Here's a reason to spread a little more sunshine on social media: Free gas.

That is the reward Chevrolet recently gave to select drivers in four cities to demonstrate the auto brand's new "global positivity system," which uses IBMWatson to evaluate how positive people are on social media.

The global campaign -- which is rolling out today -- allows participants to visit a mobile or desktop site to obtain a positivity score based on their social media posts.

Users are asked to enter either their Twitter or Facebook account names at the site. Within seconds the program returns a positivity score. The output also identifies a user's least positive and most positive posts as well as their five most used positive words. Also provided is a detailed social personality summary. At the end, Chevy recommends an experience based on the output, such as "master a new language."

To promote the digital campaign, Chevy recently took over gas stations in Buenos Aires, Cape Town, New Orleans and Calgary. Drivers uncovered their positivity scores at the pump and were awarded free gas based on their scores.

The campaign was designed and executed by The Martin Agency in partnership with Commonwealth/McCann as the strategic lead agency. The effort marks the first time Chevy has used the IBM Watson technology for an ad campaign -- and could foreshadow future partnerships. "While we can't discuss future advertising strategies, the relationship has been great for both companies and we look forward to exploring future opportunities," Karen Toor, Chevy's global content marketing manager, said in an email.

The effort will be supported with paid digital media, including targeted Facebook custom videos, she said. Chevy will also run a promoted trend on Twitter across multiple global markets.

The campaign is aimed at driving awareness for Chevy across global markets. It "helps us create consistency as to who we are as Chevrolet, bringing a unifying message around our global brand promise to 'Find New Roads,' " Ms. Toor said, referring to Chevy's tagline.

While users consent to allowing General Motors to access their social media handles, pictures and biographical information to execute the program, GM will "not retain or use this information for any other purpose," according to the terms and conditions.

Watson's technology "uses natural language processing and machine learning to reveal insights from large amounts of unstructured data," according to IBM. The Chevy campaign marks the latest example of how IBM has integrated Watson into campaigns for other brands. Last year, Watson made Ad Age's "Creativity 50" list on the heels of partnerships with brands including North Face.

For Chevy's program, Watson "can not only read the words from social posts, but it can understand them contextually," according to a description on the site.

But Watson is not perfect.

This reporter tried out the online site and earned a positivity score of 105. That is below the average score of 122, but still earned the following message: "You have a good outlook on life." (Phew.) But, curiously, the program identified as "positive" a recent tweet I made referencing an Ad Age story about McDonald's pulling Happy Meal fitness bands because they were causing skin irritation. (Not exactly positive or happy.)

A Chevy spokesman said in an email that "we recognize that sentiment analysis is not a perfect science as it's being scored by machines. We do feel confident in the experiences created through the 'Fueling Possibilities' application."

This story also appears on Adage.com.