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Interesting computer MPG observation

TimmH

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I got my jeep in mid-September, and since the computer has tracked mileage, and it has varied, from 18.3 to 20.1, and often in a single drive to work (19 miles), it could vary as must as 1 -1.3 MPG. Yes I saw 20.1 leaving my house and could see 18.7 or 18.8 during my trip.

Mid-January, tourist season started and my drives take much longer, and the computer MPG dropped to averaging around 19.3, but still varied as much as 1 - 1.3 MPG per trip.

Last week I got new wheels and 33" tires, and installed a Tazer. over a few drives, I adjusted my tire size and sync'd my Jeep speedometer to a couple GPS speedometers on my phone.

First, my computer MPG has dropped to 17.9, which I expected a drop, but what I didn't expect is that since adjusting the tire size with the Tazer, the computer only ever varies by 0.2 or 0.3 mpg anymore.

For the record, Autosist tracks my mileage as around in the 17.9 to 18.3 per tank range.

I found it strange that literally, the day I got the tire and installed the Tazer, the computer MPG stabilized. I had never had a vehicle that varied as wildly as the Jeep did.
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se7en

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Yeah, the JL computer calculation of MPG is off more than my Tacoma was, for sure. At least by .5 MPG or more. I use Fuelly to track mine and it's always off. The Taco on the other hand rarely varied, and if it did, by maybe .2 at most.
 

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One thing I also noticed with the MPG calculator. When you auto start the JL in cold weather, it counts against your MPG calculation and thus it shows the decrease when you get in and "actually" start the JL. I like the way my F-150's autostart does not alter the mpg calculation. I like this way better. While yes, auto starting costs you more gas, I would still like to see my actual driving mpg displayed on the JL that does not include autostart run time.
 

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... and sync'd my Jeep speedometer to a couple GPS speedometers on my phone.
I'm curious how this sync is done--with the tazer?

I also wonder if had you done this sync with the old tires if your vehcile's MPG readings would have also fluctuated less afterwards.
 
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TimmH

TimmH

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I'm curious how this sync is done--with the tazer?

I also wonder if had you done this sync with the old tires if your vehcile's MPG readings would have also fluctuated less afterwards.
I got the Tazer when I got the new wheels, so that's why I did it then.

I probably would not have done syncing with the Tazer on the factory tires, as I would have expected them to be calibrated correctly, AND I did confirm the speedometer vs a GPS speedometer and found it was good.

The syncing I was referring to was changing the tire size with the Tazer based on what I presumed it was, then checking the MPH vs the GPS and adjusting the tire size until the 2 matched consistently.
 

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Rubicon@2019

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I can agree that if I let the JL warm up in the morning I lose about 1 MPG. It should not calculate sitting still. I don't get it
 
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TimmH

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I can agree that if I let the JL warm up in the morning I lose about 1 MPG. It should not calculate sitting still. I don't get it
If it's running it is calculating, because you're burning fuel. That means less fuel to travel distance, so your overall MPG goes down, but I agree it seems to drop considerably for a short stationary idle.
 

Rubicon@2019

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I understand the miles to empty being effected because of the lose of fuel but it calculates MPG based on fuel used while traveling distance but yet the jeep hasn't moved. It just doesn't make sense to me.
 
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TimmH

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I understand the miles to empty being effected because of the lose of fuel but it calculates MPG based on fuel used while traveling distance but yet the jeep hasn't moved. It just doesn't make sense to me.
It calculates based on fuel used vs mileage of the vehicle (or in reality, something like the last 1000 miles).
So it would be calculating fuel used / 1000 miles.

So if you have used 2 gallons driving home and had a 20mpg average (40 miles), then let the vehicle sit idling long enough to burn a gallon of gas (Yes that's extreme). you now have burned 3 gallons but still only traveled 40 miles so your MPG just dropped to 13.3.

If the engine is running but vehicle is not moving your current MPG would b 0 MPG, and that all averages out.
 

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I understand the miles to empty being effected because of the lose of fuel but it calculates MPG based on fuel used while traveling distance but yet the jeep hasn't moved. It just doesn't make sense to me.
Miles per gallon. You drove X miles and bought Y gallons of gas. ESS exists because gas burned while idling at stop lights counts, why shouldn't gas burned idling in your driveway count?
 

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InvertedLogic

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Miles per gallon. You drove X miles and bought Y gallons of gas. ESS exists because gas burned while idling at stop lights counts, why shouldn't gas burned idling in your driveway count?
If you care about "Trip MPG" vs "Tank MPG". My old car would track both which was neat. Had an overall mpg for the tank, and a tracker for the current run cycle. That way I could see my overall combined mpg was ~20mpg, but for a specific highway trip it was 25.
 

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I can agree that if I let the JL warm up in the morning I lose about 1 MPG. It should not calculate sitting still. I don't get it
Yeah it should. You are burning gas, this is like the auto start/stop system for traffic lights. You are traveling 0 miles and still burning gas, so you are getting 0 MPG, however you are burning much less gas than when moving so it shouldn't affect much unless you let your Jeep sit idle for minutes (which is not good for it).

Do this:

Test A:
1) fill up at gas station and reset the mile counter
2) leave the Jeep idle for a few minutes (you are burning gas)
3) drive 40 miles
4) Fill it up again divide the mile counter by gallons (MPG).

Test B:
1) fill up at gas station and reset the mile counter
2) drive 40 miles
3) Fill it up again divide the mile counter by gallons (MPG).


You will get better MPG from Test B, because the gallons number (denominator) will be higher on test A vs. test B (you burned more gas for test A), while the miles number will be the same (you drove the same amount of miles - 40). Giving you a smaller number.
 

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If you care about "Trip MPG" vs "Tank MPG". My old car would track both which was neat. Had an overall mpg for the tank, and a tracker for the current run cycle. That way I could see my overall combined mpg was ~20mpg, but for a specific highway trip it was 25.
You could effectively duplicate this by resetting Trip A after every fill up for Tank MPG, and resetting Trip B each time you pull out of your driveway/parking lot and start your drive for Trip MPG. It's a little more manual, but accomplishes the same if you remember.
 

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Over 34 tanks the computer was optimistic on mileage by .4 mpg for me. I recorded the miles driven and computer-reported MPG for each tank then plugged the gallons into an excel file to find the actual. The excel file then calculated the difference between actual and computer mileage.
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