General Motors Corp. idles two Michigan car plants starting next week as it draws down rental fleet sales.

The auto maker’s Lake Orion, MI, plant that assembles the Pontiac G6 will cease production for three weeks. G6 sales were down 21.8% in January vs. year ago, Ward’s data shows.

Meanwhile, the Lansing (MI) Grand River plant, which makes the Cadillac CTS, STS and SRX, will halt production for one week to lower inventory in advance of the redesigned ’08 CTS. The facility will produce the redesigned ’08 CTS but isn’t likely to tool up until summer for anticipated fall production.

The shutdowns are in concert with GM’s 14% decrease in production for the first quarter, vs. year ago. Much of that is related to the planned 120,000-unit cut in vehicles sold to rental fleets this year.

Starting next week, GM also will reduce output of the ’07 Malibu at its Fairfax, KS, plant that also makes the Saturn Aura.