President Carter's new $615.8 billion budget came under fire on Capitol Hill again yesterday, this time from skeptical House members who challenged its $15.8 billion deficit as unrealistically low.

At a hearing of the House Budget Committee, both Democrats and Republicans complained that Carter had based his figures on assumptions that were unlikely to hold up, and that the red-ink figure would be higher.

At one point, Rep. Tim Wirth (D-Colo.) told administration witnesses his own computations showed the deficit was likely to run as high as $30 billion when the final figures were in.

However, officials said later Wirth's calculations contained several mistakes of their own, including the charge that Carter's budget did not account for government grain purchases.

The administration also denied allegations by Wirth and Rep. Delbert Latta (R-Ohio) that the Pentagon would seek an extra $3.55 billion in fiscal 1980 and $4.5 billion in fiscal 1981 to cover higher-than-expected oil prices.

Officials said the Pentagon still has not submitted any revised estimates of its fuel costs for fiscal 1980 and 1981 and that any shortfall that might emerge would be only a small fraction of those totals.

The defense budget Carter submitted contained fuel-cost estimates based on oil-price levels that were in place in September. Crude oil prices now are some $6 a barrel higher.

Yesterday's questioning provided still further evidence of congressional discomfort with Carter's new budget. On Wednesday, members of the Senate Budget Committee lambasted the deficit as too high.

Treasury Secretary G. William Miller and Charles L. Schultze, chairman of the Council of Economic Advisers, who testified at yesterday's hearing, defended the deficit estimates as the best that were available.

However, Wirth, and several others on the panel, remained skeptical of the deficit figure. Among other things, they cited $5.6 billion in proposed cost-cutting measures Carter is counting in that may not be enacted.