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Discrepancies between Energy Monitor and actual MPG

Discrepancies between Energy Monitor and actual MPG

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  #1  
Old 03-01-2006, 09:27 PM
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Default Discrepancies between Energy Monitor and actual MPG

I have been observing significant discrepancies between the MPG reported by the Energy Monitor and that calculated manually by dividing total no. miles by the number of gallons to refill my tank. Wondering if anyone else has been observing this issue.

Today, when I went to fill my gas, my Energy Monitor reported 27.1 MPG (and that only covered the miles since my previous refill). However, the manual calculation gave me only 25.2 MPG. That's 7% lower!

I did have my A/C and/or fan on for about half of the total miles travelled, but I'm really surprised that that would result in a 7% reduction in my FE. Especially since I had the setting on low for the times the A/C and/or fan were on.

Has anyone else measured such a large discrepancy? I also had my CD player playing for almost the entire miles covered. And for about 20 miles, I had the butt warmers on. But, again, I would expect that to affect the FE by nominal amounts (~1%)
 
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Old 03-02-2006, 03:56 AM
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Default Re: Discrepancies between Energy Monitor and actual MPG

There are some limitations to the accuracy of both the energy monitor and the manual method of calculation. The manual method is fairly accurate for 3-4+ tanks, but on a tank per tank basis there is a high probability that there will be a significant discrepancy. As far as the energy monitor is concerned, I have a feeling it may "miss" some portion of the data that's available because it's basically a really slow and simple computer, and thus can't handle the full amount and speed of the data, so sometimes it will drop the moment of data that may include a very high peak consumption, such as a brisk acceleration... That's my feeling, not scientifically validated, but I can compare it with some other tools for reading data from the vehicle, and the built-in display seems pretty limited.
 
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Old 03-02-2006, 02:12 PM
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Default Re: Discrepancies between Energy Monitor and actual MPG

I would imagine having your A/C, CD player and butt-warmers on, and their impact on mpg (if any), would be accounted for by the computer anyway.

Yes we've seen descrepancies that big between the computer and the "by hand" method before. Sometimes it ends up that "by hand" is the larger number of the two!

Like Schwa said, there's a margin of error in both methods. The computer has to have some degree of error, and whatever gas pump you go to may not shut itself off at the exact same time every time you fill up. Or if you go to different pumps routinely, or if you add a lot more fuel after the first click.

There are a lot of variables there...... with our HiHy, we just try to be consistent so we don't add variables on top of variables. We stop re-fueling on the first click, and occasionally our "by hand" calculation is actually higher than the computer says, but most often it's at least 1mpg less. I'll think we're going to have a tank that's 32mpg or higher, only to find out it was 30.5, etc. Or, if "by hand" is just as inaccurate as the computer, then I can assume I really *did* get 32mpg or higher!

Rick
 

Last edited by ender21; 03-02-2006 at 02:14 PM.
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Old 03-15-2006, 11:26 AM
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Default Fuel tank bladder?

We asked our dealer why the computer in our Highlander Hybrid regularly says we're getting about 1.5 - 2 mpg better than we calculate manually. The mechanic thought that the Highlander probably has the same rubber bladder in the fuel tank as the Prius, and that therefore we aren't necessarily filling up all the way. I really like this explanation, because that means we're getting better mileage than we thought (27.8 average calculated manually). Is this true? Is there a bladder in the HiHy?

If so, is there a way to go back and edit the tanks I've posted in the compare area? I did keep a record of what the computer said for each tank.

Katie
 
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Old 03-15-2006, 03:08 PM
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Default Re: Fuel tank bladder?

As far as I know, yes. In fact, pretty much all the hybrids have one.

There was a thread a few months ago in the FEH forum from an engineer, who was concerned when he kept getting lower calculations than the computer, so he did an experiment, and found it was right, and he was wrong, due to fill factors and the bladder.

I may be wrong about the HiHy, but that's my understanding as it is.

As for changing your tank data, yes, that's what the edit button is for next to your tank data, on the right, next to more. (After you've logged in if you're not)
 

Last edited by Pravus Prime; 03-15-2006 at 03:11 PM.
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Old 03-15-2006, 08:50 PM
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Default Re: Fuel tank bladder?

Well, on the assumption that that is correct, I edited all my tanks to use the computer's calculation for mpg (we've been writing that down, along with the odometer reading, on all the receipts). Went from 27.8 average to 29.1!! Is this real, though? If I take the number of miles on the odometer, and divide by the number of gallons of gas we've purchased, it comes out to around 28 still. This isn't making sense again.
 
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Old 03-16-2006, 03:29 PM
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Default Re: Discrepancies between Energy Monitor and actual MPG

I asked my dealer about this. It turns out there's a rubber bladder in the fuel tank. If it's cold when you fill up, the rubber doesn't stretch as much, so less gas goes in (and the mileage goes up). When it's warmer, the opposite is true (more gas can go in, the mileage goes down). The computer's calculation is not affected by the bladder (I think the mechanic said it uses numbers from the fuel injection?) I saw another post on this board that mentioned someone doing some calculations regarding this.
 
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Old 03-18-2006, 10:56 AM
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Default Re: Discrepancies between Energy Monitor and actual MPG

Although from all these posts, this may be pointless, I'm doing a little experiment with my current tank. For the length of this tank, I'm leaving the butt-warmers off, the A/C and/or Heating off and the radio/CD player off. I'm going to limit gunning my HiHy to very rare occasions (although this is probably the hardest part). I'm even altering my commute to work to take an expressway instead of the freeway. Basically, eliminate as many excuses as possible for that Energy Monitor from being so far off from my manual calculations.

I don't really buy the reasoning that the discrepancies between the EM and manual calcs are due to nuances in filling up the tank, because the EM has been consistently higher than the manual calculation for every tank I've filled up. If the discrepancies were due to fill up nuances, I would have expected some of the comparisons to show EM reporting lower than the manual calculation.

So far, the EM shows 28.1 mpg at about 100 miles into this tank. We shall see... Based on prior discrepancies, this should translate into a manual calculation of about 26.6 mpg.

 
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Old 03-19-2006, 09:12 AM
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Default Re: Discrepancies between Energy Monitor and actual MPG

Originally Posted by KBerryhill
I asked my dealer about this. It turns out there's a rubber bladder in the fuel tank. If it's cold when you fill up, the rubber doesn't stretch as much, so less gas goes in (and the mileage goes up). When it's warmer, the opposite is true (more gas can go in, the mileage goes down). The computer's calculation is not affected by the bladder (I think the mechanic said it uses numbers from the fuel injection?) I saw another post on this board that mentioned someone doing some calculations regarding this.
I'm pretty sure that the Highlander HH & Lexus RX do NOT have the fuel blader, like in the Prius. I know on the wifes Prius there is rubber all the way into the neck that you have to push the nozzle into when you fill up and it creates a tight seal around the nozzle, my HH is not this way and seems to be a conventinal tank and filler. I'm finding the dealers and their employees are not that familiar with the differences between the two as they are not the same in many aspects e.g. Thermos used on the Prius and not on the HH/RX.

On my fillup yesterday, the computer showed 25.7MPG and when I calculated it out I only got 21.3 MPG and it was cold here (20's) so if what you say was true I should have gotten better mileage. That was the biggest difference between the two for any tank fill since I bought the HH. The previous tank and this one I believe were packed to the same extent. Who knows what the reason is.
 

Last edited by Bob259; 03-19-2006 at 09:15 AM.
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Old 03-22-2006, 05:49 PM
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Default Re: Fuel tank bladder?

Originally Posted by KBerryhill
Well, on the assumption that that is correct, I edited all my tanks to use the computer's calculation for mpg (we've been writing that down, along with the odometer reading, on all the receipts). Went from 27.8 average to 29.1!! Is this real, though? If I take the number of miles on the odometer, and divide by the number of gallons of gas we've purchased, it comes out to around 28 still. This isn't making sense again.
I don't think it's real. Your using the same odometer reading as the car's computer. The only variable is how much gas is used. If it had to do with gas tank variation some would be higher and some would be lower. I'd edit back to 27.8. Now if your odometer is inaccurate it could be even less.

My odometer appears to be off by 2% my mpg should really be 26.7 not 27.2.
 
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