Uncle Sam, already finding it tough to sell his 6.5 percent Savings Bonds in a double-inflation economy, is about to face a lawsuit charging the Treasury with "fraudulent" promotions.

The Gray Panthers, a senior citizens lobby that played a role earlier this year in forcing somewhat-more-generous treatment on savings accounts for small depositors, plan to file such a suit with the Federal Trade Commission within a few weeks.

Robert L. Gnaizda of the senior citizens group said in a telephone interview that the Treasury "fails to disclose the impact of inflation" in selling savings bonds.

Gnaizda pointed out that promotional materials for Savings Bonds often incorporate extravagant promises such as "Make Your Dreams Come True." The point of the lawsuit will be to require full government disclosure of the impact of inflation, Gnaizda said. Buyers should be warned that a $100 bond might be worth only $50 when cashed in, he added.

Despite an increase in the interest rate from 6.0 percent to 6.5 percent on June 1, new purchases of Savings Bonds aren't keeping pace with liquidation. To make up the difference, the government has to borrow the money in regular financial market transactions.

At the last sale of Treasury bills on Monday, the government paid a record 10.531 percent for 13-week bills, equivalent to an 11 percent yield. The discrepancy between such high rates and the low rates available to small savers through bonds or savings accounts is increasing pressure for a change in the system.

According to Treasury officials, Savings Bond sales this year to date have totaled $4.991 billion, down from $5,563 billion in the comparable period of 1978. Redemptions also have increased to $5.8 billion from $4.1 billion in the same period a year ago.

A main advantage to Savings Bonds, compared with savings accounts paying about the same rate of interest, is that taxes are deferred until the bonds nature. But the Gnaizda group argues that this advantage is insignificant when compared to higher rates available to large savers.

In some cases, however, Treasury sources say, continued extension of the deferral of taxes on some of the early E bonds has amounted, in effect, to tax forgiveness. The Treasury says it does not know how much interest has been involved.

On present Savings Bonds, Treasury officials have indicated that tax deferral can be extended for at least ten years, if the bonds are "rolled over," that is, renewed.