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Discussion in 'Gen 2 Prius Fuel Economy' started by timmyjane, Apr 12, 2017.
Lie-O-Meter = 43.9
Pencil-N-Paper = 40.98
An error of ~7%
While it might seem bad, if you look at it in terms of gal per 100 miles which is linear, rather than exponential as it is in MPG,
Lie-O-Meter = 2.278 gal/100 mi
Pencil-N-Paper = 2.440 gal/100 mi
Still an error of ~7%,
but in the real world you are talking a difference of 0.162 gals per 100 miles (2.440-2.278) which is 20.736 fl oz per 100 miles or 0.207 fl oz per mile.
It does not really rate within the margin of error. Not something I'd worry about.
A 7% error is completely unacceptable in engineering terms.
The other thing to notice is that, excluding a very few cases, the L-O-M is ALWAYS optimistic and statistics cannot explain that - it must be deliberate.
I think the discrepancy between the MFD and pencil 'n paper is probably attributable to the bladder that you lucky people have. And I'd venture to say the error is in the pencil 'n paper figure given the variable amount of fuel added between tanks that the bladder affords.
My experience is MFD vs calculated is very close still optimistic most times but can be pessimistic as well. The other thing I have noticed is those tanks where I get a lot of pulse and glide in, have a higher (optimistic) discrepancy.
Fuel gauging in cars is not about engineering. It is about managing driver behavior and expectations.
That bladder issue can apply only to variations in individual tanks. It can't explain the consistent bias.
I made no comment about gauge accuracy, the comment was about L-O-M accuracy and that is an engineering concern.
I see the L-O-M as a part of fuel gauging. And as such, its accuracy is of no more concern than the fuel gauge level accuracy. It is an optional feature that nearly all cars lacked for generations. Unlike the speedometer, it is not important enough to be regulated by the government.
In many other world markets, even 7% error is well within the government mandated, and industry normal, speedometer accuracy requirement.
It's is not, but a completely separate feature from the fuel level gauge. Would you have the same opinion if your radio tuner was off by two stations? After all, no one ever ran out of fuel because the radio was off
You are ok with being deliberately lied to by software that indicates your MPG is higher than actual? I doubt most consumers would agree and in fact a number of OEMs have paid fines for doing exactly that - misstating fuel economy.
Yea, not. Even a 1% error at a fuel pump of a filling station will get someone in BIG trouble with the law in every state in the union. Misrepresenting FE by even 1% is enough to influence purchase decisions in the real world.
Back in the analog tuner days, that was normal. And I still have some analog tuners.
Let me call your bluff. Please give specific examples of car makers who have been fined for optional fuel economy gauges providing misleading results.
I am well aware of manufacturers being fined for misstating their required EPA ratings on the required Monroney label. That is not at all the same thing.
2010 Toyota Prius FCD Inaccuracy | Page 6 | PriusChat
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You are absolutely correct, it will average out over several tanks. However, what I had in mind was an individual tank calculation. Mea culpa.
Please find attached my latest "Lie-o-meter" (MFD) MPG readings from my last tankful (42.5 litres of fuel used). I'm sure I could have stretched it to 600 miles, but I wanted to refuel at a specific ESSO fuel station, so I filled up before it got too low.
New to me 2007 with just over 195,000 miles. Only paid $1,000 for it. Did not use any oil between 194000 and 195,000 when I changed oil. New air filter, and new hybrid battery installed a few weeks ago. New Yokohama touring tires, 185 65 18, inflated to 33 psi.
The only way to accurately know what mileage you are getting is:
1. Check your odometer against mileage posts on an interstate (re-calculate as needed).
2. Over several tanks, keep an accurate record of how much fuel you are using each time.
3. Do the math.
After 1,000 miles we are getting right between 44 and 45 mpg. We have a 22 mile commute to the office. It is about 75% highway. I do not baby it, but I don't floor it from every stop light either. I try to drive the same as if I was driving Mom's Buick. Highway speed is between 70 and 75. Pretty much 5 over in town, other than residential areas, where I am normally 5 under (would hate to hit a child, right?)
Don't know how accurate the fuel economy read outs are on the rest of the gen 2 prii, but ours happens to coincide with my real world calculations. Right now, it shows 44.3 since being reset 1000 miles ago.
I know I could do better by over inflating tires, and by driving 60 - 65 on the interstate. Just not worth the trade off to me. I realize a LOT of Prius owners driver the Prius much differently than they do a "regular" car, and I think that is where some of the numbers get horribly skewed, as it is not a fair comparison driving your Prius 60 on the highway, and driving your other cars 70 or better.
Still, with no more than I have in this car, 45 mpg is pretty darn good. Gas prices are almost certain to rise, at least short term. The more expensive gas is, the more the Prius makes sense.
Congrats on the new to you Prius. Some good information there.
Most people here find the MFD average MPG about 5% optimistic, but for myself I find it pretty close (but still on the high side), and close enough for me. I have had it pessimistic on a few occasions as well. I get my real figure off Fuelly, and the MFD gives me a good enough estimate as I go.
The only thing I'd pick up on is 33 PSI for your tires, not sure where you got that from, but that is under inflated, for the front anyway, unless your tire wear pattern shows otherwise. The placarded pressures are 35 PSI front and 33 PSI rear. Most people find that tire wear shows this to be under-inflated and about 37F/35R to 39F/37R to be somewhere in the right region. A lot do run even higher pressures than that chasing better MPG, but I figure that any saving in fuel is lost by premature tire wear. I run 38F/36R and find that gives me an even tire wear pattern. I have found this does also give me a good MPG, but I see that as a side benefit. I have in the past run higher pressures, but found it was giving a wear pattern that indicated over-inflation, so dropped it back Never noticed any significant MPG change in doing that.
Those are my data points, as always your milage may vary.