The tax rate applied to an imported vehicle depends on if it has a CO2 figure on the final build type approval certificate. Assuming your car was imported from Japan, as this is outside the EU it will not have a CO2 figure applied to it, and in this case vehicles are taxed as private light goods which is why a higher rate is applied. Unless you are able to obtain a type approval certificate with the CO2 emissions listed, you will be unable to challenge this decision.