Book value

Hello. You are kindly requested to help me in calculation of book value of all fixed assets in order to evade maximum income taxes. Actually i belong to lare manufacturing industry firm which posseses number of fixed assets, and among them we disposed assets viz vehicles, and at the end of financial year, we came to know that there has been a capital gain over these assets as the book value furnished to sales tax was quite less. Please tell the procedure how to keep check over this in near future and with what method to calculate book value?