If you're wondering how to compare the performance of different vehicles, then our power-to-weight ratio calculator is just what you need. In the following article, you'll learn how to calculate the power-to-weight ratio, and why it is a crucial measure of the performance of all moving vehicles.

What is the power-to-weight ratio, and how is it useful?

Different vehicles produce varying quantities of power, and weigh different amounts. For example, a Ford F-series pickup truck has a peak power output of 290 hp (216 kW) and weighs 4,069 lb (1,846 kg), while a Ford Fiesta, produces 89 hp (66 kW) and weighs 2,546 lb (1,155 kg). How can we fairly compare their performance using these numbers?

What we need is a metric that doesn't depend on the size of the vehicle. The solution is to divide the vehicle's power by its weight (or mass) to get the power-to-weight ratio. But what does that mean exactly? It is a measure of how much power the vehicle can produce per unit weight. The two most common power-to-weight ratio units are horsepower per pound (hp/lb) and kilowatt per kilogram (kW/kg). If a car has a higher power output per pound than another vehicle, it will be able to accelerate that pound of weight more quickly. An example of a quickly accelerating vehicle is a motorbike, which can have an average amount of power. However, because it is very lightweight, its power-to-weight ratio will be much greater than that for a typical car.

It's important to note that the word "weight" used throughout this article is a colloquial term for mass. So if you wanted to calculate the power-to-weight ratio of the Moon buggy, for example, you would use its mass rather than its weight as measured on the Moon, which would be one-sixth of that measured on Earth.

How to calculate the power-to-weight ratio?

We'll now explain step by step how to calculate the power-to-weight ratio of a vehicle.

  1. Look up the power of the vehicle. You can either find this value in the vehicle's owner's manual, or search for it online. Manufacturers usually quote the peak power output of their vehicles, which is OK for comparison purposes, but keep in mind that most of the time, the vehicle will be producing less power. If you can't find a value, you can always use our horsepower calculator to work it out.

  2. Find out the vehicle's curb weight. Again, this can be found either from the manual or an online source. The curb weight is the weight of the vehicle, excluding the driver, passengers, and luggage. This value is useful when comparing vehicles, though you could add the extra weight to see how the performance of your vehicle changes when it's fully loaded up for a long road trip.

  3. The equation to calculate the power-to-weight ratio is:

    power-to-weight ratio = power / weight

    You'll usually see it quoted in units of hp/lb or kW/kg, but you can use any units of power and weight you like, as long as you use the same ones when comparing vehicles.

Worked examples of calculating the power-to-weight ratio

Let's go back to the two examples at the top of the page and calculate their power-to-weight ratios (PWRs). For the pickup truck, the calculation is:

PWR of pickup truck = 290 hp / 4069 lb = 0.071 hp/lb

And for the small Ford Fiesta, the calculation is:

PWR of small car = 89 hp / 2546 lb = 0.035 hp/lb

So we can see that the pickup truck has double the power-to-weight ratio of the small car. In theory, the truck should be able to accelerate twice as fast as the car, though, in reality, other differences such as aerodynamic drag, the rolling resistance of the tires, etc., will affect this prediction.

If you are a keen cyclist and are wondering how your power-to-weight ratio compares to a motor vehicle, you first need to calculate your body's power output. You can do precisely that with our cycling wattage calculator, which also calculates your power-to-weight ratio for you.

You can now compare the performance of a wide range of vehicles using their power-to-weight ratio. Try it out on high-performance supercars, massive dumper trucks, trains, and ships. You can even try spaceships if you can find the information.

Steven Wooding
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