Hi, I am experiencing bad gas mileage on my 06 Lexus RX400h.

Hello, fuel economy is a difficult thing to assess and determine. All you can really do is make sure that everything is working as designed. Engine performance is good - no check engine warning lights, a check of on-board computer diagnostic systems shows all things working as they should. Make sure spark plugs are changed, air filters are changed as prescribed by your maintenance manual. Additionally, your hybrid battery may not be doing what it should - causing the engine, instead of electric motor, to carry more of the burden. Have your hybrid battery state of health checked. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops