With only a few days of contract bargaining left to avert a nationwide coal strike, United Mine Workers President Arnold Miller said yesterday he would consider a contract extension if the talks begin to show "substantial progress."

But he emphaisized that progress now is insufficient to merit consideration of an extension, under which miners would continue to work while bargaining continues.

The current contract between the UMW and the Bituminous Coal Operators Association, the industry's main bargaining arm, expires Dec. 6. But both sides agree that it would take at least 10 days for miners to ratify a contract - meaning an agreement would have to be reached by late this week to avoid a walkout Dec. 6 if there is not contract extension.

Industry officials indicated interest in Miller's comments but declined to react.

Even if Miller were to recommend a contract extension, there is some question whether UMW members - who have a no-contract, no-work tradition and a propensity for wildcat strikes - would stay on the job.

But one industry observer, noting the lack of normal wildcat, activity this year, indicated they might continue to work this time. Miller noted that contract extensions are not unprecedented, having been approved twice in the 1950s.

It was not clear what Miller meant by "sufficient progress." He contends that no progress has been made since talks started Oct. 6, while BCOA was reporting some progress last week. "We'd have to be moving along toward a possible settlement," Miller said yesterday.

Even if a strike occurs, it is not expected to have a major impact on national energy supplies at least for about three months, according to government and coal, steel and utility sources. UMW officials conceded yesterday that stockpiles are sufficient to withstand a long strike.

Miller made his comments in response to reporters' questions at a briefing called to discuss a UMW-drafted economic analysis, which concluded that BCOA, which represents companies employing about 130,000 UMW members, can afford a 50-percent increase in miners' compensation.

Including benefits, miners now earn $13.08 an hour, or about 35 per cent of the sale value of coal, according to the study. If labor costs amounted to 53.5 per cent of the sales value, as they did in 1969, miners would earn $19.97 an hour, an increase of $6.89, according to the study done for the UMW by Stanley Ruttenberg, an economic consultant.

Miller emphasized that the union is not asking for a 50-per-cent increase but rather is saying it could be justified economically in light of profits, hazards to workers and pay gains by workers in other industries. Miller noted that the UMW has yet to make a specific wage request. Benefit guarantees and grievance procedures, including the union's demand for a right to strike over local problems, are considered bigger obstacles to a settlement than wages.