Unions have been in talks with Jaguar bosses in the hope of persuading them not to stop car production at the Browns Lane factory in Coventry.
More than 1,100 workers face losing their jobs
Efforts to secure a breakdown of the £80m cost savings a year, which Jaguar parent Ford said will result from the planned lay-offs, have been frustrated.
"We had a full and frank conversation but Ford is still withholding vital figures," the Amicus union said.
No more than £15m can be made in salary savings, unions are arguing.
Ford was "much more forthcoming with information and it was a positive meeting but there are a number of questions that we await answers to," said Dave Osborne, T&G national secretary.
"We are yet to be presented with any evidence that convinces us that ending car production at Browns Lane will answer Ford's problems," he added.
Amicus said it wants Ford "to justify where the remaining £65m is coming from" and wants to know the cost of transferring production from Browns Lane to the Castle Bromwich site.
"We estimate that the move will cost close to the £80m they say is going to be saved," said Roger Maddison, Amicus Regional Officer.
Union officials say they considered Friday's meeting a first consultation, and would be pushing for more talks.
Hundreds of people marched through Coventry last Saturday in protest at plans to end car assembly at the plant.
Workers were balloted this week on whether to take strike action and the result will be known in mid-December.
Jaguar, now part of US giant Ford, announced the decision to end major production at Browns Lane in September.
Jaguar says cuts are necessary to secure its future, as the firm is losing hundreds of millions of pounds.
Under the plans to end production of XK and XJ Jaguars in Coventry, there will be 400 voluntary redundancies and 425 jobs will be moved to the firm's Castle Bromwich factory in Birmingham.
Jaguar will also lose 750 mostly white-collar staff, as office work is merged with Land Rover.
Jaguar has urged workers not to strike over the plans, saying it would not be in the interests of the firm, employees or customers.