Motor bicycle registrations fell 46 percent to 11,634 in June from 21,833 a year earlier and three wheelers fell 64 percent to 4,134 from 11.630.
Sri Lanka raised taxes on vehicles in February 2012 to prohibitive levels to discourage imports.
Analysts had warned that the move will hit tax revenues widening a budget deficit and putting further pressure on interest rates, the credit system and dollar peg, and delay a recovery from a balance of payments crisis, which is a monetary phenomenon.
To stop currency depreciation authorities have to stop printing money to monetize debt or sterilize foreign exchange sales by raising interest rates.
Lorries (trucks), whose taxes were cut held up better growing 8.96 percent to 2,542 from a year earlier. Lorries were also down from a March peak of 3,425.Sales of Maruti/Suzuki, who make small cars and is Sri Lanka's most popular brand fell to 318 units in June from 1,334.