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Acura MDXs are are not known for fuel efficiency at any speed. I don't expect to win any fuel efficiency contests with Prius drivers. Nor do I care. I bought mine for space and utility, compare gas mileage. The handling , acceleration and quiet ride are good bonuses for owners of this fine beast.

That said, I have noticed that my 2005 is increasingly harder on gas than it used to be. It burns gas like a supersonic jet. When I bought the car, it was getting around 15-18 around town and 21-24 on the freeway. City mileage is about the same, maybe 1mpg off, but the highway is taking a beating as I have never gotten higher than 19 mpg.

What could be causing this? I have my own ideas, but am curious of anyone has experienced a similar drop in fuel efficiency.

I changed my plugs at 207,000 miles and I also noticed that my temperature gauge is reading at about 1/3 between the C and H. Could that be a factor?
 

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A thermostat not working right certainly can impact mpg if it isn't allowing the engine to get to full operating temp. Valve adjustment is another possibility if it has not been done.
 
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Your lower mpg figures are about what mine gets on a good day. I have a Scan Gauge that can calculate per trip fuel efficiency. even when I zero it and the car out, during a trip they can deviate substantially. But by the end of a tank, the Scan gauge usually knows within 0.5 gallons the total gas used, which is what it uses for calibration . So I tend to trust it more than the in car mpg gauge. I get much wider swings in trip MPG than I ever got in my manual transmission Nissan Pathfinder.

As Pkrface said valve adjustment and low temp can cause low MPG - as well as low tire pressure if you don't have pressure monitor, vacuum leaks that frequently end up tripping engine light, clogged PCV and EGR valves, and drift of some sensors like MAP sensor, temp sensor or throttle position sensor. Human factors can influence it like restless throttle foot, or other changes in driving patterns. And if you live in a smog area with high oxygenated summer gas it can make a difference. I'm going through these now trying to get my MPG up a bit without throwing tons of parts and money at the issue.

One point on the temp gauge, most cars and the MDX included, put the temp gauge at 1/3 for many reasonable temperatures so they don't get a lot of customer concern inquiries. I've seen mine as low as 186 degrees and as high as 207 degrees without the temp gauge moving at all. That's another interesting thing about the MDX. The actual temp (once fully warmed up) varies during a trip a lot more than my Pathfinder. Though that could be the result of better temp sensor placement in the MDX rather than any real difference.

Sent from my SM-N950U using Tapatalk
 

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Another thought, are you sure your transmission is making it all the way to 5 (overdrive) on the highway? If you aren't sure, try driving in 4 on the highway to see if the car seems the same or different.

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The common things that can cause that kind of gas mileage loss...

1) Dirty air cleaner element. On mine, I couldn't crack 20mpg on my loooooong road trips (averaging close to 80mph). The final element was tossing the K&N air filter and replacing it with a paper element.
2) PCV clogged / dirty (cheap and easy to swap - probably do it "just because")
3) Valve adjustment (this seemed to add a mpg or two when I did it at 170,000 miles). The PO had neglected to do it, apparently.
4) Worn upstream (pre-cat) O2 sensors. These determine the fuel flow, and can make a big difference.
5) Regional fuel blend (as mentioned previously)
6) Tire pressure (I used to run mine close to 40psi)
7) Long idle times warming up / cooling off the vehicle prior to moving
8) Teenage boy driver in the house... ;-)
 
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