Depends on your arrival/departure time in STT as to whether you'll make the car barge to STJ and back for your return flight. Any night time arrival will not accommodate you for catching a barge nor will very early departure schedules.

See the barge schedule: https://www.vinow.com/general_usvi/interisland_ferry/

Not all car rental agencies allow their vehicles to go to St. John so best to check.
Budget and Dependable Car (www.dependablecar.com) are your best bets for renting in STT to go to STJ, IMO.

Not all/many STJ car rentals allow their cars to go to STT so best to check on that end, if you wished to visit STT while on STJ..

The only rental car company on STT which doesn't allow their vehicles to go to St John is Hertz. A couple of others have restrictions on compacts being taken there but allow Jeeps/4WD vehicles to be taken there. No STJ agencies allow their cars to be brought to STT.

The advantage to renting on STT, and particularly if you have a larger party, is being able to save money stocking up on supplies on STT. Plus no hassle of taxiing, moving luggage around, etc. both going to and coming from.

Enjoy a great vacation!

I've read on other forums that STJ car rental agencies will charge for another day if you don't bring car back by 10am. If I rent a car on STJ at 4:30 pm and pay for 8 days would they charge another day if I returned the car by 2pm on the 8th day?
Steve

Best to call them directly for that answer and be sure to get the name of the person that responds so if there is any question about the response upon arrival (and do check again) when getting the car and upon its return, you have covered your bases.

I just checked a few of the rental agreements for car rentals on St. John and they do charge for returns after 10 am or noon. One had a charge of $100 for excessive sand and $150 for a wet seat. Make sure you read the agreement thoroughly before you sign it.
Steve

I was there in March. I got a Jeep with 2500 miles on it from Amalie car. They met me at the airport for both pick up and drop off. It was pricey, but worth every penny. Great vehicle, and perfect for the cray hills. I took it to STJ, to the top of the mountain where the sky ride thingy (or whatever it's called), all over, and it was perfect.

And the people were really good to work with.

What sky ride thingy in STJ?

STT, not STJ, my post was ambiguous, my bad. The tram thing over near the larger cruise port. We went on a day with no cruise ships and drove up because it wasn't operating.

Sky ride? Cable car? Why can't I remember the name of it? The thing that goes up the mountain with the great view.

Thought so but was confused since you said STJ.
The sky ride/Paradise Point Tramway has been closed for awhile now.
Don't know what they are up to or if/when it shall reopen.

The Tramway has been undergoing renovations and is scheduled to reopen at the end of this month.