The 2018 Ford Mustang, like pony cars in general, pairs best with a manual transmission. Its stoplight-racing, smoke-'em-if-you-got-'em attitude begs for three pedals of driver involvement and a slick, row-your-own shift lever between the front seats—regardless of engine choice. But with Ford and General Motors having co-developed a new 10-speed automatic, the latest's Mustang's mid-cycle update provided an opportunity for Ford's seminal pony car to receive the brand's newest transmission. While the automatic remains an option we'd skip out of personal preference, the result is indeed a quicker and more flexible Mustang.

Ford's version of the 10-speed 'box made its debut in the second-generation F-150 Raptor for 2017, later rolling out into lesser F-150s as well as the Ford Expedition and Lincoln Navigator. (GM's applications currently include V-8–powered Chevrolet Camaros and versions of its full-size trucks and SUVs with the optional 6.2-liter V-8.) In the Mustang, the 10-speed replaces the previous six-speed automatic and costs $1595 in both the four-cylinder EcoBoost and V-8 GT models. For this review, we tested fastback coupe variants with each of the available engines.

Both of our test cars featured their respective optional Performance packages, which add a Torsen limited-slip differential and, in EcoBoost models,...