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Problem with vehicle's calculation of MPG

9K views 16 replies 12 participants last post by  stop-eject 
#1 ·
There is a problem with the mpg calculations on my 2018 Pacifica Hybrid.

Before resetting the Trip B display, I always record both the trip data and the average mpg from the dashboard display. The Trip A display has not been reset since I purchased the vehicle new four months ago, so it should show miles driven and mpg over the life of the vehicle.

I have noticed that the average mpg figures are mathematically inconsistent with the trip data. The average mpg figure seems to assign too much weight to the most recent miles rather than being a true cumulative figure. (In fact, as shown below, the same can be said for the trip mpg itself--instead of showing mpg for the entire trip, it also seems to assign too much weight to the the most recent miles.)

I entered the data into a spreadsheet and calculated the number of gallon-equivalents for each 'trip' based on the number of miles driven and the reported mpg for the trip. Then I calculated what the cumulative mpg 'should' have been after each trip (assuming that the mpg reported for the trip itself was correct).

Here are the results:

Trip....Trip...Displayed...Calculated
Miles..MPG..Avg. MPG..Avg. MPG

..17.0...-----...12.8.........12.8
625.1...73.6...70.8.........65.4
387.8...32.0...34.6.........46.9
..88.7...78.4...43.7.........48.5
138.5...30.4...35.0.........45.5
..77.5...32.1...33.9.........44.4
..21.3...25.2...32.6.........43.9
..93.3...63.6...40.4.........44.8
424.0...36.0...35.2.........42.4
162.8...35.9...35.0.........41.8
182.4...28.7...30.2.........40.3

Note that the cumulative mpg reported in the dashboard display keeps jumping around a lot (column 3), but the cumulative figure that I calculated myself (column 4) becomes more stable over time, as one would expect from a cumulative figure.

The sum of the trip miles is equal to the odometer reading (which confirms that the trip miles are accurate and also that I have remembered to record the trip data every time I reset the meter).

The other trip display, which has not been reset since I purchased the vehicle, shows the same lifetime mpg as the dash display. This suggests that the trip meters have the same problem with calculating mpg as the dash display. Thus, even the mpg for individual trips in column 2 is unreliable. (I have noticed that, even after the Trip B display is up to a couple hundred miles, a short drive with low mpg will cause the displayed trip mpg to plummet, and then a short drive with high mpg will bring it back up again by an implausible amount.)

This is a significant issue. If I had not done these calculations, I would think that I was getting just 30 mpg instead of 40 mpg. Even though the 40 mpg figure is unreliable too, it makes me feel a lot better about my $45,000 investment than the even less reliable 30 mpg figure on the dash display.

Chrysler needs to realize that some of the people who buy hybrids are data junkies, and that accurate mileage figures are important to us.
 
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#2 ·
I track my gas Pacifica and I find with highway millage the computer is always very close, less then .5 mpg off. City driving is not as accurate for some reason but it has never been off by more then 1.5 mpg, and even then its typically within 1 mpg.

I track every tank and fill up.
 
#3 ·
#4 ·
I agree with @stop-eject. The displayed average miles per gallon is pretty much useless. They are taking the electricity used when the gas engine is off and equating it to a theoretical energy equivalent amount of gasoline. But they don't take into account where the electricity came from. If it all came an external source then that might make some sense. But if it came from regeneration from decelerating or going down hill then that energy ultimately might have come from the gasoline engine and has already been measured. So they count it twice. The only use of the displayed MPG is as a game -- if almost all of your power comes from electricity then you can get that MPG up over 70 or 80 MPG and people take pleasure in seeing how high they can make it go.

I figure the only true way of measuring efficiency is to calculate the actual cost per mile to run the car by measuring both the gas and electricity you put in. Gas is easy to measure by keeping a log as you have done. Electricity is trickier because the car won't tell you, but you can do it by using a smart WiFi connected charging station that measures your electricity usage such as ChargePoint or JuiceBox. I just passed my one year anniversary with the car and these are my results:
Miles traveled: 4729
Miles on electricity according to the tripmeter 3925
Miles on gas according to the tripmeter: 804
Miles per gallon according to the tripmeter: 75.8
Gas used: 36.2 gallons
Average MPG ignoring electricity cost: 171
Electricity used: 1400 kwh
Electricity cost: $98 (7 cents per kwh when charged at off-peak rates)
Gas cost: $106
Total gas and electricity cost: $204
Cost per mile: 4.3 cents
Average cost per gallon of gas: $2.93
Gas cost if I had not ever charged the car: $433
Savings: $433 - $204 = $229

I don't think these results are representative of what most people get. I don't drive as many miles as most. Most of my driving is around town within the electric range and I only made three trips out of town during the year. Also my electricity cost is lower than most due to the strong emphasis Arizona Public Services puts on encouraging people to keep their peak hour electricity usage low. If you can do all your charging during off-peak hours, which isn't hard to do, you can save a lot.
 
#14 · (Edited)
Edit: mpge is somewhat useful but real life cost savings matter more to the consumer (me). My Pacifica Hybrid is costing me close to 5 cents per mile when running on roughly 80% electric and 20% gas hybrid. My 2019 Pacifica gas version cost me an average of 21 cents per mile at $5/gal. For my usage profile, fuel for the hybrid version is 4x cheaper. It’s about like paying $1.50/gallon for gas. Some EV hybrid haters get all caught up on the modest difference in highway gas mileage between a hybrid and ICE gas model, choosing to ignore the combined average use cost and plug-in cost savings. My EV use is higher than most, but as a former Pacifica gas and current EV hybrid owner, my 4x fuel cost savings is REAL. I liked both vehicles, but my wallet loves the plug-in.
 
#7 ·
PacHy needs to measure the voltage and current going to the motors to operate. So the data is already there. The modern electronics is very accurate- you can buy any $3 voltmeter on ebay and it will deviate less than 1%.
There is one caveat here. Measuring the power drawn from the HV battery doesn't account for the charging losses, that can be between 5 and 15%.



The gas consumption measurements rely on the fuel injectors. They are calibrated for flow rate, recording how long they stay open during each engine cycle provides the data to calculate the fuel economy.
 
#6 ·
I totally agree... I reset A with every fill-up and leave B alone until I get close to 10K and the automatic reset to zero. The displayed average on B is weighted towards the most recent miles as it frequently is within 1mpg of the average on A.

I kept a spreadsheet for the first 7 months, but since I didn't buy an EVSE that tracks my electricity consumed by the car and then my hubby "lost" receipts during a road trip, I eventually gave up on calculating an accurate MPGe. On the other hand, I delight in only filling up every other month (or during a long trip), and the kids in my family gave my PacHy it's nickname of "The Magic Car" after playing with the rear screens.
 
#8 ·
I found this problem long time ago. It is true to all my Chrysler made vehicles. So far I've had Chrysler Town & Country 2012, Jeep Grand Cherokee 2014 and Chrysler Pacifica Hybrid. The average of Trip A & B always very close regardless to milage you've accumulated with A or B counters. The calculation problem is not strictly Pacifica related problem in my opinion...
 
#9 · (Edited)
I bought my Pacifica Hybrid with a dead charge, full tank of gas and 110 miles. I drove home, charged it and reset the average to see how good I could make the "milage?" number look on the all electric average. If you fart around it calls the "gas milage" 80 something, but 60 something is realistic if you drive normal. I tried reseting the milage when the electric was used up and have a hard time seeing 32 mpg in the hybrid mode. If you stop and start much and go slow in traffic it looks about as terrible as as my old gas Town and Country. Bottom line is my electricity off peak hours is 5 cents per kilowatt hour. It's less than a dollar to charge the van, so every 100 miles charged is less than 3 dollars vs around 20 bucks in my old Town and Country or about 15 bucks in Pacifica Hybrid mode. Currently I've used about 1/3 tank of gas for about 420 miles traveled with the battery depleting now and again. The dash average says 45 mpg right now and I haven't reset it for hundreds of miles. If I filled the tank with 8 or 9 gallons? being 2/3 full of gas the 45 mph would be about right at this point.
 
#10 ·
There is a problem with the mpg calculations on my 2018 Pacifica Hybrid.

Before resetting the Trip B display, I always record both the trip data and the average mpg from the dashboard display. The Trip A display has not been reset since I purchased the vehicle new four months ago, so it should show miles driven and mpg over the life of the vehicle.

I have noticed that the average mpg figures are mathematically inconsistent with the trip data. The average mpg figure seems to assign too much weight to the most recent miles rather than being a true cumulative figure. (In fact, as shown below, the same can be said for the trip mpg itself--instead of showing mpg for the entire trip, it also seems to assign too much weight to the the most recent miles.)

I entered the data into a spreadsheet and calculated the number of gallon-equivalents for each 'trip' based on the number of miles driven and the reported mpg for the trip. Then I calculated what the cumulative mpg 'should' have been after each trip (assuming that the mpg reported for the trip itself was correct).

Here are the results:

Trip....Trip...Displayed...Calculated
Miles..MPG..Avg. MPG..Avg. MPG

..17.0...-----...12.8.........12.8
625.1...73.6...70.8.........65.4
387.8...32.0...34.6.........46.9
..88.7...78.4...43.7.........48.5
138.5...30.4...35.0.........45.5
..77.5...32.1...33.9.........44.4
..21.3...25.2...32.6.........43.9
..93.3...63.6...40.4.........44.8
424.0...36.0...35.2.........42.4
162.8...35.9...35.0.........41.8
182.4...28.7...30.2.........40.3

Note that the cumulative mpg reported in the dashboard display keeps jumping around a lot (column 3), but the cumulative figure that I calculated myself (column 4) becomes more stable over time, as one would expect from a cumulative figure.

The sum of the trip miles is equal to the odometer reading (which confirms that the trip miles are accurate and also that I have remembered to record the trip data every time I reset the meter).

The other trip display, which has not been reset since I purchased the vehicle, shows the same lifetime mpg as the dash display. This suggests that the trip meters have the same problem with calculating mpg as the dash display. Thus, even the mpg for individual trips in column 2 is unreliable. (I have noticed that, even after the Trip B display is up to a couple hundred miles, a short drive with low mpg will cause the displayed trip mpg to plummet, and then a short drive with high mpg will bring it back up again by an implausible amount.)

This is a significant issue. If I had not done these calculations, I would think that I was getting just 30 mpg instead of 40 mpg. Even though the 40 mpg figure is unreliable too, it makes me feel a lot better about my $45,000 investment than the even less reliable 30 mpg figure on the dash display.

Chrysler needs to realize that some of the people who buy hybrids are data junkies, and that accurate mileage figures are important to us.
I just bought my PachyHy and already have notice the Average mpg inconsistency. The mpg should not be fluctuating as drastically as I add more miles on the trip taker. The average mpg must be connected to recent miles driven and the difference in gas consumption from the tank. I wish Chrysler gave us the constraints in the manual.
 
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