road, about 15 percent
slower. That figure is closer to what you will see.

Rollout, and why two sources disagree

Drag-strip timing starts the clock after the car has moved one foot. Tesla and
several other manufacturers quote 0-60 on that basis. It is worth about a quarter
of a second, which on a three-second car is ten percent, and it is the single
largest reason two reputable sources report different times for the same
vehicle. Our database records which convention each figure uses.

Why electric cars break the formula

Run an electric car through a combustion model and you overestimate its 0-60
by around 60 percent. Three things cause that, and none of them is extra
power.

An electric motor makes peak torque from zero rpm, so there is no waiting for
the engine to come on song. There is one fixed gear, so no time is lost changing.
And the battery sits low and flat, which helps the car put its torque down. The
calculator uses a separate set of constants for electric cars, and on our data
that cuts the error from 15.6 percent to 5.6 percent.

The offsetting problem is that a quoted combined motor output is not what the
battery will actually deliver on a warm day at half charge. Electric cars are the
noisiest part of our dataset for that reason.

How accurate this is

We fitted the model against 283 vehicles with a published 0-60 time, held back
a fifth of them at random and measured the error only on the cars the model had
never seen.

Half of those land within 4.9 percent of their published figure.
At five seconds that is about a quarter of a second. Nine in ten land within 17.5
percent.

The range shown beside the result is an 80 percent interval, and it is
rechecked every time the tool is built. It currently contains 80.6 percent of the
cars we can verify, which is what an 80 percent range is supposed to mean. It is
wider for high-powered rear-drive cars, because those are genuinely harder to
predict, and the tool widens it rather than hiding the fact.

Seven constants in the model are fitted to that data. Everything else is
either exact physics or a published engineering figure, and the two are kept
separate on purpose. One of the fitted numbers is worth pointing at: the
conversion between the SAE gross horsepower ratings used in America before 1972
and modern net ratings came out at 0.76, close to the figure the industry has
always quoted. Nothing told the model to land there.

The weakest number is grip. It absorbs tyre compound, surface preparation,
temperature and driver technique in one coefficient, and it varies across the
calibration set. Where this tool is wrong, grip is usually the reason.

If you want the other half of the picture, what these cars cost when they
were new and what that money is worth now, our
car inflation
calculator runs the same kind of arithmetic over eighty years of prices.

Questions

How do you calculate 0-60 from horsepower?

You cannot do it from horsepower alone, and that is the honest answer. The physics floor is simple enough: if every horsepower reached the road the whole time, the time to 60 mph would be 0.0000608 x weight x 3600 / horsepower, which for a 3,500 lb car with 300 hp is 2.55 seconds. The same car really takes about 5.5 seconds. The gap is gearing, traction, drag and the fact that an engine only makes peak power in a narrow band. This calculator models those separately rather than hiding them in one fudge factor, which is why it also needs to know the drivetrain and the gearbox.

How do you calculate power-to-weight ratio?

Divide one by the other. In the United States the usual form is pounds per horsepower: curb weight divided by horsepower, so a 3,500 lb car with 300 hp is 11.7 lb/hp, and lower is quicker. The rest of the world tends to use the ratio the other way up, as horsepower per ton or kilowatts per tonne, where higher is quicker. They are the same number. Use curb weight, not gross vehicle weight, and add about 180 lb if you want to match the way a magazine measures it, because their figures include a driver.

What is a good power-to-weight ratio?

Around 20 lb/hp is an ordinary modern car. Under 12 lb/hp feels genuinely quick. Under 8 is sports car territory, under 5 is supercar territory, and the quickest production cars ever built sit near 2.5. For reference, a 1970 Chevelle SS 454 is about 11 lb/hp on its gross rating and a 2016 Miata is about 15 — and the Miata is not far behind to 60, which tells you the ratio is not the whole story.

Does power-to-weight ratio determine 0-60 time?

Less than most people think, and less the more power you have. Below roughly 60 mph a car is limited by either power or grip, whichever runs out first, and which one binds depends on how much power there is. Across the cars we checked, an ordinary car over 14 lb/hp spends about a third of its run to 60 limited by grip rather than power. A modern car under 7 lb/hp spends about 84% of it that way. Past a point, adding power stops buying time, because the tyres cannot take any more of it.

How accurate is this 0-60 calculator?

We fitted the model against 283 vehicles with a published 0-60 time, held back a fifth of them at random, and measured the error only on the ones the model had never seen. Half of those land within 4.9% of their published figure — about a quarter of a second at five seconds — and nine in ten land within 17.5%. The range shown beside the result is an 80% interval, and it is checked on every build: it currently contains 80.6% of the cars we can verify, which is what an 80% range is supposed to mean.

Why is my car slower than the calculator says?

Almost certainly because the published times this model is calibrated on are professional launches on a prepared surface at an optimal temperature, and your driveway is not that. The tool shows both figures for exactly this reason: the magazine number, and a street number about 15% slower that assumes a cold road and a normal launch. Beyond that, tyres, fuel, altitude, temperature, how full the tank is and whether anyone else is in the car all cost tenths.

Do manufacturers subtract rollout from 0-60 times?

Many do. Drag-strip timing starts the clock after the car has moved one foot, and Tesla among others quotes its figures on that basis. It is worth roughly 0.25 seconds, which on a three-second car is ten percent, and it is the single largest reason two sources disagree about the same vehicle. Our dataset records which convention each published figure uses, and the model is calibrated against true standing starts so that cars quoted both ways can be compared.

Why are electric cars quicker than their horsepower suggests?

Three reasons, and none of them is extra power. An electric motor makes its peak torque from a standstill instead of somewhere near the redline, so there is no waiting for the engine to come on song. There is one fixed gear, so no time is lost changing. And the battery sits low and flat, which helps the car put its torque down. Run an EV through a combustion model and you overestimate its 0-60 by around 60%, which is why this calculator uses a different parameter set for them.

Why is a 1970 muscle car slower than its horsepower suggests?

Partly the tyres and partly the horsepower figure itself. Before 1972 American engines were rated SAE gross, measured on a stand without accessories, air cleaner or production exhaust; the same engine rated the modern way gives about three quarters of the number. The bias-ply tyres of the period also had roughly a quarter less grip than a modern radial, so a big-block on a cold road spent much of the run to 60 turning power into smoke. A 450 hp rating from 1970 is not a 450 hp rating from today.