New rules will give mutual fund owners two versions of the after-tax performance of their funds--one assuming that the owner sold the fund, and one assuming otherwise. The two versions are intended to show how the fund compares to others on an after-tax basis, using the top tax rate of 39.6 percent. However, few people pay that rate. The new Securities and Exchange Commission (SEC) rules, pushed by Congress, require that the SEC tell mutual funds how to report after-tax returns, and SEC investment management division chief Paul Roye says that the comparison is between one fund and another on a historical basis. Mutual funds that advertise themselves as "tax efficient" will have to include the two after-tax numbers in October, and all funds will have to include them by February. The SEC chose the maximum tax rate because the one-year calculation is based on short-term capital gains, and because reflecting every tax bracket would be too complicated. Opinion is divided on whether the SEC should have chosen the year-and-a-day calculation instead. Developing the actual numbers is complicated, so most fund companies have hired others to do the calculations.