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The error that you you see when hand calculating between fill-ups is introduced because the calculations have no knowledge of how much energy is in the gallon of gas you buy and deltas during filling up. Ethanol is blended with gasoline in varying amounts and ethanol contains less energy, the higher the ratio of ethanol, the less MPGs or the more fuel used.

I have found over the past year, my average MPG versus my hand calculation deltas occur during the time when the switch from/to summer/winter blends. If you know when that happens, reset your average MPG and the calculations will be more accurate with respect to time.

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Wow. This response wins this thread. :Wow1:

The error that you you see when hand calculating between fill-ups is introduced because the calculations have no knowledge of how much energy is in the gallon of gas you buy and deltas during filling up. Ethanol is blended with gasoline in varying amounts and ethanol contains less energy, the higher the ratio of ethanol, the less MPGs or the more fuel used.

I have found over the past year, my average MPG versus my hand calculation deltas occur during the time when the switch from/to summer/winter blends. If you know when that happens, reset your average MPG and the calculations will be more accurate with respect to time.

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198 Posts

DATE Hand EVIC Difference

03/30/14 18.33 20.0 +1.67

04/18/14 16.81 17.8 +0.99

04/22/14 16.78 18.4 +1.62

04/27/14 13.17 13.4 +0.23

05/05/14 15.69 17.1 +1.41

05/13/14 17.67 19.3 +1.63

05/27/14 17.83 19.9 +2.07

06/03/14 17.51 19.2 +1.69

06/12/14 16.46 18.2 +1.74

06/19/14 15.90 17.7 +1.80

EVIC has been reset after every fill up. Doesn't appear to be getting any more accurate as we have returned to "summer blend" season.

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So there is no mechanical flow meter somewhere in the fuel line measuring the flow rate of fuel being consumed?Just about every model year forum has a discussion about the accuray of average MPG. Since I have first hand knowledge of how the calculations are done I will say the accuracy is right on, plus or minus the display digit count and number rounding in the calculations. Fuel consumption is the sum of the count and duration of the fuel injectors. To calculate the MPG the distance traveled and the fuel consumed yields an instant MPG which is used to average the overall MPG over time.

Jim

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I must say, however, that the EVICs indicated range (@ 65 miles till empty) was spot on, assuming I have 32 gals usable. The low fuel warning came on with about 75 miles till empty.

Jim

I have 3000 mi. On my 2014 Ram ED and my EVIC is right on with my hand calculation.

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386 Posts

That's messed up.

The error that you you see when hand calculating between fill-ups is introduced because the calculations have no knowledge of how much energy is in the gallon of gas you buy and deltas during filling up. Ethanol is blended with gasoline in varying amounts and ethanol contains less energy, the higher the ratio of ethanol, the less MPGs or the more fuel used.

I have found over the past year, my average MPG versus my hand calculation deltas occur during the time when the switch from/to summer/winter blends. If you know when that happens, reset your average MPG and the calculations will be more accurate with respect to time.

There's no error introduced with hand calculations because of fuel BTU's

MPG= miles per gallon. distance measurement/ volume measurement.

If you meant to say the computer messes it up, then there again, its injector volume, duration and frequency, giving a total fuel volume calculation again. distance per volume again.

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I hope he's not trying to pin the difference we see on some error in our hand calculations. As you said, our calcs have nothing to do with the energy content of the fuel. If I read his post right, they are estimating a fuel flow without actually measuring it with a flow meter. However, his explanation didn't really make clear to me how the energy content of the fuel affected that either.That's messed up.

There's no error introduced with hand calculations because of fuel BTU's

MPG= miles per gallon. distance measurement/ volume measurement.

If you meant to say the computer messes it up, then there again, its injector volume, duration and frequency, giving a total fuel volume calculation again. distance per volume again.

The only error in our calculations is the slight variation in how full we get the tank each time. The emptier the tank before the fill-up (i.e. the more gals used in our calculation), the less significant that error is. Averaging a series of fill-ups drives that error close to zero.

Jim

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If you reset the EVIC after each fill-up, you will never get your average MPG over time. Quit resetting it.EVIC has been reset after every fill up.

It isn't needed with fuel injection. Knowing the fuel line pressure, the injector flow rate, the duration the injector is open, plus the count of injector openings equals the amount of fuel used.So there is no mechanical flow meter somewhere in the fuel line measuring the flow rate of fuel being consumed?

Jim

Excellent, variations between average and hand calculations assuming the same energy level of the fuel should be with in 5% or better.

Correct, but that isn't what I said, the delta is seen because when you fill up there is no way to know how close to full you are with respect to the last fill up. The temperature of the gas you pump also controls the volume since gas expands as temperatures rise. Granted these are small amounts but the sum of all factors does affect the result.That's messed up.

There's no error introduced with hand calculations because of fuel BTU's

MPG= miles per gallon. distance measurement/ volume measurement.

A trend lately has been to display the average MPG over the last 50 miles driven in an attempt to make more accurate the average displayed to reduce complaints.

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198 Posts

I reset one trip meter and have left the other one alone. That being said I find it laughable that you say I will never get my average MPG over time. I know it because I hand calculate. When I want to figure out the actual range that I have left on my current tank I look at the trip EVIC mpg and subtract 1.75 mpg it to use in my calculations-this will be much closer than what the EVIC reports. Pretty ridiculous in this day an age that a 5% margin of error could be considered "excellent".If you reset the EVIC after each fill-up, you will never get your average MPG over time. Quit resetting it.

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116 Posts

I have noticed the same on mine. I reset Trip Meter A daily, Trip Meter B has never been reset. The mpg in Trip B does not seem to indicate my usage since new. I log all my fuel and distances for work so my spreadsheet includes calculations for average consumption since almost new (disregard the fuel and driving after delivery)

That being said my last couple tanks of gas have been less than 1 mpg different from the EVIC. Perhaps a function of summer blends and location use.

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