Av MPG Readout vs Calculated

[Longshot270]

Originally Posted by ticopowell

Your math makes perfect sense to me, and unless it was recording wrong on something, the longer it's in the more accurate it is, but I still dont know why my average mpg's change so much. I am driving back tomorrow and Ill report on how they do, but I expect them to drop to about 12 or less going up parley's canyon (pretty big canyon here), then raise to about 15-17 as I hit the park city exit, then lower to 14 by the time I am to Evanston... and this is without ever resetting the Gryphon. the only time I have ever seen it reset is when I unplug it when going to the dealer to get free oil changes. If anyone's Average mpg's are more steady than mine then please let me know, and even though I got an oil change a little more than a week ago (aka not much time calculating) I have seen this same behavior every time I drive my truck ever since I got the Gryphon, and it's been both trucks too... maybe I have a bad hardware something that doesn't record as many points of data... Oh well though, I have had mine for almost a year in this truck, and it was almost a year in my other truck too, and I am not planning on changing because of a miscalculation in my percieved mileage... I know I suck at getting good mileage, but I didnt get a truck to get mpg's, I got it because it's useful, and safe, and also fun ... oh and BTW it hasn't helped get any girls either... maybe that will change in texas...

To make it steady out you just have to leave it a long time. The influence earlier was from JUST 25 seconds, if you are cruising along at 60, that is not even half a mile...now consider you can get around 400 miles per tank, and the average doesn't really steady out until after you've driven through the second tank. Because of the swings from regular to 40 mpg

I haven't tested this in a while but I bet if you used the cruise control all the time the average may be more accurate. Part of the inaccuracy is from letting off the pedal while rolling. All the time you spend braking is counted as 40 mpg per reading. I think back and the computer rarely uses 0 throttle so you wont have nearly as much 40 mpg getting thrown into the mix. Only problem is you can't use cruise control in varying traffic.

The only reason I look for ways to improve mileage is because I'm cheap and need to stretch the tanks.

You might be able to get some girls out in Del Rio, last time I was passing through I wanted to take my time.

[88Racing]

Ok Jim,
I'll agree on the overwritting data but the most accurate data comes from 4 tank fulls of gas ...that's the optimized amount of samples according to Bill with out reseting each time....

[ticopowell]

So I just got back from utah, and I have had at least 5 tanks of gas since the last time the gryphon was unplugged, and it still varied from 14.x to 17.x on the average mpg's I had it in cruise control most of the trip except for the stops and when I made sure to not hit the people going 10 under the speed limit , I have a little fun program on my phone, and on the way back from Utah I gained 18,431 feet and lost 16,069 feet, with a tailwind I got about 15 mpg's (done by hand) and my gryphon shows about 16.5ish... so the trend continues haha .
My worry still is that mine is fluctuating too much, aka more than it should and more than everyone else's is... but I am happy about the 15 mpg's, I got only 13.7ish on the way to utah... :/.

The trick in Del Rio is to figure out how to outshine all the other brand new 2nd Lt's that are girl hunting! haha

[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.