Accuracy of MPG Readings
 

Hello,
1. I was curious how does the accuracy of the NAV radio mpg display compare to your tank readings?

2. Do they get closer as your tank refills get higher in number?

3. How does the DTE display compute your remaining mileage?

I just filled up my second tank on my FWD 07 a little over 800 miles now. The DTE was reading 22 miles left, and it took 13.2gal to fill up and I had 401.2 miles since my 1st fill up = 30.3mpg

However - the NAV display is showing 32.3mpg - Why the big discrepency? I had reset the display at the beginning of the tank because I wanted to start fresh with my driving vrs my wifes driving of the car.

After filling up the DTE read 463mte - So is this computed from the NAV mpg in some way?

Thanks from a new happy FEH owner (2 weeks today!)

Re: Accuracy of MPG Readings
 

Congrats on the new FEH! I have had my '06 for 11 days now. I love it. Our average econ is 28.8 right now. To respond to your question, I don't know for a fact, but DTE must be calculated by fuel level divided by average econ, based on a full tank reading from the fuel level sending unit being 15 gallons.

Re: Accuracy of MPG Readings
 

1. There's a thread on the subject after extensive tests, the Nav econ was found to be more accurate then by hand calculating, mostly due to differential fills. (You don't always put the same amount in that you used.) If you do a search, you can find the thread.

2. Not sure I understand the question.

3. The MTE, or Miles to Empty display is calculated on your FE by 13.7 (Or some similar number) gallons of gas. A few people have calculated that, and I'm sure they'll probably chime in. That way, when you hit empty, you still have some fuel left.

Re: Accuracy of MPG Readings
 

Hi Gary

Reset you Nav average at fill up for a good way to determine your MPG average. The Scangauge is the best way to measure the entire tank IMO. Check out a thread I just posted on CleanMPG. http://www.cleanmpg.com/forums/showthread.php?t=1343

GaryG

Re: Accuracy of MPG Readings
 

When my 2005 was very new ( less than 5000 miles ) the Nav screen fuel economy was ALWAYS about 1 or 2 MPG HIGHER than actual. Since this was computed over many, many tanks, it was not due to my methods of refill at the pump.
Lately, it has been much closer, like within 0.5 MPG of what I pump.

I do agree that it is IMPOSSIBLE to fill up exactly the same way twice in a row, as there is at LEAST 3 gallons of air space in the tank.

Some people have filled up with 17 gallons when parked on an incline, and I have filled up with even more than that when I used a gas can to slowly "dribble" gas in, letting air burp out.

14 gallons is a typical fill up, with 15 or 16 gallons possible. More than that is a bit tricky.

MTE 0 = you have about 1.0 to 1.5 gallons of usable gas left.
If you are in the city, don't feel afraid to take it to MTE 0 all the time.

So while I think the accuracy of the screen is +/- 2 MPG, your accuracy of pumping at the gas station is worse than that.

Most old-timers on here ( had the car a year plus ) trust the screen if you reset it every fill up. The screen only holds about 2000 minutes of data, then data is tossed out... so you really can't keep a "running total" so you might as well reset it every fill-up.

Re: Accuracy of MPG Readings
 

I dont have NAV, only messgae center. Is it possible to reset the average econ on that?

Re: Accuracy of MPG Readings
 

Thanks for the replys - I guess that are no real clear cut answers.

I would think that if you took your total mileage over lets say 10 tanks and divided by your total gallons you should be close to your nav avg? This would let you average out the air pockets, etc in your tank fill-ups?
This would have to be the most accurate right?

On this tank I didnt reset my NAV average - yet I'm still getting 1mpg swings especially on short trips. I would think with 400miles in the computer that it would take more than a 5-10 mile trip to swing the overall average by 1mpg?

If it stores 2000min of driving/mpg info - Wouldnt keeping the data give you the best overall actual number to match your tank data? Especialy since it would be averaged over 3-4 tanks? I would think the swings would be slow - unless you totally change your driving techniques and the old data gets replaced with the new data?

Am I missing something?

I've searched for the old threads, and it doesnt seem like their is a conclusion?

Without the programer for the NAV it might be hard to figure out how often they sample the data or if they avg out your instant readings over the whole past 60seconds before showing the history data on the screen?

Thanks for answering the questions!

Gary

Re: Accuracy of MPG Readings
 

You are right that there is no concensus on how the darn thing works.
However, there is concensus that yours behaves just like all the rest out there.
It holds more than 15 minutes of data, and no more than 2000 minutes.
That's all we know for sure.
It may be something in between.
There are some hints it may be 1000 minutes of driving, or collects 2000, then dumps the oldest 1000 to make room for a new 1000 minutes.
All possibilities.
In the long run ( over the whole tank ) the computer is pretty close.
-John

Re: Accuracy of MPG Readings
 

Just a couple of observations on the MTE...

First, I've gone 20kms plus after getting MTE=0 and found the tank still had 3 litres or so left (another 40-50kms). Nice to know if I really need it. This must be a universal thing cause all the jeeps I've ever owned got right around 70kms after the low fuel light came on.

Also, I have noticed that the MTE appears to be tied to the avg MPG at last shut down and doesn't closely follow the current mpg. Example...I was running an average of ~35mpg at fill up and was getting a reading of 650kms to empty. Before I shut down however I changed my driving style drastically to mpg of around 28mpg (I added four adults...three sets of golf clubs and increased hwy speed 35km/hr to 140km/r). The MTE DID NOT change drastically however until I restarted the vehicle. When I shut it off it was showing 630 MTE or so and at start up this changed to 550 or so.

Re: Accuracy of MPG Readings
 

Good observations, and I concur. The MTE is closely tied to fuel tank level x recent fuel economy, and re-adjusts often, even while driving, but most observable when you stop and restart.
-John