Craving a new car that weighs two and a half tons? You've got something like 12 months to get down to your local Cadiallac or Oldsmobile dealer to get one. After that, it'll be a used two and a half tonner or none.

A year from now, the last of the Motor City's super giants with acres of hood and mucho horses - the Cadillac Eldorado and Oldsmobile Toronado - will be gone.

Cadillac Division general manager Edward C. Kennard told newsmen at the National News preview here that he expects to sell about 50,000 1978 Eldorados.

But they won't be advertised as the last of the breed because it might not go too well with the forces in the federal government now pushing for even tougher mileage requirements than are already on the books.

Curb weight of the 1978 Eldorado is close to 5,100 pounds. The cars closest to that weight are the Lincoln Town car at 4.870 pounds and the Chrysler New Yorker ar 4,775 pounds.

Most, if not all, other 1978 GM cars weigh less than 4,000 pounds. That is simply an indication of how GM's huge financial resources let it lead the vast program of reducing the size and weight all U.S. cars to make them more fuel efficient.

Kennard carefully refused to talk about what the 1979 Eldorado would look like, arguing. "Why get people excited about something we don't have to sell.?"

There's little of the GM new-car news this year at Cadillac. It's the other car divisions, which will be showing sharply reduced "A body intermediates. Tuesday through Friday, will generate have most of GM's news this month.

Still Kennard floated a number of surprising items:

There will be almost certainly be a diesel powered Cadillac - using the Oldsmobile engine - something after the first of the year.

Cadillac shipped parts for 1,100 Servilles to GM Iran since April, but doesn't see any particularly large future in exporting.

Cadillac experts to top a projected record 327,000 1977 model sales with 345,000 1978 model sales.