Credit card companies would have to disclose interest rates and fees in clearer, easier-to-understand language under new consumer-protection rules proposed the Federal Reserve Board that could take effect by the end of the year, the Washington Post reported today. The new rules would require companies to tell customers 45 days before terms of a credit card contract are changed, compared with 15 days now. The rules would also expand the list of changes requiring advance notice to include those involving penalty interest rates, which often range above 30 percent. The proposed rules, which the Fed unveiled yesterday after a 2 1/2 -year study, would be most significant change to the nation’s truth-in-lending regulations in 26 years. Companies would also be required to spell out that low rates on balances transferred from another credit card apply only to that balance, not to new purchases.