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Getting good hybrid mileage really depends on letting the car warm up. For the first 10 or 15 minutes the car's priority is getting the cats and all the other powertrain components up to operating temperature; so the engine runs almost constantly. If you take many short, infrequent city trips (like I do during the week) then you'll be getting about 13-14 mpg. My LS430 over the same route gets about 12 mpg for whatever it's worth. Once warmed up; out on the open road 25+mpg is possible. I've seen 28-29 mpg on the way to Palm Springs; although this is at speeds well above the legal limit. I'm sure someone who was more afraid of the law could top 30mpg easily. Usually I find accelerating under battery power to be not that efficient because then you have to run the motor at some point to charge the traction battery. It's actually better to accelerate quickly to about 30/40 mph; then ease off the gas and let the electric motor take over. Coasting gently to a stop and using as much regen braking as possible is the most efficient way to charge the battery. Coming home from the gas station through lightly crowded city streets late at night I saw 37mpg reported this way. Amazingly efficient car for the power and weight; usually at least 10/20% more efficient than the LS430 over the same route.

Personally I would have the transmission fluid drained and filled. DO NOT do a flush. There is no such thing as lifetime, that is a sales pitch. I guarantee you the fluid is reaching end of life.