Av MPG Readout vs Calculated

[Jim Allen]

I'm sure most of you have noted that if you check mpg the old fashioned way, the average MPG readings on the Gryphon will be significantly higher than the calculated MPG from miles driven/gallons used. The F-150 is pretty easy to fill consistently but even when the the two values are recoded over a year and averaged separately, I show an average of 14.7 mpg using a g/mile calc vs 16.6 mpg from recording the screen average at every fillup (and reset each time). That's about a 11+ percent difference. Yes, my tire circumference is measured to the millimeter and loaded into the custom settings. My speed and odometer have been checked against mile markers, radar units and GPS and are accurate.

Comments? Other observations?

[ticopowell]

I have noticed about the same thing... but with worse mpg's.. , but from what I understand it is just an error inherent in the gryphon... it was never designed to be 100% accurate in the readout of mpg's, only to give you an idea of what you are getting. My truck is also GPS accurate on the gryphon's speedo, and ive seen about a 2 mpg difference every time. I dont record them, but I check my mileage at almost every fillup, and its consistently the same amount low.
That's all I got, anyone else know any better than me?

[88Racing]

If a true comparison was to be made then.....

Don't reset the Gryphon each time.....let it go for a year....

[ticopowell]

wait... we can reset the Gryphon's average mpg's? Ive never seen nor needed to do this because it fluctuates enough that when I am driving around town it shows 14-15 mpg's(actually getting about 12.5-13mpg's) and when I drive up to denver airport I have seen it climb up to 18, and my trip here to SLC this weekend it got over 20 at a few parts, but I only got 13.5 mpg's averaged out through the first tank of gas... , so I guess my question is why does it fluctuate like that? Ive never seen it stay on the same number for more than a few hours and that is when I was driving home through the flat part of wyoming... and a hours is pushing it, if I hit a big hill it would drop the average mpg's by 1 or 2 on the way up, then they would go back up to whatever they were after a few miles of flatter driving again... oh and fyi this was all on winter gas, summer gas you can add 1ish mpg's to all the numbers except the 18 and 20... just my observations...

[Jim Allen]

Resetting the Ave. mpg (done by hitting "enter" when the unit is on) because Bill C. that was the way to show the most accurate MPG. When reset, it will go to whatever the instant MPG is reading at that moment and then start averaging input data. IIRC, Bill said that the error stacks up as data is stored. The unit can store only so much data and dumps or overwrites as it goes along.

I just spoke with a guy who uses the built-in Ford unit in a similar truck and he said his reads optimistically by 2-3 mpg as well.

I watch instant mpg religiously, to control my driving (try to accelerate while keeping it above 9mpg!) but I'm about to put another parameter in place of the average.

I think people don't like to talk much about this because they like the "braggin'" right they get ( : < ).

[Longshot270]

If recording more increases your % error then you shouldn't even be watching it to begin with. Your chance of error goes down when the number of samples goes up.

Here is a quick example of why short term averages are bad. For easy math (and since I'm not positive on the official rates other than the data stream rate) we'll say the programmer takes 5 readings per second.

Lets say you have two different calculations going. One set of calculations has a small number of readings at 1000 readings and the other has a large number of readings at 10000 readings. If the current average is 16 mpg for both then it is easy to see how a single stretch of road could impact the mathematical formula.
You climb up a hill with an IECON reading of 5mpg for the 10 seconds it takes to climb up that hill. Then you slowly roll down the other side of the hill with zero throttle input (maxing out the IECON at 40mpg) dragging on the brakes (to not speed and ram the little old lady ahead of you) for 15 seconds. Your average in this section of road is 26 mpg according to the programmer.
Factor that into the "small" calculation and once you are back on the road your average is 17.1. The large equation comes out to 16.1. If you then compare to the "full" amount of 100,000 readings, you end up with 16.0, or no noticeable change. Keep in mind, this is the impact of only 25 seconds of drive time, now consider what happens when you are cruising through traffic or hilly areas where you are switching between the gas and brake pedals as well as changing speed often. If you were able to freely roll down the hill to shorten the recording time, the average would not be thrown off as much. Unfortunately the computer cannot account for elevation changes, lack of throttle input, resistance from wind and tires, etc. Until then the average will not be completely accurate except under ideal conditions.