When comparing car catalogs,
you might come across the term
"power-to-weight ratio."
The power-to-weight ratio quantifies the relationship between a car's weight and its maximum output.
Rather than just looking at the maximum output,
it makes it easier to compare "how much weight that horsepower is moving."
The calculation method is very simple.
However, when actually comparing cars,
should you use the curb weight or the gross vehicle weight?
What's the difference between kg/PS and PS/t?
Which output should you use for EVs and hybrids?
Does the same power-to-weight ratio mean the same acceleration performance?
There are several points you should know.
This time, rather than a general explanation of "what is the power-to-weight ratio," we will focus on how to actually calculate it yourself and compare cars.
Power-to-weight ratio calculation formula
The power-to-weight ratio commonly used in Japanese automotive articles is calculated as:
Curb weight ÷ Maximum output
The unit is generally
kg/PS.
For example,
for a car with a curb weight of 1,200kg
and a maximum output of 120PS,
1,200 ÷ 120 = 10.
The power-to-weight ratio is
10kg/PS.
Simply put,
this means "1 horsepower is responsible for 10kg."
GAZOO also explains the power-to-weight ratio of general automobiles as "vehicle weight per horsepower."
Is a smaller kg/PS better performance?
When comparing under the same conditions, a smaller kg/PS means the car has more output relative to its weight.
For example,
Car A
1,000kg
100PS
would be
1,000 ÷ 100 = 10kg/PS.
On the other hand,
Car B
1,000kg
200PS
would be
1,000 ÷ 200 = 5kg/PS.
Both cars move a 1,000kg body:
Car A with 100PS.
Car B with 200PS.
Therefore, if you only compare output and weight, Car B with 5kg/PS is more advantageous.
The important thing is not just
"whether the horsepower is high,"
but to look at
"how many kg 1 horsepower is responsible for."
First, let's actually calculate three cars
Calculating makes the concept easier to understand.
| Vehicle | Curb Weight | Max Output | Power-to-weight ratio |
|---|---|---|---|
| Car A | 1,000kg | 100PS | 10kg/PS |
| Car B | 1,500kg | 150PS | 10kg/PS |
| Car C | 1,500kg | 200PS | 7.5kg/PS |
Car A and Car B have a maximum output difference of 50PS.
However, since the weight also differs by 500kg, both calculate to 10kg/PS.
On the other hand, Car C has the same weight as Car B, 1,500kg, but with 200PS, it becomes 7.5kg/PS.
Calculating this way, you can see that
you cannot simply say "150PS is always more advantageous than a 100PS car."
You can easily calculate it on your smartphone
The calculation method is simply
Weight ÷ Horsepower.
For example, if the catalog states:
Curb weight 1,420kg
Maximum output 180PS
You would enter
1420 ÷ 180
into a calculator.
The answer is approximately 7.89.
Therefore,
it would be approximately 7.9kg/PS.
By performing the same calculation for the car you want to compare, you can easily compare the balance of weight and output.
What's the difference between kg/PS and PS/t?
When researching power-to-weight ratios,
you might see not only kg/PS
but also PS/t.
Here, caution is needed.
kg/PS is a way to look at
"how many kg per horsepower."
A smaller number indicates higher output relative to weight.
On the other hand, PS/t is a way to look at
"how many horsepower per ton of vehicle weight."
Here, a larger number indicates higher output relative to weight.
In other words, the interpretation of the numbers is reversed.
How to convert from kg/PS to PS/t
For example,
let's say there is a car with 5kg/PS.
Since 1 ton is 1,000kg,
1,000 ÷ 5 = 200.
Therefore,
it is 200PS/t.
Conversely,
if it's 200PS/t,
1,000 ÷ 200 = 5,
so it's
5kg/PS.
In other words,
as kg/PS decreases, PS/t increases.
When looking at overseas websites, do not just compare the numerical values; always check the units.
Which "weight" should I use?
This is very important when actually performing calculations.
Automobile catalogs may list
curb weight
and
gross vehicle weight.
When comparing the power-to-weight ratios of general passenger cars, it is generally easier to compare by using the "curb weight" under the same conditions.
On the other hand, if you use the gross vehicle weight for only one car, the conditions will not be consistent.
When comparing power-to-weight ratios,
it is important to use the same type of weight.
Also pay attention to "dry weight"
For overseas sports cars and supercars,
Dry Weight
may be used.
Dry weight may have different conditions than typical curb weight, which includes fuel and various fluids.
Therefore,
comparing the curb weight of a Japanese car
with the dry weight of an overseas car
directly may result in power-to-weight ratios with different underlying conditions.
If the numbers seem exceptionally good,
it is accurate to check "which weight the calculation is based on."
What happens if you calculate it with people in the car?
The power-to-weight ratio listed in catalogs is not exactly the same as the actual driving conditions.
In reality,
the driver,
passengers,
luggage,
fuel,
etc., add weight.
For example,
a car with a curb weight of 1,000kg
and a maximum output of 100PS
has a power-to-weight ratio of
10kg/PS.
If you add
a 70kg driver,
a 60kg passenger,
and 20kg of luggage,
the actual weight to be moved becomes 1,150kg.
1,150 ÷ 100 = 11.5,
so it becomes
11.5kg/PS.
The maximum output has not changed.
However, the weight to be moved has increased.
This calculation is also easy to understand when considering why
"it feels light when driving alone, but acceleration feels heavy with four people."
How much does an extra 100kg change things?
For example,
for a car weighing 1,200kg
with 150PS,
1,200 ÷ 150 = 8kg/PS.
If occupants and luggage are added, increasing the weight to 1,300kg,
1,300 ÷ 150 = approx. 8.67kg/PS.
Even just 100kg can change the power-to-weight ratio.
Especially for lighter cars with smaller outputs, they are relatively more susceptible to the effects of occupants and luggage.
Conversely, what if you lighten it by 100kg?
If the same car,
1,200kg
150PS,
is lightened by 100kg to
1,100kg,
it becomes
1,100 ÷ 150 = approx. 7.33kg/PS.
Without increasing the output at all,
it improves from 8kg/PS to approx. 7.3kg/PS.
This calculation makes it easier to understand why weight reduction is emphasized in sports cars.
However, this does not mean we recommend removing safety equipment or necessary parts from street-legal cars for weight reduction.
Manufacturers design vehicles considering safety, rigidity, quietness, durability, and more.
Comparing with increasing horsepower
Consider the same 1,200kg car.
With 150PS,
it's 8kg/PS.
If the output is increased to 180PS,
1,200 ÷ 180 = approx. 6.67kg/PS.
On the other hand, if the weight is reduced to 1,000kg while keeping 150PS,
1,000 ÷ 150 = approx. 6.67kg/PS.
The power-to-weight ratio is the same in terms of calculation.
Therefore, if you only look at the power-to-weight ratio,
increasing output
and
reducing weight
both work to improve the numbers.
How to calculate the power-to-weight ratio for EVs?
The basic concept is the same for EVs.
You divide the vehicle weight by the maximum output.
For example,
for an EV with a vehicle weight of 2,000kg
and a system maximum output equivalent to 400PS,
2,000 ÷ 400 = 5kg/PS.
However, there's a caveat with EVs.
For cars equipped with multiple motors (front and rear),
separate output figures may be listed, such as:
front motor alone,
rear motor alone,
or the entire system.
To compare the power-to-weight ratio of the entire car, you generally need to use the value corresponding to the total system output available for the vehicle.
You should not compare the output of a single motor with the system output of another car.
The same caution is needed for hybrid vehicles
For hybrid vehicles,
multiple figures may be listed, such as:
engine output,
motor output,
and system maximum output.
Here,
simply adding engine maximum output + motor maximum output
may not always be correct.
This is because the maximum output of the engine and motor may not necessarily occur at the same time.
If the manufacturer announces the vehicle's total output under a name like "system maximum output," it is more appropriate to use that value as a basis for comparison.
When comparing, it is important to
use figures based on the same criteria.
What if PS and kW are mixed?
In recent catalogs, output may be displayed in kW.
In this case, you need to unify the units of the two cars being compared.
If one is in PS
and the other is in kW,
you cannot compare them as kg/PS.
You can convert both to PS,
or
convert both to kW and compare them as kg/kW.
When comparing search results or manufacturer data,
it's important to look not just at the output number, but also the unit.
If the power-to-weight ratio is the same, is the acceleration also the same?
This point is particularly important.
The answer is:
Not necessarily.
For example,
Car A
1,000kg
100PS
10kg/PS
Car B
2,000kg
200PS
10kg/PS
have the same power-to-weight ratio.
However, their actual acceleration performance may not be exactly the same.
This is because actual cars have factors like:
torque curve,
gear ratio,
transmission,
tires,
drive system,
air resistance,
drivetrain loss,
and throttle response.
The power-to-weight ratio is simply
an indicator for comparing the ratio of weight to maximum output.
It does not represent the overall performance of the car with a single number.
Especially launch acceleration cannot be determined by power-to-weight ratio alone
For starting from a standstill, how much driving force can be transmitted to the tires is crucial.
Here, factors like:
engine or motor torque,
1st gear ratio,
final drive ratio,
tire diameter,
tire grip,
and drive system
play a significant role.
For example, even if the power-to-weight ratio is excellent, if the tires spin, that power cannot be transmitted to the road surface.
Conversely, if the driving force can be efficiently transmitted to the road surface through all-wheel drive, it can be a great advantage when starting.
Difference from low-end torque
Power-to-weight ratio and "low-end torque" are also different metrics.
The power-to-weight ratio primarily uses maximum output.
On the other hand, the feeling of power in city driving is also related to
how much torque is produced in the commonly used RPM range, such as:
1,500rpm,
2,000rpm,
2,500rpm.
Therefore,
a car can have a very good power-to-weight ratio,
but be docile at low RPMs.
Conversely,
a car might not have a particularly outstanding power-to-weight ratio,
but feel powerful in city driving due to high torque at low RPMs.
If you want to know more about torque curves, you can find detailed information in "What is robust low-end torque? The correct way to view maximum torque and torque curves."
Why doesn't it match 0-100km/h times?
Cars with a good power-to-weight ratio tend to have faster 0-100km/h times.
However, there isn't a perfectly linear relationship.
0-100km/h acceleration is influenced by factors such as:
traction at launch,
number of shifts,
shifting speed,
gear ratios,
tires,
drive system,
and engine/motor output characteristics.
Therefore, it's possible for Car A to have an advantage in kg/PS, but Car B is actually faster in 0-100km/h.
The power-to-weight ratio is not
"a formula for calculating 0-100km/h times."
It's not a number to predict top speed either
Even if the power-to-weight ratio is excellent, it does not necessarily mean the top speed will be higher.
At high speeds, air resistance becomes extremely important.
Factors such as:
body shape,
frontal projected area,
drag coefficient,
gear ratio,
and the speed range where maximum output can be generated
are also relevant.
In other words,
for launch,
mid-range acceleration,
high-speed acceleration,
and top speed,
the influencing factors differ.
The power-to-weight ratio is not a universal indicator of speed.
Practical use when comparing cars
The power-to-weight ratio is very useful for
initial comparisons,
rather than as a final conclusion.
For example, if you have two potential cars for purchase:
First, research their curb weight and maximum output.
Calculate the kg/PS for each.
Next, look at the maximum torque and torque curve.
Then, examine the gear ratios and drive system.
Finally, consider the actual acceleration performance and test drive feel.
This approach helps you avoid making a simplistic comparison like,
"200 horsepower means this one is definitely faster."
Common calculation mistakes
When calculating the power-to-weight ratio yourself, there are a few common mistakes.
The most frequent ones are:
Not aligning weight standards.
Not aligning output units.
Assuming kg/PS and PS/t represent numbers in the same direction.
Simply adding engine and motor outputs for hybrids.
While the calculation itself is simple,
which numbers you use for the calculation
is crucial.
Basic rules for interpreting power-to-weight ratios
Here's what you should remember:
For kg/PS, smaller numbers mean higher output relative to weight.
For PS/t, larger numbers mean higher output relative to weight.
When comparing, use the same weight standard.
Unify the output units.
For EVs and hybrids, check the output standard for the entire vehicle.
And,
do not determine actual acceleration performance solely by the power-to-weight ratio.
If you keep these points in mind, it becomes a very useful metric for catalog comparisons.
Next Science's perspective on "using numbers"
Numbers are extremely helpful when looking at car performance.
However, you cannot judge everything by a single number.
The power-to-weight ratio is
an excellent way to view the combination of two factors: maximum output and weight.
On the other hand, the actual driving feel experienced by the driver also involves factors such as:
torque curve,
throttle response,
gear ratio,
friction and drivetrain losses,
and tires.
That's why,
the power-to-weight ratio is not "the answer to a car's speed," but rather
"a tool for reading catalog specifications more accurately."
This way of thinking will be easy to understand.
Summary: Calculating weight ÷ horsepower changes your perspective
The kg/PS power-to-weight ratio commonly used in Japan can be easily calculated as:
Curb weight ÷ Maximum output.
For 1,000kg and 100PS,
it's 10kg/PS.
For 1,500kg and 200PS,
it's 7.5kg/PS.
When comparing using kg/PS, a smaller number indicates greater output relative to weight.
However, what's truly important is using the calculation results correctly.
Compare using the same weight standard.
Do not mix PS and kW.
Understand the difference between kg/PS and PS/t.
For EVs and hybrids, confirm the output standard.
And,
do not conclude acceleration performance solely based on the power-to-weight ratio.
Keep these points in mind.
Next time you find two cars that interest you,
first try calculating "curb weight ÷ maximum output."
It should make it easier to compare car performance than just looking at maximum output figures alone.