Highway gas mileage

[PilotguyPA28]

I bought an RX350 a few months ago with 25,000 miles.   It now has 30,000.  I just went on a 1,000 mile 4 day trip.   My average for the entire trip was right at 22.5 mpg.   About 95% was highway and I drove 75 most of the time.  I was hoping to get closer to 24-25.   Any ideas why it was 22.5?   Would driving 70 give me the 24-25?   Some of the time I even drove 78-80 but mostly 75.   

 

Thanks in advance.   I tried searching and didn't see anything and felt this has to have been asked before.   I also saw that it hits "low fuel" when there  must be 3-4 gallons left.  

[Les Lex 2018]

Here are actual numbers on an 18 RX FWD:

29 MPG at 55 MPH Interstate.

27 MPG at 65 MPH Interstate.

26 MPG at 70 MPH Interstate.

I can’t extrapolate 80 MPH but guess you are getting poor MPG at that speed.

MPG to MPH is not a linear function.  Drag gets a lot worse the faster you go.

No reason to worry about how much fuel you have left in the tank unless you are trying for a record.

I found that the driver’s bladder gets full long before the gas tank gets empty.

[RickC5]

Our 2015 with 30K miles gets 24-26 at 65-75 mph, but there are MANY variables that have to be considered, such as: outside temperature, how heavy your right foot is, is the AC on, how much slowing/accelerating due to traffic, how much ethanol in the gas you use (15% ethanol = 15% worse mileage), etc.  I'm sure there are more things to consider.  You get the idea.