My initial guess on the MPG issue is with the exhaust system. I had a similar issue with a different vehicle years ago. At 137k miles is the FJ running with the original catalytic converter, muffler, and exhaust? This can also help explain the sluggish throttle or acceleration. I've done some research in this area and the best options I was able to come up with are either the TRD exhaust system or the Borla dual exhaust upgrade.