President Reagan's tax revision would increase the federal deficit if it were enacted as written, according to the findings of two economic forecasting firms.

The two firms differ on the specifics and the timing of the deficit increase, but economists from both firms will tell the House Ways and Means Committee today that the plan would bring in less revenue than current law.

Data Resources Inc. believes that expensive business depreciation provisions and higher interest rates would raise the deficit in the long run, while Wharton Econometrics will testify that slow economic growth and a big tax cut for individuals would cause the plan to bring in less federal revenue in the near term.

A third firm, Chase Econometrics, found in running the plan through its econometric models that the deficit would be slightly lower than currently projected if the tax overhaul were enacted. The firm based that calculation on an anticipated reduction in interest rates, which would lower the cost of servicing the national debt.

The "revenue-neutrality" of the tax plan is a crucial issue as Congress considers the proposal. Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.) has said he is worried that the administration proposal would lose $12 billion over the next five years, a relatively small amount compared to the receipts projected for that period -- and a small amount compared to the losses predicted by the forecasting firms.

Roger Brinner, chief economist for DRI, said yesterday the deficit would be $220 billion by 1995 under the plan -- $50 billion more than if the tax proposal were not enacted. He emphasized that DRI's baseline projection, which assumes no tax changes, is based on some future cuts in federal spending.

Brinner said that the 1.5 percent increase projected by the administration in the level of gross national product by 1995 is not out of the question, but that even that expansion wouldn't bring in enough additional tax revenue to offset the loss from the proposed depreciation system which would result in larger tax losses as years passed.

"I think on fairness, the president gets a 'B.' On simplicity, he gets a 'C.' On growth, he didn't even attend the class," Brinner said.

The proposed depreciation system, under which the value of business investments would be indexed for inflation, wouldn't start affecting revenue until after 1991 or 1992, Brinner said. Because of inflation indexing, the new system would be "back-loaded" -- business would get more benefit later in the lives of the assets they buy.

Interest rates would be slightly higher because national saving -- a combination of personal, business and government saving -- would decline as a result of higher government borrowing due to the revenue loss. If the quantity of saving is smaller, the "price" of borrowed money -- interest rates -- would be expected to rise, Brinner said.

Treasury Department spokeswoman Kim Hoggard said the department would not comment on the economists' findings because officials had not seen their figures.

Unlike DRI, Wharton's econometric simulations of the effect of the tax plan supported the Treasury Department's projections for revenue on the corporate side. But revenue from individuals would be less than projected.

"It's not revenue neutral," said chief economist Nariman Behravesh. "We think the Treasury has underestimated the magnitude of the cuts going to individuals."

The difference occurs, he said, because some of the prospective elements in the plan, such as taxation of the increase in value of life insurance policies, would bring in new revenue more slowly than the administration predicts. And, Behravesh said, Wharton is more pessimistic about economic growth in general than the administration, which also results in lower revenue forecasts.

Wharton expects the deficit to rise and then fall back to the predicted baseline in later years. From 1986 to 1990, the plan would lose $40 billion, compared with the administration's prediction of $12 billion. In 1989, Behravesh said, the deficit would be $30 billion higher than it would be absent the proposed tax changes. But as the tax cut for individuals stimulates growth, the deficit would fall again to within $10 billion of the baseline.

Chase, the third firm, said it believes the deficit would decline because of lower interest rates, which in turn would spring from a short-term slowdown in economic growth. Leon Taub, Chase vice president, predicted that GNP would grow by 1 percent less than otherwise, because of the slow phase-in of personal tax-rate cuts and curtailment of some business investment incentives.

Also yesterday, representatives of small-business groups told the House Small Business Committee that the tax proposal would help very small firms that pay taxes using individual rate schedules. They praised changes from the Treasury Department's original version that restored lower rates for small firms. But they protested the continued complexity of the plan, changes in accounting rules and cutbacks in employe fringe benfits.

A hopeful note for the tax plan also was sounded yesterday, as House Speaker Thomas P. (Tip) O'Neill (D-Mass.) said he hoped to work out an arrangement with the Senate that would permit a tax bill to be passed by both houses before Thanksgiving. O'Neill indicated he would be willing to hold the House past its planned adjournment date of late November if enactment of tax legislation were close.

Staff writer Margaret Shapiro contributed to this report. 1986 to 1990, the plan would lose $40 billion, compared with the administration's prediction of $12 billion. In 1989, Behravesh said, the deficit would be $30 billion higher than it would be absent the proposed tax changes. But as the tax cut for individuals stimulates growth, the deficit would fall again to within $10 billion of the baseline.

Chase, the third firm, said it believes the deficit would decline because of lower interest rates, which in turn would spring from a short-term slowdown in economic growth. Leon Taub, Chase vice president, predicted that GNP would grow by 1 percent less than otherwise, because of the slow phase-in of personal tax-rate cuts and curtailment of some business investment incentives.

Also yesterday, representatives of small-business groups told the House Small Business Committee that the tax proposal would help very small firms that pay taxes using individual rate schedules. They praised changes from the Treasury Department's original version that restored lower rates for small firms. But they protested the continued complexity of the plan, changes in accounting rules and cutbacks in employe fringe benfits.

A hopeful note for the tax plan also was sounded yesterday, as House Speaker Thomas P. (Tip) O'Neill (D-Mass.) said he hoped to work out an arrangement with the Senate that would permit a tax bill to be passed by both houses before Thanksgiving. O'Neill indicated he would be willing to hold the House past its planned adjournment date of late November if enactment of tax legislation were close.