Paying $50,000 to Warner Bros.-Seven Arts to use the name and likeness of their Road Runner cartoon character (as well as a "beep, beep" horn, which Plymouth paid $10,000 to develop), and using the Chrysler B platform as a base (the same as the Belvedere), Plymouth set out to build a back-to-basics muscle car. Everything essential to performance and handling was beefed-up and improved; everything nonessential was left out. The interior was spartan with a basic vinyl bench seat, lacking even carpets in early models with an automatic transmission, and few options were available - just the basics such as power steering and front disc brakes, AM radio, air conditioning (except with the 426 Hemi) and automatic transmission. A floor-mounted shifter (for the four-speed) featured only a rubber boot and no console so that a bench seat could be used. The earliest of the 1968 models were available only as 2-door pillared coupes (with a B-pillar between the front and rear windows), but later in the model year a 2-door "hardtop" model (sans pillar) was offered. The Road Runner of 1968-1970 was based on the Belvedere, while the GTX was based on the Sport Satellite, a car with higher level trim and slight differences in the grilles and taillights.
The standard engine was an exclusive-to-the-Road Runner 383 CID (6.3 L) Roadrunner V8 rated at 335 bhp (250 kW) and 425 lb·ft (576 N·m) of torque. Its extra 5 hp (4 kW) rating was the result of using the radical cam from the 440 Super Commando and a .25 raise in compression to 10.5:1 (vs. 10.25:1 with the 330 hp (246 kW) 383). When air conditioning was ordered, the cars received the 330 hp (246 kW) version, as the radical cam specs of the 335 bhp (250 kW) version didn't create enough vacuum to accommodate a/c; and there were concerns of overrevving which would grenade the RV-2 York compressor. For an extra $714, Plymouth would install a 426 CID Hemi rated at 425 bhp (317 kW) and 490 lb·ft (664 N·m) of torque. Combined with low weight, the 6-passenger Road Runner could run the 1/4 mile in 13.5 seconds at 105 mph (169 km/h). It would prove to be one of the best engines of the muscle car era, and the Road Runner one of the best platforms to utilize it.
The standard equipment transmission was a four-speed manual with floor shifter and Chrysler's three-speed TorqueFlite automatic was optional. Early four-speed '68 Road Runners featured Inland shifters, which were replaced by the more precise Hurst shifters during the course of the model year.
Plymouth expected to sell about 20,000 units in 1968; actual sales numbered around 45,000. This placed the Road Runner third in sales among muscle cars with only the Pontiac GTO and Chevy's SS-396 Chevelle outselling it. Dodge debuted the Road Runner's cousin, the Super Bee, as a mid-1968 offering after seeing Plymouth's success with the Road Runner, along with demands from Dodge dealers for their own low-priced muscle car as the Dodge Boys started the model year with the higher-priced Charger R/T and Coronet R/T - both of which were priced similar or higher than the Plymouth GTX.