Transmission or Vacuum Pump

If it was an issue with the fuel line and sender unit, I suspect your Montero would be doing other strange things like bucking at times, seeming to lack power, say on hills, or when you are trying to pass another vehicle on the highway.

That you are burning extra gasoline does tell me something and it also leads me to a question. First, it tells me that your SUV is likely over-revving and using lots of gas because you are running in gears that are well over a 1:1 gear ratio. In other words, your engine is running too fast for the speed you are traveling. For example, if you look at the RPMs when your SUV is in first, the chances are good that you are running well above 4,000 rpm — likely higher — to maintain a speed. And, just maintaining that speed is likely straining things pretty well.

With that said, it leads me to this question: does your Montero seem to be straining with every shift? In other words, is it overrevving all the time? If it is, then I am sure I have your answer. Could you let me know, when you have a moment?