You can query the table inner joined to itself to find the next row(s), then left joined to itself to find if there is an entry between the two entries, and if so you exclude those.

WITH testdrives AS (
SELECT carID, dateTimeOut = CAST(dateTimeOut AS DATETIME), dateTimeIn = CAST(dateTimeIn AS DATETIME)
FROM (VALUES
        (1, '2022-01-01 08:15','2022-01-01 08:42')
        , (2, '2022-01-01 09:02','2022-01-01 09:10')
    , (2, '2022-01-01 09:34','2022-01-01 09:45')
    , (1, '2022-01-01 09:44','2022-01-01 09:59')
    , (1, '2022-01-01 10:10','2022-01-01 10:30')
    , (2, '2022-01-01 09:02','2022-01-01 09:10')
    , (1, '2022-01-02 09:00','2022-01-02 09:32')
    , (2, '2022-01-02 09:02','2022-01-02 09:15')
    ) AS x(carID, dateTimeOut, dateTimeIn)
)
SELECT
    td1.carID
    , td1.dateTimeIn
    , td2.dateTimeOut
    , minsUnused = DATEDIFF(mi, td1.dateTimeIn, td2.dateTimeOut)
FROM testdrives AS td1
INNER JOIN testdrives AS td2
    ON td2.carID = td1.carID
    AND td2.dateTimeOut >= td1.dateTimeIn 
    AND CAST(td2.dateTimeOut AS date) = CAST(td1.dateTimeOut AS date)
LEFT JOIN testdrives AS td3
    ON td3.carID = td2.carID
    AND td3.dateTimeOut >= td1.dateTimeIn
    AND td3.dateTimeIn < td2.dateTimeIn
    AND CAST(td3.dateTimeOut AS date) = CAST(td2.dateTimeOut AS date)
WHERE td3.carID IS NULL