Believe it or not. The MPG of my wife's M35X with fully charged original factory battery(over 9 year old now) has been improved at least 20% recently...
PS: Right now is winter. Usually we get worse MPG on VQ35HR during cold winter...
You may ask whether the condition of M35X battery is good or not. In my opinion it's still GOOD. Why? The battery still can receive charge very well(That's why it put a lot of load on car) and still provide good power when start-up even though most of the trips are very short...
Believe it or not. My wife M35X's MPG get better now. (10% better) I should do it earlier. We had waste some money in gas before. (The generators/alternators in cars are extremely low efficiency comparing with electrical power from local power company)