MPG computer lies - badly!

[StuartV]

First, this thread is NOT about the absolutely atrocious mileage I'm getting. There are already enough threads on the Hemi's bad mileage.

This is about how the computer that displays my fuel economy is lying. Lying like a rug! Liar, liar, pants on fire!

I've got about 1050 miles on my new truck. I have been tracking the mileage using Fuelly since I filled it up when I first got it home. It had about 220 miles on the clock then, so Fuelly says I have been tracking fuel economy for 813 miles, so far.

Whenever I fill up, I reset Trip B, AND I reset the Fuel Economy display in the EVIC (center dash display).

Yesterday, I filled up after driving 383 miles on the previous fill-up. It took 30.518 gallons. That is 12.55 MPG. But, at the point of filling up, the EVIC Fuel Economy display said I had been getting 14.5. That is a pretty huge discrepancy.

I reset Trip B and the Fuel Economy display when I filled up (as I always do).

After filling up, I immediately got on I-85 and headed north, through Richmond, towards DC. When I got home (after all highway driving), I filled up again, just to check it. I had driven 160 miles and it took 10.890 gallons. That is 14.7 MPG. But, the Fuel Economy display AND the Trip B display both said I had been getting 17.2. Again, a huge discrepancy.

We all know that the exact way you fill up, how level the truck is, etc, etc can affect mileage calculations. But, with these distances and amounts of fuel, the discrepancies that could come from those variables is going to be in the tenths of a mile per gallon - not 2 whole MPG.

Over 813 miles, those discrepancies pretty much wash out. One fill-up might have been a little short, for example. But, the total amount of gas put in is exact, as is the number of miles driven. So, the only 2 variables that affect anything are exactly how full it was at the very start of that 813 miles and exactly how full it was after the very last fill-up.

So, Fuelly says that my average over 813 miles is 13.2 MPG and my best on any one fill-up was 15.8 MPG. All of which is simply to validate the specific "hand calculated" numbers from yesterday. 12.5 for 3/4 city driving and 14.5 for all highway seem completely in line with the overall history.

And the amount of optimism shown in the onboard computer is really disappointing.

I have seen a bunch of posts on here from people talking about getting 20+ MPG with their Hemis. Some of them have explicitly acknowledged that their statements were based on what the on-board computer says. So, uhhh.... yeah. You guys reporting what your on-board computer says may as well save your breath - unless you also hand-calculated and are reporting how the two numbers compare.

Personally, my new truck and my last truck (an '09 4th Gen) are the same - CrewCab, 5'7" bed, 4x4, Hemi (new one has eTorque), bed cover, stock sized tires, and 3.92 rear end. I was really hoping to get a decent bump in fuel economy with the new truck, just because of the 8-speed tranny and the eTorque (and having 193K less miles on it). And the fact that new truck has air suspension and lowers itself at speed where my old truck had coil suspension and actually had a 2" lift/level on it. I THOUGHT, from the on-board computer, that the new truck was doing a little better than the old truck. I am pretty bummed to see that that was all lies. The new truck is definitely doing no better than the old truck and maybe slightly worse.

 

[Ram92131]

Mine has been spot on a couple times, has over-reported once or twice around 1.5 mpg, and has even under reported at least once. But usually it’s around .5mpg over. In your case I’d suggest running the tank empty instead of filling after 10 gallons. Your hand calc will be somewhat inaccurate regardless of how carefully you click off the pump. Better to spread that error out over 2x or 3x the gallons (if you have the 33gal tank). Your statement about overall mileage over 813 miles is valid, but since you reset the mileage meter each fill up you can’t compare it to anything.

Your overall mileage over that time seems in line with what I had been getting that early on. My mileage as recorded by Fuelly has improved a little but it isn’t the big step change other have claimed. I’m at 15mpg average now and mostly highway miles.

 

[alow]

I've found mine to be pretty accurate over all. Every now and then it drifts but its usually within 1 mpg.

 

[JR2085]

Over our first 2k miles on our 5th Gen, we have averaged 17.8 MPG's and an average speed of 30.3 MPH. Most times when I hand calculate, the on board computer is .5 MPG short, rather than over stating.

 

[GRAYWOLF]

This is my 4th consecutive ChryCo product, and they have all had the tendency to run high. This truck run from .5 to 1.5 mph high. (I am comparing to Fuelly calculated mileage).

I have noticed that if I run most of my tank around town, then finish it on the highway, it is more disparate. It is as if the average is based on the last xx# of miles, not the entire run since last reset...

Sent from my SM-G965U using Tapatalk

 

[Ram92131]

This is my 4th consecutive ChryCo product, and they have all had the tendency to run high. This truck run from .5 to 1.5 mph high. (I am comparing to Fuelly calculated mileage).

I have noticed that if I run most of my tank around town, then finish it on the highway, it is more disparate. It is as if the average is based on the last xx# of miles, not the entire run since last reset...

Sent from my SM-G965U using Tapatalk

I was thinking similar. When I fill up after having just run around locally, up and down hills, stop and go, the meter drops very quickly and when I fill, seems more accurate more often. The one time it was way off I recall had been a tank including some light off-roading, and finished with a highway jaunt with large elevation loss. That tank was over reported by a lot.

 

[Neurobit]

Mine's pretty much spot on all the time, to within .2 to .3 MPG. I calculate every tank manually with Fuelly.

Cheers,

 

[SpeedyV]

Mine has been spot on a couple times, has over-reported once or twice around 1.5 mpg, and has even under reported at least once. But usually it’s around .5mpg over. In your case I’d suggest running the tank empty instead of filling after 10 gallons. Your hand calc will be somewhat inaccurate regardless of how carefully you click off the pump. Better to spread that error out over 2x or 3x the gallons (if you have the 33gal tank). Your statement about overall mileage over 813 miles is valid, but since you reset the mileage meter each fill up you can’t compare it to anything.

Your overall mileage over that time seems in line with what I had been getting that early on. My mileage as recorded by Fuelly has improved a little but it isn’t the big step change other have claimed. I’m at 15mpg average now and mostly highway miles.

This^^^. Mine's been all over the board. I think it was within a tenth once or twice. It was low (under-reporting) one time. Most commonly, it's optimistic. Whether it shows high-15's, low-15's, or high-14's, it typically ends up hand-calculating to the high-14's...pretty much in line with my Fuelly lifetime average.

 

[StuartV]

Mine has been spot on a couple times, has over-reported once or twice around 1.5 mpg, and has even under reported at least once. But usually it’s around .5mpg over. In your case I’d suggest running the tank empty instead of filling after 10 gallons. Your hand calc will be somewhat inaccurate regardless of how carefully you click off the pump. Better to spread that error out over 2x or 3x the gallons (if you have the 33gal tank). Your statement about overall mileage over 813 miles is valid, but since you reset the mileage meter each fill up you can’t compare it to anything.

I'm not sure what the second part of the last sentence means.

But, as far as running it to empty and the 10 gallon fill-up, yes, I normally run to about empty before filling up. Yesterday, filled up after only 160 miles because it was all highway, on cruise control, and I wanted to see what it did.

And even if my fill-up variation for that fill-up was a full gallon, that would still mean a MPG of 16.2, versus the computer showing 17.2. Still a large variation.

I wonder why so many of you are getting such accurate results and I am not. Do any of you that are getting good accuracy also have the 3.92 rear end? I see several sigs that say 3.21. I wondering if my issue has something to do with having the 3.92.

I am glad to see so many people here using Fuelly, though. That should definitely give some long-term data that is useful.

 

[spaightlabs]

I try not to get too worked up about it - what am I gonna do, stop driving my brand new truck?

I also try not to worry too much about what mileage other guys say they are getting...doesn't affect me.

I wish I got 30 MPG going 82 on the hwy. I think Tesla is coming out with an electric pickup truck soon, so maybe that's in the future.

 

[danielle2019]

I’m one that reported over 20 mpg who couldn’t care less about how manual calculations may differ.

For you guys that do, what is the point? I’m genuinely curious. Why the obsessive tracking and calculating of gas mileage? What do you do with this information?

Why not just enjoy your nice truck, instead of manufacturing things to be stressed about??

 

[StuartV]

I’m one that reported over 20 mpg who couldn’t care less about how manual calculations may differ.

For you guys that do, what is the point? I’m genuinely curious. Why the obsessive tracking and calculating of gas mileage? What do you do with this information?

Why not just enjoy your nice truck, instead of manufacturing things to be stressed about??

I love my new truck. I want to know everything I can about it. How it works. How it performs. What I can do with it and what I can't.

That includes knowing whether I can really drive from A to B on one tank of gas or if I need to plan to stop.

Also, tracking the mileage may help me spot a problem before it leaves me on the side of the road.

And, knowing my REAL fuel economy will help me decide, in 2 or 3 or 4 or 5 years, if I want to trade this truck in on a diesel. I have been driving 20K miles per year for the last 10 years. I work from home. Last year, I drove that PLUS another 10K miles on my motorcycle.

I want to know how much all my driving costs. If I were getting a real 22 MPG in this truck, the impetus to get a diesel would be a lot less. If I'm getting 15 on the highway, then a diesel may happen sooner.

 

[Neurobit]

I’m one that reported over 20 mpg who couldn’t care less about how manual calculations may differ.

For you guys that do, what is the point? I’m genuinely curious. Why the obsessive tracking and calculating of gas mileage? What do you do with this information?

Why not just enjoy your nice truck, instead of manufacturing things to be stressed about??

Must be the engineer in me. While being OCD about many other things, I really don't obsess over MPG, unless entering odometer and gallons into an app (Fuelly) once per fill up qualifies.
That's all really. Doesn't mean I don't enjoy my truck. Love seeing how the MPGs change over time (or a road trip), how many times I fill up on a particular year, money spent, etc.

Cheers,

 

[ddharris]

If you are getting 12 miles per gallon you have a vehicle problem or a brutal right foot.
I eco mode the **** out of this truck. Computer says 17.8 mpg (3/4 gone)on this tank so far mostly city with some interstate.
Now when I was letting " seat in" I was getting in the 14's.

 

[ddharris]

I give it gas off a red light then roll into eco mode until next stop. You have to " want" to get good gas mileage in a truck. Not fun but it's one or the other.

 

[Neurobit]

If you are getting 12 miles per gallon you have a vehicle problem or a brutal right foot.
I eco mode the **** out of this truck. Computer says 17.8 mpg (3/4 gone)on this tank so far mostly city with some interstate.
Now when I was letting " seat in" I was getting in the 14's.

If this was directed at me, I have neither.
The issue in my case, is that my commute is less than a mile ea way, which has killed my MPG in every vehicle I've owned. Doesn't bother me. The upside is that I fill up every three weeks or so.

Cheers,

 

[SpeedyV]

If you are getting 12 miles per gallon you have a vehicle problem or a brutal right foot.
I eco mode the **** out of this truck. Computer says 17.8 mpg (3/4 gone)on this tank so far mostly city with some interstate.
Now when I was letting " seat in" I was getting in the 14's.

I couldn't baby this truck much more, and there's no way I'm getting 17's without cruising a long, long ways at 55 mph. But I'm guessing my truck is much heavier than yours...what does your payload sticker show?

 

[ddharris]

I have neither.
The issue is that my commute is less than a mile ea way, which has killed my MPG in every vehicle I've owned. Doesn't bother me. The upside is that I fill up every three weeks or so.

Cheers,

That will do it. Short trips are brutal on MPG.

 

[ddharris]

I couldn't baby this truck much more, and there's no way I'm getting 17's without cruising a long, long ways at 55 mph. But I'm guessing my truck is much heavier than yours...what does your payload sticker show?

I have the quad cab with 3.92 gear.

 

[JJRamTX]

The other thing to factor in is wheather or not you are actually getting the amount of Gasoline that it states at the pump. I have calculated and also done the 5 gallon gas can fill and noticed some stations cheating you on your fuel purchase and some giving away a little free.