good day all, perhaps someone can help, i have notice on my 2007M35x very poor fuel efficiency, 14-15 miles in high way per gallon. i had cleaned the throttle body and seen a bit of improvement, however i see, when i'm driving the rpm is bet 12 rpm and when i slowdown is about 10rpm, is this normal? should i replace the throttle body and if so, what would you recommend, since i have seen some bet $65.00 and $170.00 or perhaps any other alternatives

thanks

Old cars just get worse MPG. Cleaning the throttle body can help, as will making sure your spark plugs are functioning properly, and doing a fuel service to clean the injectors as well as seafoam in the intake to help remove excessive carbon buildup inside the engine.
Other than those things, I feel like all the other stuff to make the engine get back to peak efficiency is just too much money to be worth doing on old cars. Things like taking the engine apart and honing the cylinder walls, replacing the piston rings, replacing any worn bearings you find during disassembly, removing all the valves from the cylinder heads and cleaning them up and doing a bit of grinding to reseat them perfectly again, cleaning out the engine entirely of all carbon buildup. An engine rebuild like that is likely going to cost as much more more than the car is worth. Even if you did that, the engine was never rated for very good MPG to begin with, so you are only going to get up to 20-22mpg highway even with all brand new rebuilt stuff. You'd never make back that much money in gas savings.