MPG Decline Over Time

f1racer328

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So it's pretty known that these rovers get atrocious MPG. As far as this post is concerned, I'm talking about the 5.0 V8 (2013 MY), although I'm sure the V6 isn't much better.

Anyways, I've owned my Discovery for 51,000 miles now, and have charted every single fuel up. (Date, gallons, odometer)

As a result, I have a great history of my car and everything I've done to it. I bought it with about 43k miles on it and now have 94k miles. $10,000 worth of fuel later and almost 3 years of ownership.

Anyways, what I've been seeing is a decline in my average MPG. I do a bit more towing now, so the second chart has any MPG below 11 phased out, but it is still on a steady decline.

I have not replaced my spark plugs or cleaned my fuel injectors. Those are the things I'm really wondering about, so I might do something about that soon. I did have a pretty bad wheel bearing a while back, and replaced the front as a set, and the dealer had replaced one of the rear bearings a long time ago. All of the wheels seem to spin freely and without much resistance.

I also got larger tires, and expected a hit from that, but it didn't really change much (255/55/19 to 255/60/19)

Besides the additional towing, driving habits haven't changed much. I moved to a hotter climate which runs the viscous fan more, so that could be impacting this as well, but even my 100 day 200 day average is still on a decline.

Anyways, here are photos of the graphs, I've also attached PDFs if anyone prefers that format.

Not sure if this is interesting or if anyone has their own data like this, but that'd be awesome if anyone else keeps track of this kind of stuff like I do. Engine probably needs some work, and I'm sure I could swap out all of the other fluids as well.

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Here's the chart with nothing under 11 MPG taken into account

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PaulLR3

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I don't have any exact data, but I know that after 132,000 miles my average mpg is lower than ever. And that is just based on the onboard mpg calculator that isn't that accurate. When the LR4 was new, I could hit 20 mpg on a long distance highway trip. Now 18 mpg is about max, despite being really well maintained. New spark plugs at 90K, always premium top tier gas, replaced bad hubs, bearings, dragging brake calipers and more.
 

Stuart Barnes

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How many horses have escaped? I wonder if anyone has dynoed one of these trucks.

Engines wear, things get tired and dirty, yes I’m referring to you inlet valves, power drops off, more loud pedal for a given speed and acceleration.

All pointing to a slow decline, a bit like me really.

You’d be really complaining about mpg if it were a petrol land cruiser.

But keep on roverin :)
 

Stuart Barnes

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@f1racer328: If you have a GAP tool or the like, take a look at your fuel trim data. Dirty injectors should show up as an increase in fuel trim (increases time open on the injector to compensate for lower flows).


Do you know the rough baseline for the trim values? As I’m not sure of that one.
 

greiswig

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I did a quick google search but didn't turn up anything: I wonder what is a "normal" loss in MPG for a vehicle correlated with odometer reading?

To the OP, great data capture!
 

jlglr4

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Do you know the rough baseline for the trim values? As I’m not sure of that one.

Ideally, fuel trims should be as close to zero as possible at idle or any steady speed. I think you generally want to see within +/- 5%, but I believe most shops will consider anything +/- 10% to be within normal range, provided there are not drivability issues.

Obviously, it’s not a specific diagnostic. Anything that contributes to inefficiencies/inaccuracies in the fuel mixture can affect the fuel trims. So, little vacuum leaks, aging o2 sensors, fuel pressure, even valve problems can all contribute. But different problems will affect fuel trims differently. For instance, I believe dirty injectors will cause positive fuel trim at both high and low RPMs, but vacuum leaks typically show up with high fuel trims at idle that diminish with increasing engine speed.

I’m not so well versed to know all the particulars, but it’s a useful piece of info to see how your ECM is trying to compensate for whatever is going on.
 

ktm525

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to the OP: Same fuel blend? Up here we get "winter gas" which cuts mileage. Fortunately we still have some sources which are 100% gasoline and none of this alcohol crap.
 

f1racer328

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Live in Phoenix Arizona, so fuel blend is probably always the same. I usually put in 91 octane fuel from Costco (Top Tier)

I do have a GAP Tool and have checked my fuel trims. They are actually negative for long term when I checked last (-.781% Bank 1, -2.34% Bank 2)

Short term trims change all the time but when I checked last they were both positive (.781% and 3.12%)

I even looked at the fuel injector pulse widths and they are all nearly identical.

I might start playing around with Sea Foam. I'm sure the inlet valves aren't clean. I tend to drive more with a heavier foot, and towing gets my engine temps nice and hot, but I'm not sure if driving style will prevent carbon buildup.

I guess my hitlist in the next few months will be fuel injector cleaning, spark plugs, and trying to get carbon off of the inlet valves. Maybe O2 sensors.

If I do anything I will try and do a (mostly) scientific measurement of a before/after. Ideally same stretch of road/speed/weather conditions.

If I had money to piss around I'd dyno it.....
 

ktm525

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Start easy. Can of BG44K. Then plugs. BG sells an intake tract leaning service as well which may get some of the junk off the backside of your intake valves.

Then the expensive stuff.
 

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