To the best of my knowledge and an email from one of the star's in the film the Drive In scenes were filmed at the Pickwick Drive In in Burbank. It is no longer there and the screen burned to the ground. Dont mess with RUBY! Pickwick Drive-In 1100 W. Alameda Burbank, CA 91506
I'm not sure if (out of all the So Cal drive-ins,) there were some with the same design and layout. But after watching Ruby again, there are the same details seen in Ruby & other movies filmed at the Pickwick.
The white picket fence around the projection booth, separate from the snack bar.
(Blue Thunder & Grease)
The blue paint on the bottom skirting of the screen tower. (Grease)
The open air windows of the snack bar (Christine)
The box office had expanded between filming Ruby & Blue Thunder. But in Ruby, it appears the cars veer off to the right into the field. Which you can see from the aerial shots from Blue Thunder.
According to the director's commentary with Piper Laurie. They thought the drive-in was located in North Hollywood, but couldn't remember the name.