What is the RideEyed Performance Index (rPI)?

How do you compare the overall performance between different cars?

Comparing Technical Specifications

Horsepower

One end of the spectrum is using a single technical specification factor. The most popular specification is horsepower. It's a single, simple-to-compare, whole number that is an important (if not the most important) factor in overall performance. Nevertheless, weight differences can negate even significant horsepower differences.

FIA Performance Factor

The opposite end of the spectrum is a mathematical formula using many technical specification parameters. Used for classification of race cars into performance groups, the Federation Internationale de l'Automobile (FIA) requires around 40(!) technical specification factors for their Performance Factor calculation. These specifications pertain to weight, engine, aerodynamics, drivetrain, and structure. Many of these specifications are not readily available for most production cars.

Comparing Performance Tests

0-60

A popular way to compare performance is a car’s time accelerating from 0-60mph. However, the time can vary significantly, depending on variable testing conditions such as weather, road surface, location elevation, driver-assist features, driver technique, and even driver weight.

Lap Time

Similarly, track lap times can measure car performance, but are also dependent on variable testing conditions. In addition, logistics make it difficult to test every car at a single track location with identical conditions during each test.

Factors to Consider

Assessing overall performance using instrumented tests can be too variable or too difficult to minimize the variances, so let’s focus on readily available technical specifications. Horsepower alone is not enough, and the FIA’s ~40 factor math formula requires too much information that cannot be easily gathered.

When I consider a car’s performance, I focus on horsepower, torque, weight, and drivetrain. A car’s horsepower, torque, and weight are usually available somewhere online. As for assigning a numerical value to drivetrain type, I defer to the FIA Performance Factor values: 1.0 for FWD, 1.1 for RWD, and 1.3 for AWD.

I used to consider the transmission type back when manuals were usually faster than automatics, but now some automatics are faster than manuals. Also, the only information I could find that assigned a performance value to different types of transmissions were random online car 0-60 calculators that were opaque on any specifics. The FIA lists Performance Factor values for transmissions, but only considers manual and sequential gearboxes.

The RideEyed Performance Index

With the focus narrowed to four performance factors, let’s combine them into a single, easily comparable number. The formula below describes how to calculate the RideEyed Performance Index, or rPI:

Yes, it oversimplifies as an indicator of a car's performance. Similar rPIs between two cars may not always 100% accurately describe which car has more performance. Nevertheless, the rPI still provides a more descriptive single-number relative datapoint than any of the four values alone, all from easily obtainable data.

Although the factors in this performance index use US customary units, the index itself is just a single, whole number, where a higher number indicates more performance. The 1,000 multiplied at the end just shifts the index from a decimal that’s less than one to a number that can be rounded to a wide range of whole numbers.

Let's use my favorite car, the 2005 Lotus Elise, as the first rPI example:

To also factor a car's price when considering performance, see What is the RideEyed Performance Value Index (rPVI)?.

For an up-to-date list of cars with an rPI, see rPI and rPVI Rankings.