how to calculate power to weight ratio

Calculating the power to weight ratio (PWR) of a vehicle is an important metric that helps determine overall performance. The power to weight ratio measures how many horsepower an engine can produce relative to the total weight of the vehicle. Vehicles with a higher power to weight ratio can accelerate faster and climb hills more easily than vehicles with a lower ratio. Knowing how to accurately determine PWR is useful when comparing vehicles and understanding performance capabilities.

In this comprehensive guide, we’ll cover everything you need to know to calculate power to weight ratio for any type of vehicle. We’ll start with an overview of what power to weight ratio is, then look at how to determine:

  • Engine power
  • Curb weight
  • Gross vehicle weight
  • Finally, how to use those numbers to calculate overall PWR

We’ll also provide some real world examples to help illustrate the concept. Let’s get started!

What is Power to Weight Ratio?

Power to weight ratio (PWR) is a calculation that measures how many horsepower an engine can produce relative to the total weight of the vehicle.

It provides a numerical representation of a vehicle’s power output versus its mass, giving an idea of how quickly a vehicle can accelerate under power.

The formula for power to weight ratio is:

Power to Weight Ratio = Engine Power (hp) / Vehicle Weight (lb)

The result is expressed in hp/lb, or how many horsepower are available for each pound of vehicle weight.

For example, a vehicle with 300 hp and a weight of 4000 lbs would have a PWR of:

300 hp / 4000 lbs = 0.075 hp/lb

Generally, a higher power to weight ratio indicates better performance:

  • Vehicles with a high PWR can accelerate faster, since more power is available per pound of weight.
  • They can also climb slopes and hills easier due to increased power available.
  • A lower PWR means less power proportional to weight, therefore slower acceleration.

When comparing vehicles, calculating power to weight ratio gives an applicable metric to help understand real world performance differences.

Next, let’s look at how to determine engine power and vehicle weight in order to calculate overall power to weight ratio.

Determining Engine Power

The first number needed for the PWR calculation is engine power, specifically horsepower (hp) produced by the engine.

For standard production vehicles, this horsepower rating is provided by the manufacturer. But there are also a few ways to independently determine engine horsepower if needed:

Use Manufacturer HP Rating

For most production cars, trucks, and motorcycles, the manufacturer provides an advertised engine horsepower rating. This is the easiest way to get engine hp.

Some common places to find the advertised hp rating:

  • Window sticker or Monroney label – Required by law for all new cars sold in the US, lists engine hp along with other details
  • Manufacturer website – Specification pages will list engine power
  • Owner’s manual – Will provide hp rating in technical specifications
  • Product brochures – hp rating is always highlighted in marketing materials

As long as the vehicle is stock and unmodified, the manufacturer’s advertised hp can be used for the PWR calculation.

Dynamometer Testing

If an independent measurement of engine power is needed, a chassis dynamometer (dyno for short) can be used. This device measures power output directly from the engine across the RPM range.

Dyno testing requires specialized equipment but is an accurate way to verify actual engine performance, especially for modified vehicles. The peak hp measured on the dyno would be used for power to weight ratio calculations.

Calculations from Torque and RPM

Horsepower can also be derived from mathematical formulas using torque and RPM measurements. This method requires knowing the engines torque output across its RPM range.

Torque (lb-ft) x RPM / 5,252 = Horsepower

While doable, this approach takes more effort and precision instrumentation to generate usable hp numbers for the PWR formula.

For most applications, the manufacturer’s advertised hp or dyno tested figures will be suitable for determining engine power.

Determining Vehicle Weight

The next key component is the vehicle’s weight in pounds (lb). There are two primary weight ratings used:

  • Curb weight – The total weight of the vehicle with all standard equipment and fluids, no occupants or cargo.
  • Gross vehicle weight rating (GVWR) – The maximum allowable weight designated by the manufacturer including all passengers, cargo, and added equipment.

Curb Weight

A vehicle’s curb weight is its baseline mass as built and delivered from the factory without any occupants or cargo onboard.

Some ways to find manufacturer published curb weight specs:

  • Window sticker – Lists curb weight along with other details
  • Manufacturer website – Look for technical specification pages
  • Owner’s manual – Often provides curb weight in the specs section
  • Weigh vehicle at a scale – Measuring directly with the vehicle empty provides curb weight

Curb weight provides a standard weight basis for comparing PWR between vehicles. It represents the minimum mass that must be accelerated and climbed hills.

Gross Vehicle Weight Rating

GVWR is the maximum loaded weight designated by the vehicle manufacturer. It includes:

  • Curb weight of the vehicle
  • Weight of all occupants and cargo
  • Any added accessories like roof racks, hitches, etc

GVWR is specified on a placard inside the driver’s door jamb. It can also be found in the owner’s manual.

Calculating PWR using GVWR shows the worst case ratio – when the vehicle is fully loaded. This represents real world power available during commuting, hauling, towing, etc.

Choosing Curb Weight vs GVWR

For standard performance comparisons between vehicles, published curb weight is commonly used in the PWR formula. This provides an even baseline not accounting for variable payload.

But for understanding realistic on-road performance, using the GVWR is more applicable. The worst case heavily loaded ratio better represents real world driving dynamics when accelerating or climbing grades.

With either curb weight or GVWR collected, we can now calculate complete power to weight ratios.

Calculating Power to Weight Ratio

With engine horsepower and vehicle weight determined, the actual PWR calculation is straightforward:

Power to Weight Ratio = Engine Power (hp) / Vehicle Weight (lb)

Let’s walk through some examples for both curb weight and GVWR:

Curb Weight PWR Example

A 2020 Toyota Camry has:

  • Engine power: 203 hp
  • Curb weight: 3,572 lbs

The PWR based on curb weight is:

203 hp / 3,572 lbs = 0.057 hp/lb

GVWR PWR Example

A Ford F-150 pickup has:

  • Engine power: 395 hp
  • GVWR: 7,000 lbs

The PWR based on GVWR is:

395 hp / 7,000 lbs = 0.056 hp/lb

Let’s look at one more example comparing two vehicles:

Comparing PWR of Two Vehicles

Vehicle A:

  • Power: 300 hp
  • Curb weight: 3,300 lbs

Vehicle B:

  • Power: 400 hp
  • Curb weight: 4,500 lbs

Calculating PWR for each:

Vehicle A:

300 hp / 3,300 lbs = 0.091 hp/lb

Vehicle B:

400 hp / 4,500 lbs = 0.089 hp/lb

Although Vehicle B has a more powerful engine, Vehicle A has a slightly higher PWR indicating better acceleration and climbing ability per pound of weight.

This demonstrates how power to weight ratio provides a useful performance comparison between vehicles.

Power to Weight Ratio Values

As the examples illustrate, typical passenger car power to weight ratios range from 0.03 – 0.08 hp/lb for curb weight. Lower end economy cars have PWRs around 0.03, while high performance sports cars reach over 0.07.

For SUVs and trucks using GVWR, power to weight ratios often fall between 0.04 – 0.065 hp/lb. Heavy duty trucks can be even lower.

Here are some rough PWR guidelines:

  • Economy cars: 0.03 – 0.05 hp/lb (curb weight)
  • Family sedans: 0.04 – 0.06 hp/lb (curb weight)
  • Performance cars: 0.06 – 0.08+ hp/lb (curb weight)
  • SUVs: 0.04 – 0.06 hp/lb (GVWR)
  • Pickup trucks: 0.035 – 0.06 hp/lb (GVWR)

As with any parameter, higher PWR values indicate better performance potential. But the ratios must be calculated consistently using either curb weight or GVWR for an accurate comparison.

Real World Power to Weight Examples

To help provide context, here are some real world power to weight ratio examples for various popular vehicle models and trims:

2020 Toyota Camry LE 4-cyl

  • Horsepower: 203 hp
  • Curb weight: 3,572 lb
  • PWR: 0.057 hp/lb

With the 4-cylinder base engine, the Camry LE has economy car level PWR. Performance is adequate but not fast.

2020 Camaro SS V8

  • Horsepower: 455 hp
  • Curb weight: 3,747 lb
  • PWR: 0.121 hp/lb

Boasting over 450 horsepower in a relatively lightweight package gives the Camaro SS a very high PWR of 0.12. It provides incredible acceleration and speed for a muscle car.

2020 Ford F-150 XL Regular Cab V6

  • Horsepower: 290 hp
  • GVWR: 6,050 lb
  • PWR: 0.048 hp/lb

As a work truck with base V6 engine, this F-150 configuration prioritizes payload and towing over acceleration. The PWR is respectable but not Fast.

2020 Porsche 911 Carrera S

  • Horsepower: 443 hp
  • Curb weight: 3,254 lb
  • PWR: 0.136 hp/lb

With massive power and light weight, the 911 Carrera S unsurprisingly produces an extremely high PWR. Few street vehicles can match its combination of performance and precision.

These real world examples show how power to weight ratio gives insight into actual vehicle performance capabilities. When numbers are crunched consistently, PWR provides a useful metric for comparisons.

Conclusion

To summarize, here are the key steps covered to calculate power to weight ratio:

  1. Determine engine horsepower
    • Use manufacturer rating, dyno test, or torque/RPM calculation
  2. Identify vehicle curb weight or GVWR
    • Curb provides base weight, GVWR gives maximum loaded weight
  3. Plug horsepower and weight into formula:
    • PWR = Engine HP / Vehicle Weight

The resulting power to weight ratio provides an objective measure of relative vehicle performance. Comparison between vehicles is only valid using consistent curb weight or GVWR figures.

While not a definitive indicator, PWR gives perspective on real world acceleration, speed, and climbing ability. Combined with other factors like drive type, aerodynamics, and gearing, power to weight ratio helps reveal the full performance picture.

Understanding this simple but important calculation gives car enthusiasts and drivers another tool to analyze and compare vehicles. We hope this detailed overview gives you the knowledge to accurately determine power to weight ratio. Now get out there, crunch the numbers, and gain additional insight into vehicle performance.

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