THE new annual rate of interest payable on tax reserve certificates will go up from 4.56 per cent to 5.04 per cent, according to the Inland Revenue Department. The increase will be effective from tomorrow. Interest on tax reserve certificates is calculated in monthly steps. The new rate is $0.42 per month per $100. Interest will be credited as before in respect of complete months between purchase and surrender in payment of tax. It is only credited when certificates are used to pay tax and no interest is due where the principal value of a certificate is repaid by a cheque.