General Motors Corp. is getting very creative with production schedules and model-year timing in order to meet federally mandated corporate average fuel-economy (CAFE) regulations.

GM halted production of its 1998 full-size Chevrolet Tahoe/GMC Yukon and Chevy and GMC Suburban in January after a four-month model run and switched over to '99 models to keep from having too many low-mileage '98s upset its CAFE ratings. Last fall it bolstered suspensions on four-wheel-drive Suburbans. The effect was to increase the gross vehicle weight to more than 8,500 lbs. (3,856 kg.). The government regards anything over that weight as a medium-duty truck and therefore immune from CAFE standards.

Now comes word that CAFE is going to cut short GM's '98 model year run for its big pickup trucks. Also, GM will produce more V-6 engines and fewer V-8s.

Chevrolet and GMC were to have dropped their full-size C/K pickup trucks this summer to make room for the redesigned Silverado and Sierra. Now it will build both old and new models, mostly in half-ton versions rather than three-quarter- or 1-ton, which get fewer miles per gallon.

CAFE requires the average fuel economy of each automa-kers' fleet of cars and trucks to meet prescribed minimums. CAFE for cars is 27.5 mpg (8.5L/100 km) through 2000; for trucks it's 20.7 mpg (11.4L/100 km).

The law does, however, allow an automaker to go back three years and borrow credits from years when it exceeded the requirement or to go forward three years and borrow credits if it expects to do better than the standard.

In 1996 GM's truck average was 20.9 mpg (11.2L/100 km), besting CAFE by 0.2 mpg. In 1997, however, its truck average was only 20.2 mpg (11.6L/100 km). For 1998, it forecasts its truck average at 21.2 mpg (11.1L/100 km), giving it a 0.5 mpg credit. But it will take some intriguing gyrations to get there.

The '98 C/Ks will become '99's without any change in architecture. They will be built through December, even though the redesigned '99 Silverado/Sierra goes into production this spring in Oshawa, Ontario.

Chevy also has cut short production of its 1998 G-Vans (the Express passenger van and Chevy Van commercial van), ending the run on March 13. Chevy started taking '99 orders March 31 and will begin building the '99s May 18. The low-mileage full-size vans threatened CAFE.

Production of the '98 Chevy Venture minivan was to run through June but its run has been extended through Aug. 28, 1998. Production of the '99s will begin Aug. 31. That means a late-fall arrival for the new models. Chevy is continuing '98 Venture output because mileage from minivans, considered light trucks, is relatively good and helps raise GM's CAFE for trucks.