Dear Stanley, The pictures of your car on the VintageBentleys.org web site suggest it is a sporting 3 litre in good order. Replacement of tyres will depend on the condition of the wheels, if they are good, and do not need replacing, then new tyres of a similar size could be fitted. Availability depends on where you are. If in USA, many people like locally made tyres, but they don't look very vintage. In uk, there is choice, with Blockleys for the sporting minded, if in your size, but they may not be, BF goodrich make a very nice 6.00 x 19, maybe also in 20"

As your car is 1925, it would originally have had beaded edge wheels, but these days, they are less used, because they are expensive, wear out quickley and give a harder ride! Hope this has helped initially. I believe that some VB's were fitted with 20" wheels when new, around 1926, but have no further info.