On street parking in Belfast is limited to 2 hours and so is no use to you for the duration of the tour, except on Sundays when on street parking is totally free.
We recommend a choice of 2 high-rise carparks in the city centre: Hi-Park in High Street and Castlecourt Carpark. Both are a 5 minute walk from city hall (where the tours start). The High Street carpark is also convenient for the end point of both tours. Parking should cost no more than £13-15 for 3 hours in either carpark. If you have any other questions about car parking, Visit Belfast has a good map of all city centre parking options.