There may have been any number of reasons for this decision, and I am guessing cost was a big factor. GM probably decided it could make more money on its high-tech V6, while getting better fuel economy numbers as well.
As the new CAFE rules ramp in, I expect we will see a move away from V8 engines in cars and light duty trucks. I predict the automakers will try to meet the early CAFE standards in approximately this order:
- By adjusting powertrain mix. This is relatively easy and can be done now. Push consumers towards smaller engines, swapping I4s for V6s and V6s for V8s. Build fewer large engines, and bundle them with expensive options, to increase sticker price (and reduce demand). The I4 will become the most common powertrain in small and mid-sized vehicles. Timeframe: now.
- Adding technology to up-market engines. A base "appliance" sedan will be sold with a naturally aspirated I4 in the 1.8-2.5L range. The sport or luxury model would get an I4 with lots more power from turbocharging, direct injection, and other currently available tricks. Timeframe: 3 years out.
- Reducing weight. This one is tricky because doing it too aggressively can cause higher crash fatalities, one one hand, or dramatically increase vehicle cost on the other. Aluminum unibodies are fine if you are building $60,000 Jaguars, but they are not currently feasible for a $25,000 family car. Low hanging fruit include going to aluminum engine blocks and more high-strength steels for bodies. I think you will also see some tricks with interiors, such as thin mesh seats. There will be a move away from large, low profile wheels towards lighter weight designs. NVH may suffer as sound insulating materials are pared down to a competitive minimum. Timeframe: 3 years out.
- Reducing size. To reduce weight, and still have decent performance from a smaller engine, the key is to build a smaller vehicle. I expect more designs like the Honda Fit, where a high roof and upright seating position is used to compensate for a reduction in length. Large vehicles will still be built, but they will be up-market and low-volume. Timeframe: 5 years out.