I seem to be getting about 9 mpg in my 2004. It now has 73k miles. I usually use 91 octane gas, but my wife has put 87 octane in it here and there. I'm also afraid the PO's may have done the same, but who knows. All I know is that 9 mpg sucks and that it should be better. I do drive a bit heavy and most all is in town. HWY seems to get me about 14 mpg.
One thing I must admit- at 66k I changed the oil using a good Castrol Synthetic along with a Mobil1 filter, and I did an engine flush w/ tranny fluid per forum sugg. however i'm now at 73k miles- 7k later on the same oil!
It was oversight on my part, and i promise not to do it again. Could worn oil be a cause here, and have i done irreversible damage to the engine?
Also, is it maybe time for an induction flush and throttle body cleaning perhaps? I lifted the throttle body and it really dosent seem that bad in there? Air filter looks good, and i'm not sure what else to check.

 

I think you know the answer to your own questions. Use 93 octane gas only. If you have not already done it do the 60k service and also change your gear oil in all 3 boxes and change the ATF. I doubt you hurt your engine but your mileage is getting up there and you need to start keeping up with the maintenance or things will go downhill fast.

 

Just for the sake of conversation the wife took the D II to Reno and back to Tahoe this involves going up and over an 8000 ft pass down to 5200 ft and back up and over an 8000 ft pass to get back to our house at 6400 ft. Long story short this was all highway approx. 140 miles total with a full tank of 87 octane and I was shocked today to see how much gas was left !
Then I drove 24 miles today in town(hitting every red light I could) and burned up an entire T-REX in FOSSIL FUEL !! I talked to my wife after this in amazement(this is her 3rd Rover) these trucks gets reasonable MPG up on the highway cruising along but around town these trucks just get crappy MPG no matter what you do. The only way to get decent MPG around town is if all of the guys/gals from the UK send us all Diesel engines to replace our 4.0's for free.
One other note I run 87 octane for a scientific reason involving the altitude we live at.

 

Even at high altitudes you should use 91 octane correct?

On a side note I changed all my O2 sensors, did and induction cleaning and changed my wires and the effect on my mileage seemed negligible. These trucks are gas guzzlers though 9mpg seems extreme unless you have a lead foot and live in Manhattan.

 

You need to let your wife know she will be riding a bike the next time she puts in regular and any head work that comes from this down the road comes out of her money.
As for the truck, when was it last tuned, both plugs and wires, how hard is it driven and how much air pressure in your tires?

 

Altitude lowers the air density and in turn compression, and increases detonation margins in internal combustion engines, but only slightly at your (and my) altitudes. You still need to run premium (91 octane) fuel to maintain detonation margins and to keep the engine knock sensors from continually retarding the ignition timing on your engine, which reduces power, makes for less complete combustion, and more engine deposits.

Regards,
Dave

 

it sounds time to do a complete tune up and fluid swap,,,

change your plugs and wire. induction cleaning. TB cleaning. change ALL the fluids to full synthetic. pressure up your tires. 91 octane ONLY!!! take anything out of the truck you are not using that is heavy. weight is a killer. if you have a roof rack and are not using it, take it off. dont drive the thing like its a ferrari,,,its not. try and avoid stopping at lights by slowing to them and avoiding a complete stop. dont speed on the highway.

i see 14/18 from my 04. you should be somewhere in that ballpark

 

what about 93 Octane, is that alright to use? most pumps around where I live have 87, 89, or 93. i know it says 90-92 recomended, but usually 93 is whats available. i do go with 91 if its available.

 

If you use premium you will be fine. The ECU's don't take advantage of above 91 octane. Even in a performance sports car you need to do ecu mods to take advantage of above 91 usually.

 

youre O2 sensors are probably going bad.