I don't know if this has been previously posted but all the major rental car companies are now located in a new central parking garage east of the airport between NW 21st/25th streets and 37th (Douglas Rd)/42nd (Le June Rd) avenues, about 1.5 miles away.
Access to and from the terminals is by an automated, free people mover. The people mover terminus is centrally located on the 3rd (uppermost level) near the parking garages at the F-Terminal. It can be a long walk, especially from the J and D terminals, even with the moving sidewalks.
Rental car exit from the structure is easy once you reach it. Depending on where your rental company is located on the three levels you might have to make an entire loop inside the garage to reach the exit. You can make a left turn onto 21st Street to get to 37th Avenue (Douglas Rd) or a right turn to get to 42nd Avenue (Le June Rd).
Rental car return can be a real pain and I had lived in Miami for 22 years. The best way is to use only Le June Road from the north or south and follow the signs. If you try to use the old approach from 37th Avenue to NW 21st Street you will have to make an almost complete loop (over one poorly marked mile) of the area around the rental car garage.
If you try to make a shortcut by turning onto NW 25th Street you will have to make a U-turn at a posted "No U-Turn" location. I noticed a Miami-Dade Metro police car parked on NW 39th Avenue during the day, just out of sight of anyone who would be making the illegal U-turn on 25th Street.
Allow at least 15-30 minutes of extra time for rental car return now.