The Hybrid Scorecard is an online tool created by the Union of Concerned Scientists (UCS) to help consumers determine the true value of a hybrid vehicle. The Hybrid Scorecard examines a hybrid vehicle’s forced features — upgrades and option packages that are required on a hybrid model — to a non-hybridized version and gives the vehicle an environmental score as well as a hybrid value ranking.

Today, the UCS has made its first update to the Hybrid Scorecard. As part of this update, the organization makes a statement about the benefit of smaller gasoline engines being used in some of today’s luxury hybrid models — specifically the Mercedes-Benz S400 Hybrid.

Engineers at Mercedes-Benz downsized the standard eight-cylinder engine from the non-hybridized S400 to a six-cylinder engine for the hybrid version. Although the system used in the Mercedes-Benz S400 hybrid is considered a mild hybrid because it cannot run on electric power alone, the vehicle is able to achieve a noted improvement in overall fuel economy. That's because the engine, itself, was downsized.

Combining the hybrid drivetrain with a smaller engine allows the S400 hybrid to have a smaller environmental footprint. The UCS gave the vehicle a 5.3 environmental score and awarded it with a very high hybrid value ranking.

On the opposite end of the spectrum is the new BMW ActiveHybrid X6. The UCS calls BMW’s first hybrid a “muscle hybrid” as it combines the hybrid drivetrain with an eight-cylinder engine to increase the vehicle’s horsepower and torque. The BMW ActiveHybrid X6 is the most powerful hybrid on the market.

However, its power comes at a greater environmental cost. The UCS gave the luxury hybrid SUV a 4.4 environmental score. The Lexus RX 450h, another midsized luxury hybrid SUV, earned an environmental score of 7.4. This puts the BMW ActiveHybrid X6 environmental score on par with full size SUV hybrids like the Chevy Tahoe and GMC Yukon.

To make matters worse, the BMW ActiveHybrid X6 comes with $10,000 in forced features over the non-hybrid model. These mandated upgrades have led the UCS to give the luxury hybrid a very low hybrid value. For comparison, the Lexus RX 450h only had $250 in forced upgrades and the Mercedes-Benz S400 hybrid had no forced upgrades.

While hybrid vehicles typically perform better from an environmental perspective than their non-hybrid counterparts, it is important that consumers look at the true costs of hybridization. Automakers would also be well-served to examine the overall value of their hybrid models.