I've been driving my new 2012 JK Unlimited Rubicon to establish baseline performance before starting the project.
As I wrote in 4Wheel Drive Magazine, the '12 is a better Jeep than the '07-'11 JKs are.
The press mule I drove for the feature in the magazine returned excellent fuel economy. On one 220-mile trip around Southern Utah that included over 50 miles of dirt track, the tester returned 20.9 average mpg over the whole trip!
I'm not getting that kind of mileage with mine. If I coast down a hill, I can get upwards of 20 mpg, but normal driving around town is returning about 13-14 mpg. No, I'm not mashing the go pedal either. Disappointing.
The 3.6 VVT V6 is an improvement over the 3.8, but it has less torque off-idle than the 3.8 V6 did. The 3.59 first gear of the WA580 automatic transmission compensates for this, but the 3.6 doesn't shine until the RPMs reach about 4,000, when the JK feels like it has a small block V8 in it. The WA580 is worlds better than the 42RLE, though. It makes the JK much more pleasant to drive anywhere, on- or off-road.
Should you trade your '07-'11 JK for a new one? Maybe. The '12 is better. Is it enough better? If my JK's mileage doesn't improve, I'm already thinking V8.