The car Michael's brother was driving is a coupe with the small rear side glass, but the one in the junkyard was a sedan with the larger side glass.
During one chase the striping on the road is yellow, but in a second take the striping is white. During the time of filming the US was changing from white road striping to yellow.
The car that Michael's brother is driving at the beginning of the movie is a Ford Club Coupe. The one that Michael eyes in the in the junkyard is a Ford Custom 2 door sedan. Both are the same year, 1951.
When "The Kid" shows his car to the sheriff, the fuel induction setup appears to be four two-barrel carburetors on a small-block Chevy. Later, when "The Kid" is working on his car at the garage, the engine has a Holley four-barrel carburetor.
The engine in the real '34 Ford was just a plain 302 Ford with a single four-barrel carb. To make it appear more of a hot-rod engine, the engine actually shown to Sheriff Roy Childress is the 327 Chevy from the yellow '32 coupe from American Graffiti (1973) with four two-barrel carbs.
When the Kid is first arrested, he tells the sheriff that his speedometer is off because of his bigger tires. Bigger tires would cause a speedometer to read low not high.
Several scenes depict the vehicles' tires squealing on dirt. Tires cannot squeal on dirt.
(Wrong: tires squeal very well on packed dirt, as race track visitors know. In addition the early bead-lock style rims and tires weren't very tightly fitted, and the tire beads could lose traction upon the wheel, squealing loudly.)
Although the film is set in 1958, at several points cars from the early and mid-1960s can be seen in the background, including the cab that takes the people from court to their cars (1964 Checker Manhattan).
Until 1961 all road striping in California was white. Many shots show yellow center striping. It may have been possible for the double yellow lines to be present, but single-dashed striping was white until the rules changed in 1971. California, as with most of the US, repainted from 1971-75. The film is set in 1958.
Clearly seen throughout the film is the black and gold California license plate on McCord's coupe. This type of plate was not issued until 1963. In addition, the serial prefix of the plate (WLA 285) is a 1967-68 series.
One of the pinball machines in the diner is clearly named "Doodle Bug". That pinball machine was released in 1971, much later than the story's 1958 setting.
When Michael is in the junkyard, walking along the side of the wrecked
car his brother was killed in, you can see the reflection of a large studio light in the side window of that car for a brief moment.