Av MPG Readout vs Calculated

[Jim Allen]

I recall Bill telling me to reset every tank but what you say makes sense, Longshot. I'll leave it alone for a while and see how it goes.

A related phenomenon I recently noted is that my mpg readouts were significantly reduced for a while after I disconnected the battery to erase the adaptive memory. I reverted from an AEM CAI back to stock, so I reset my Gryphon to L1 (because Bill had put the special CAI tune in L2) and disconnected the battery to start at square one again. For about 35 miles or so, driven over a road I know well the reading I should be getting, the (instant and average) mpg were down 25 percent or so. At least it started that way. It gradually worked back up to normal levels as I drove. Bear in mind I use that stretch of road as a "track" for testing mpg when I get new test product to play with, so I am very familiar with what I should be getting on the instant reading, even by the sections of road. By the time I was 2/3s of the way, it was more or less back to normal. Engine readapting?

[Longshot270]

Originally Posted by Jim Allen

I recall Bill telling me to reset every tank but what you say makes sense, Longshot. I'll leave it alone for a while and see how it goes.

A related phenomenon I recently noted is that my mpg readouts were significantly reduced for a while after I disconnected the battery to erase the adaptive memory. I reverted from an AEM CAI back to stock, so I reset my Gryphon to L1 (because Bill had put the special CAI tune in L2) and disconnected the battery to start at square one again. For about 35 miles or so, driven over a road I know well the reading I should be getting, the (instant and average) mpg were down 25 percent or so. At least it started that way. It gradually worked back up to normal levels as I drove. Bear in mind I use that stretch of road as a "track" for testing mpg when I get new test product to play with, so I am very familiar with what I should be getting on the instant reading, even by the sections of road. By the time I was 2/3s of the way, it was more or less back to normal. Engine readapting?

The reason it starts out low then builds back up to the normal range is from accelerating then cruising. When you accelerate you are throwing low numbers into your average. As you cruise you are adding a longer list of normal values. As your normal value count increases the % error from the abnormal values (acceleration values) will diminish. Eventually you'll have enough "good" values that the "bad" values will not be significant. It would only take 21 good readings to make one bad reading disappear from the average. That is why you can make that average shoot up by flooring it for a second or two then letting off and rolling. It only takes a few seconds to go from 30 to 70 but the truck is heavy enough to roll for a while. If you roll long enough, the low readings get displaced by the high ones.

I'll get a datalog later on today to demonstrate this. It'll give me another reason to drive to Lowes for some stuff.

[Jackpine]

Jim - This is a pretty old subject, but I can tell it's still confusing people. I've just done a bit of research and I'll try to clarify some points (but not all of them)!

First, it would be nice if the Gryphon used a simple "Miles Driven/Fuel Used" calculation for Avg Economy, but it doesn't. The reason is that the truck has no "Fuel Used" sensor or even a simple "Flowmeter". Forget the gas gauge, we all know it's set up to really just warn us if we're close to empty. Instead, the fuel economy calculations seem to depend on an "inferred" value that the PCM determines by looking at the MAF sensor values: http://forum.gopowerhungry.com/63-post7.html

This method of determining fuel used has at least two inaccuracies and maybe more:

1. The MAF sensor is not particularly accurate - it's just a heated wire that is cooled by the action of airflow which alters its electrical resistance. Inaccuracies are then corrected by input from the O2 sensors allowing the PCM to increase/decrease the injector delivery as needed to maintain proper A/F ratio in "closed loop" operation.
2. As Bill said in the cited post, the inferred value of fuel delivered was based on the ideal A/F ratio for "real gasoline", and E10 gas causes the engine to operate at a richer A/F ratio. E85 requires a VERY rich mixture.
3. The inferred value for fuel used may not account for additional enriching during "open loop" (hard acceleration or cold engine).
4. Any accumulated "flow" calculation is inherently less accurate than a corresponding measurement of total volume.

Now, I can't find where Bill told me this, but here's how the "Avg Economy" is calculated. The "Inst Econ" value is sampled each second. These are then averaged using this formula:
((Current Avg Econ x Sample Count) + Current Inst Econ Reading) / (Sample Count + 1)

Clearly, if the "Avg Econ" has recently been reset, the "Sample Count" will be small and each new "Inst Econ" value will have a fairly significant impact.

Once the sample count reaches 100,000 (at one sample per second this is around four full tankfuls assuming normal fuel consumption), the Sample Count is held constant at 100,000 and each new Inst Econ value should have a fairly low effect (you continually multiply by 100,000 and divide by 100,001).

So, to provide a sense of how this works out - assume the current "Avg Econ" is 15.5 MPG and you stop at a stoplight where the Inst Econ drops to zero. This is what you will see for Avg Econ in the next second:

1. Sample Count = 10: ((15.5 x 10) + 0) / 11 = 14.091 (10 seconds after reset)
2. Sample Count = 100: ((15.5 x 100) + 0) / 101 = 15.347 (1 minute, 40 seconds after reset)
3. Sample Count = 1000: ((15.5 x 1000) + 0) / 1001 = 15.484 (16.67 minutes after reset)
4. Sample Count - 25,000: ((15.5 x 25000) + 0) / 25001 = 15.4994 (6.94 hours, 451 miles at 65 mph and about 30 gallons used)
5. Sample count = 100,000: ((15.5 x 100000) + 0) / 100001 = 15.4998 (27.78 hours of samples)

And, if you sit at that stoplight, this is what you'll see at the end of each second stopped:

Samples = 10: 14.0909, 12.9167, 11.9231, 11.0714, 10.3333, 9.6875, 9.1176, 8.6111, 8.1579, 7.7500
Samples = 100: 15.3465, 15.1961, 15.0485, 14.9038, 14.7619, 14.6226, 14.4860, 14.3519, 14.2202, 14.0909
Samples = 1000: 15.4845, 15.4691, 15.4536, 15.4382, 15.4229, 15.4076, 15.3923, 15.3770, 15.3617, 15.3465
Samples = 25000: 15.4994, 15.4988, 15.4981, 15.4975, 15.4969, 15.4963, 15.4957, 15.4950, 15.4944, 15.4938
Samples = 100000: 15.4998, 15.4997, 15.4995, 15.4994, 15.4992, 15.4991, 15.4989, 15.4988, 15.4986, 15.4985

You can see that to get a true Average MPG value, you do NOT want to reset at each fillup.

There's one more problem with this method of calculation though. It's an average of discrete "snapshot" values, rather than a single calculation of distance/fuel. And, it gives each value identical "weight". It's a bit like the error you get by calculating an "average of averages", where each average may have used a different sample size. Still, without a sensor for real fuel consumption, I don't see an alternative.

- Jack

[Jim Allen]

Great info, Jack. I will keep the mitts off the reset!

I'll now have to go back and retest my calculated MPG and compare it to the AVG mpg from the Gryphon to determine an error. Considering that if the unit holds only 100,000 samples, over a 360 mile tank of fuel, resetting at fillup would have skewed the initial reading bigtime but by the end of that tank, it should be pretty stable.

[Jackpine]

Originally Posted by Jim Allen

Great info, Jack. I will keep the mitts off the reset!

I'll now have to go back and retest my calculated MPG and compare it to the AVG mpg from the Gryphon to determine an error. Considering that if the unit holds only 100,000 samples, over a 360 mile tank of fuel, resetting at fillup would have skewed the initial reading bigtime but by the end of that tank, it should be pretty stable.

Once upon a time I suggested that probably the Gryphon Avg Econ would differ from the true Econ by a fairly "fixed" amount. I don't know if this is true, because I only monitor Inst Econ. However, if it does, then it's simple to just apply the correction mentally.

Inst Econ certainly gives you a feel for "excess" usage or economical usage. However, I find I just tend to drive the way it "feels right" to get me where I want to go and to hell with the mileage.

I've been monitoring mileage using a spreadsheet. Over 5671 mostly highway miles of non-towing, I averaged 15.1 mpg. Towing my 3600# trailer over 3559 miles, I got 12.6 mpg.

- Jack

[ticopowell]

Originally Posted by JackandJanet

Once upon a time I suggested that probably the Gryphon Avg Econ would differ from the true Econ by a fairly "fixed" amount. I don't know if this is true, because I only monitor Inst Econ. However, if it does, then it's simple to just apply the correction mentally. That is what I have noticed, I dont know the % but it seems to be about 2 mpg's difference

Inst Econ certainly gives you a feel for "excess" usage or economical usage. However, I find I just tend to drive the way it "feels right" to get me where I want to go and to hell with the mileage. HAHA I wish I could say that... getting 13 mpg's doesnt let me... well my wallet wont let me at 13 mpg's

I've been monitoring mileage using a spreadsheet. Over 5671 mostly highway miles of non-towing, I averaged 15.1 mpg. Towing my 3600# trailer over 3559 miles, I got 12.6 mpg.

- Jack

Just thought I would comment on your post

[Jim Allen]

I pretty much drive by the instant economy. Kinda like one of those vacuum "Econo-Mizer" gauges they used to sell It's not so important what the actual number is, I just try to keep it as high as possible. On my truck, it's amazing how much less fuel it uses at a steady 55 than above 60.

[Jackpine]

Originally Posted by Jim Allen

I pretty much drive by the instant economy. Kinda like one of those vacuum "Econo-Mizer" gauges they used to sell It's not so important what the actual number is, I just try to keep it as high as possible. On my truck, it's amazing how much less fuel it uses at a steady 55 than above 60.

YES! But - I've posted this before. When I added the Gotts Mod, I found I could get about the same gas mileage in the 70 MPH range that I used to only get at 55-60. (Of course that lower speed mileage went up too.) Why? I don't really know.

I also believe you can get better mileage if you don't use the speed control on the highway, because it will try to keep you at your set speed going downhill and then will use more gas going uphill rather than using the energy gained in the downhill "coast". But again, I'm lazy, so I tend to use the speed control.

- Jack

[Jim Allen]

I can't see how the Gotts do much for FE. Or most CAIs. I ran for several years with a CAI on my 5.4L and, while there were some apparent initial upticks, really nothing much improved over the long term. But a tuned CAI, which replaces everything with a more flow-freindly runner, would have a better chance of making a change (good or bad) than the Gotts, which only makes changes in one area. I also had a chance to do some limited steady-state fuel flow tests on an engine dyno (4.0L Jeep) and couldn't see any remarkable differences. Would love to get a Modular on an engine dyno for about a week!

My truck is the 8200GVW and has 4.10:1 ratios. At any level of modification, it always does better at lower speeds. I think the Modulars like low rpms the best