Computer estimate way off... 52.5 actual vs. 45.4 displayed

Re: Computer estimate way off... 52.5 actual vs. 45.4 displ

with the flexible bladder in the tank. the amount that you filled, may not be the same as the amount that the car used. these kind of errors usual normalize over many tanks.

right around 25mpg at start seems correct. with the engine warming is loses its ability to stop to preserve fuel. if the car get 50mpg with the engine being off 1/2 the time it would make sense with the engine running 100% of the time the mpg would be half normal. 50/2= 25.

 

Re: Computer estimate way off... 52.5 actual vs. 45.4 displ

I just had my 2nd fill up, and had a similar discrepancy. 47.3 MFD mpg, 471 miles driven. Calculated mpg 54. In an earlier post, I suggested tracking/comparing the amount of fuel to fill up against the "calculated fuel burned" (miles driven / MFD mpg). For this go around it was 8.738 pumped versus 9.96 calculated. My first fillup was 9.414 pumped versus 8.58 calculated. Even taking into account the flexible bladder and vagaries of pumping gas, this seems odd to me. I fully expect the issues are how I pump.

But, what bothers me the most is that on the first tank, I filled up when I hit one bar. On this tank, I "pushed" it, having the blinking bar for 20 some miles. Yes, it filled up with a lot less gas! I have to figure out how to fill up in a way that I can trust I'm being consistent. In other vehicles, I've always been able to hear the gas going into the tank and could hear it getting close to the top. With the Prius, I hear nothing.

How *does* the MFD calculate mpg? Does anyone know? Should we expect it to be accurate and our calculations to merge to it? People talk about it averaging out in the long run - that implies that either the MFD calculations change (or some variable it uses averages out over time) or our own calculations change (or our habits of pumping).

All of these threads about mpg and differences b/w MFD and calculated seem like a lot of hand waving. Not that I care too much, I know I'm getting great gas mileage. It's just the engineer/scientist in me that is unsatisfied.

mtc (sorry about the long, dangling thread)

 

Re: Computer estimate way off... 52.5 actual vs. 45.4 displ

I see in the other related thread that some good information is coming out. For my particular situation, I've discovered that I did indeed underfill the tank. My first two tanks didn't start dropping on the gauge until 150 miles or so. Yesterday, after 50 miles, it dropped a "box". I have to figure out how I can avoid underfilling.

 

I think you'd be hard pressed to find any of us 20k plus drivers with a discrepancy between computed and calculated MPG with more than 2%. Per tank, ignore your own calculations. Trust the car.

-m.

 

i would definitely track both data methods for comparison purposes if any. you can see by my signature that they are slowly converging and are now nearly identical.

something i stopped doing almost immediately and might help you (although i didnt notice a difference) is that i dont top off my tank. i stop at first click, round off the dollar amount and call it good. there was a rumour (i call it that because no gas station owner will verify this that i have asked) that the overflow detector may drain gas back during auto shut off. granted the tube is very small but the more times you dribble gas in after shutoff the more potential you have to lose some of the gas you paid for.

 

Re: Computer estimate way off... 52.5 actual vs. 45.4 displ

Trust the car's displayed mpg. The bladder is too variable for guessing how much gas was actually pumped in or used.

I have over 36,000 miles. I have done 92 fillups. My calculated mpgs have ranged from 7.5 mpg higher than the car's display to 19.1 mpg below the car's display.

Overall, the difference between the calculated mpg and the car's display mpg is a whopping 0.1 mpg.

Go with the display value, but track you're calculated value for fun.

 

I'm so glad to read this thread. I expected my first tank to be lower due to break-in. But I was surprised that the 50.0 on the computer compared to only 43.9 "on the pump"... US mpg.

I got 360 miles after being 7 ticks down. It had been on the 7th tick for a while; I was expecting to drop to 2 before pulling into the station. The first tick went down at about 100 miles. Not sure if that could gauge how full the dealer really had it. How far do others typically go on the first tick of the gauge?

I'm tracking both pump and computer mpg's. Over time, the number of miles driven divided by the sum of the gas should trend out for the bladder, variance in fuel backpressure shutoff, etc.

Gary

 

<div class='quotetop'>QUOTE(efusco\";p=\"81744)</div>

In addition to Evan's comments...
The MFD usually displays about 2.5% (plus/minus 1.5%) better mileage than actual.
The calculated mileage can be varied by...
- bladder effect
- filling level
- different pump
- different posture
- different temperature

It is too impatient to judge the accuracy by a single tank.
Please try to calculate at least three times average.

Regards,
Ken@Japan

 

First tank blues explained

When I received my car (henceforth referred to as Barbarella) it had 3.2 miles on it with a 3.2 mpg on the display.

I reset the display on the way out of the dealer, and displayed an average on 52.3 for the next 457.6 miles

My first tank (yesterday) was 9.690 gallons at $2.059. This averages out to be 47.554 mpg for the tank.

If you take into effect the time at the dealer, basically burning off 1 gallon before I picked it up, everything becomes clear. The remaining 457.6 miles consumed 8.690 gallons of gas, resulting in a 52.658 mpg for my driving.

Even buying "discount" 87 octane gas at Hess, I'm hovering closer to 54 mpg so far. Comparing that to the 18.5 I averaged in my former car (a V8), I feel like I'm winning the game.

Still waiting for my vanity plate: HYLIFE. I did think about HYBQED, but figured too few people know any latin.

 

Re: Computer estimate way off... 52.5 actual vs. 45.4 displ

I am in kind of the same boat, was really disappointed about my MPG, but we had a warm up last week and my MPG shot up about 6MPG which was great to see.

Looking forward to getting out on the highway once the weather gets nice!