2006 and 2007 Impalas, 3.5 v-6, rented a year apart for 1400+ mile trips.

Speeds kept at posted limits – 65 & 75 on highway and interstate. Full use of cruise, occasional A/C.

Mountain passes through CO, ID, UT, MT, OR.

Average mpg for 2006 – 30-32.
Average mpg for 2007 – 32-34.

Recently tested 2008 LTZ with 3.9. Computer indicated mpg hovering just under 30.

Note that the final drive ratio (like the old ‘rear end ratio’ for FWD) for the 3.5 car is tall – something like 2.79 or 2.89. The 3.9 car is lower – something like 3.29. The rpm’s were noticeably higher at the same speed than the 3.5 car.

While the 3.9 car felt much stronger, it’s not much more powerful, and the lower gearing could account for the better acceleration and lower mileage. The cylinder deactivation didn’t seem to do much.

I’d like to get a 3.9 car and have the shop change it to the final drive ratio of the 3.5 car. Too bad GM doesn’t offer options like that any more.