Pontiac's intermediate line was restyled again for 1966, gaining more curvaceous styling with kicked-up rear fender lines for a "Coke-bottle" look, and a slightly "tunneled" backlight. The tail light featured a rare louvered cover, only seen on the GTO. Overall length grew only fractionally, to 206.4 in (5,243 mm), still on a 115 in (2,921 mm) wheelbase, while width expanded to 74.4 in (1,890 mm). Rear track increased one inch (2.5 cm). Overall weight remained about the same.
The GTO became a separate model series, rather than an optional performance package, with unique grille and tail lights, available as a pillared sports coupe, a hardtop (without B-pillars), or a convertible. Also an automotive industry first, plastic front grilles replaced the pot metal and aluminum versions seen on earlier years. New Strato bucket seats were introduced with higher and thinner seat backs and contoured cushions for added comfort and adjustable headrests were introduced as a new option. The instrument panel was redesigned and more integrated than in previous years with the ignition switch moved from the far left of the dash to the right of the steering wheel. Four pod instruments continued, and the GTO's dash was highlighted by walnut veneer trim.
Engine choices remained the same as the previous year. A new engine was offered that saw few takers: the XS option consisted of a factory Ram Air set up with a new 744 high lift cam. Approximately 35 factory installed Ram Air packages are believed to have been built, though 300 dealership installed Ram Air packages are estimated to have been ordered. On paper, the package was said to produce the same 360 hp (270 kW) as the non-Ram Air, Tri Power car.
Sales increased to 96,946, the highest production figure for all GTO years. Although Pontiac had strenuously promoted the GTO in advertising as the "GTO Tiger," it had become known in the youth market as the "Goat."