I would leave US at 7 AM. That should put you in Miami by 10:30 - 11:00 AM, which will give you plenty of time to return the rental car without undue stress.
The
Golden Glades interchange mentioned above is north of Miami and you'll have to watch the signage carefully to get from the Turnpike to I-95
SOUTH. You will join I-95 at roughly NW 163 Street.
Stay on I-95 SOUTH and you will eventually come to two more interchanges -- the first is with FL
112/ I-
195, pass through that one southbound. Just FYI, the signs for that interchange will also say Airport and Miami Beach, but this is the
wrong interchange for where you want to go.
Very quickly, the next interchange will come up and you have to be positioned for it. That interchange is with FL 8
36/ I-
395.
- If you have to go to the MIA Rental Car Center, as soon as you pass the first interchange, get in the RIGHT lane. That will take you to FL 836 WEST, heading to the airport. As soon as you get on 836, you are going to have to move several lanes to the left because the right two lanes exit at our major hospital complex. Follow the signs to the airport and then follow signs for the Rental Car Center.
- If you are going to the Port first, as soon as you pass the first interchange, get in the LEFT two lanes. That will take you to I-395 EAST toward Miami Beach. Watch your lane signage and once you get on the bridge, move to the left lane for the Port tunnel. Once you're in the tunnel, left lane.
Here's a clip of the area, with the Airport to the left (west) and the Port to the right (east), with I-95 going top to bottom in the middle. The road crossing to Miami Beach labeled A1A is actually I-395.
For some obscure reason, Google Maps does not show the Port tunnel. However it is right after the bridge, just where the road curves, about where the Jungle Island pin is.