|
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%) |
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.
|
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 |
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 |
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) |
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.
|
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.
|
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. https://www.greenhybrid.com/compare/.../signature.png |
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.
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.:confused: Who knows what the reason is. |
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.
My odometer appears to be off by 2% my mpg should really be 26.7 not 27.2.:omg: |
Re: Discrepancies between Energy Monitor and actual MPG
Okay, I finally filled up my tank after spending the last one without the radio, CD player, A/C and heater... no gunning the engine, altered work route to take expressway instead of freeway (note: I did have to turn on the fan for about 15 min). So, my efforts with hypermiling produced the following numbers:
Manual calculation (total miles / gallons filled) = 26.17 mpg Energy Monitor calculation = 27.5 mpg => Energy Monitor 5.0% higher than the "actual" mpg So, how does this discrepancy compare to the tank prior to this where I had the CD player playing all the time, drove on the freeway for work commute, gunned it here and there? The numbers there are: Manual calculation = 24.8 mpg Energy Monitor calculation = 26.1 mpg => Energy Monitor 5.2% higher than the "actual" mpg Namely, the discrepancies to within expected uncertainties are identical. I measured similar discrepancies for other tanks as well. In other words, you should expect your Energy Monitor measurements of your MPG to be about 5% higher than the actual values. (Would be interesting to know what percentage of the numbers stored in the mileage database are based off the EM versus manual calcs). I do have one measurement that indicates that this discrepancy can be additionally about 2% higher when you've used the A/C and/or have a trip with lots of elevation changes from climbing/descending hilly streets. I'll keep track of this during the summer months to see if this is real. Anyways, I'm going back to enjoying my HiHy; cranking up my CD-player, my butt-warmer and occasionally outgunning those obnoxious Suburbans. Hypermiling is fun... but only up to a point. Always holding back 268 hp is just plain cruel. As long as I know the EM is always about 5% higher, I can at least know where I really stand (i.e. keep the EM above 26.25 mpg (=> 25 mpg actual)... https://www.greenhybrid.com/compare/.../signature.png |
Re: Discrepancies between Energy Monitor and actual MPG
The first manual calc I did on our HiHy came out at 26.1 MPG while the computer was showing 25.3 MPG. Don't forget that ultimately gas mileage is nothing more than the miles driven divided by the amount of gasoline used in gallons. This is true for 10 miles, a full tank, or a year's worth of driving (the longer you go, the better the average).
In order for these two numbers to match, the computer must be reset at the station and the tank must be filled up to exactly the same amount as the previous fill up. This is very hard to do consistently even with the autoshutoff feature on the gas pump. I would be curious to see how the computer and the gas station data compare after one year, or even six months (I'll never know, I always reset mine). |
Re: Discrepancies between Energy Monitor and actual MPG
THIS IS A GOOD TOPIC. MY HIHY DID 24.8 ON THE SCREEN AND 24.4 ON MANUAL CALCULATION. tHIS IS THE FIRST TEST.i WILL COMPARE THE NEXT 10 TANKS TO SEE HOW MUCH IT CHANGES. JUST I MORE SQUEEZE ON THE GAS NOZZLE COULD CHANGE IT SOME.
|
Re: Discrepancies between Energy Monitor and actual MPG
The reason for the discrepency is because of a cummulative sampling error. When you drive in city/suburban conditions, frequent speed changes result in the sampling error. Your computed MPG will be different than your displayed MPG. If you do a long highway trip, at constant speed, you'll find your display matches your computed MPG, within a two or three percent.
|
Re: Discrepancies between Energy Monitor and actual MPG
I contend that the onboard FE display is going to be much more accurate from a tank to tank basis than hand calculation because the data source and the way the data is compiled for the vehicle's calculation of MPG is much more accurate than the source data available to us for calculation.
The car calculates MPG based upon fuel metered to the engine over distance traveled. We calculate FE based upon tank fill up over distance traveled. We and the car see the same distance traveled, however the onboard computer knows how much fuel was metered into the ICE. The only thing we know is how much fuel we were able to put in the tank. Bladder or not, that tank fill can vary from filling to filling. A few ounces one way or the another can have a huge impact on FE on vehicles which get exceptional economy. On the other hand, the metered fuel is a known value which is stored with the onboard computer system, compared to the known distance traveled and the vehicle comes up with an accurate average based on values that are fully known with no "fill-up variance" error. Now, if all gas pumps shut off at the same place and time on a vehicle, or we had a trans parent filler indicator on the side of the vehicle so we could see the actual fuel level in the tank, then we could refill the tank to the same point each time we refilled and do a calculation. I would suspect that if we could do that, we would get an FE number that was quite similar to what the vehicle calculated. |
Re: Discrepancies between Energy Monitor and actual MPG
Originally Posted by JeromeP
I contend that the onboard FE display is going to be much more accurate from a tank to tank basis than hand calculation because the data source and the way the data is compiled for the vehicle's calculation of MPG is much more accurate than the source data available to us for calculation.
The car calculates MPG based upon fuel metered to the engine over distance traveled. We calculate FE based upon tank fill up over distance traveled. We and the car see the same distance traveled, however the onboard computer knows how much fuel was metered into the ICE. The only thing we know is how much fuel we were able to put in the tank. Bladder or not, that tank fill can vary from filling to filling. A few ounces one way or the another can have a huge impact on FE on vehicles which get exceptional economy. On the other hand, the metered fuel is a known value which is stored with the onboard computer system, compared to the known distance traveled and the vehicle comes up with an accurate average based on values that are fully known with no "fill-up variance" error. Now, if all gas pumps shut off at the same place and time on a vehicle, or we had a trans parent filler indicator on the side of the vehicle so we could see the actual fuel level in the tank, then we could refill the tank to the same point each time we refilled and do a calculation. I would suspect that if we could do that, we would get an FE number that was quite similar to what the vehicle calculated. The fill-up variance disappears with more tanks in calculating your cumulative average. Unless the fuel pumps at the gas stations are inaccurate, the owner calculated accumulated FE (and GH database) should eventually match the onboard value. I don't know for sure but I don't think the onboard actually measures fuel use with a precise fuel meter, but does a fuel use estimate based on several other control sensors like the o2 and MAF along with fuel injector pulse durations. |
Re: Discrepancies between Energy Monitor and actual MPG
I agree with what a couple of other posts say. I also see substantial differences on a single tank between the calculated mpg and the mfd mpg. Over time though the actual gallons used will not lie. I have a little less than 15K on my 05 Prius and have noticed up to 10% or even more single tank differences. I also suspect the computer is missing negative data on the real time caclulations. I have another idea that cold weather is causing the differences. I have seen numbers varying less than 2% since it warmed up here in Dallas over the last 2 tanks (59.8 calculated vs 60.6 mfd and 59.5 calculated vs 60.6 mfd). That's about 850 miles of driving.
I've kept a tank by tank log since purchase (mfd figures added 1/1/06 forward) and plan to send it to Toyota after I have a full 6 months of the variance. Just to see what they say. :) |
Re: Discrepancies between Energy Monitor and actual MPG
Did a google looking for info on the accuracy of the fuel pumps at gas stations and found this.
http://www.npr.org/templates/story/s...toryId=5304258 NPR All Things Considered talkd to a gas station inspector that actually checks accuracy. Bottom line, there is slight variation, which overall goes in the favor of the customer, though it is certainly possible to get significantly less or more on occasion.:omg: One tip, fill at the slower notch to get maybe slgihtly more than at the fastest notch on the handle. |
Re: Discrepancies between Energy Monitor and actual MPG
So, let's look at some data that I've collected over the last 4 tanks of gas, starting from the most recent first (this is a AWD HiHy):
Manual (mpg) / Energy Monitor (mpg) / %Difference ------------------------------------------------------------- 26.9 / 27.7 / 3.0% 26.2 / 27.5 / 5.0% 24.8 / 26.1 / 5.2% 25.2 / 27.1 / 7.5% These tanks were filled up at different gas stations and different pumps. Each tank had different driving conditions (A/C, %Freeway, Gunning, etc). The most recent tanks had the least % of freeway driving, and is interestingly correlated to a lower %Difference. However, there is insufficient data to claim that the correlation is statistically significant. As you can see all 4 tanks show that the EM is consistently higher (~5.1%) than the manual calculations. This data shows good evidence that the EM reports systematically higher mpg estimates than that calculated by the manual method (total miles/gallons of gas). The NPR report provided from the previous post indicated that they measured discrepancies of typically 1/2 cup per 5 gallon or about 0.6% discrepancy, which is far lower than the 5% discrepancies from these measurements. I find it unlikely that the different gas stations represented by these measurements are all systematically calibrated to report 5% more gallons than actually delivered. (Although it would make an interesting conspiracy theory.) I don't see how that could go unnoticed over time... I'll keep monitoring this to keep adding to the statistics. https://www.greenhybrid.com/compare/.../signature.png |
Re: Discrepancies between Energy Monitor and actual MPG
Hi Gang.
We just bought our Toyota Highlander Limited 4x4 about a week ago. The whole thing we wanted was better environment, gas mileage, and of course the high resale value of toyota, amongst other pluses in quality. We paid $43k which included : 42000 MRSP -3k off = 39000. We got gap and extended warranty = 2k . We paid taxes title insu which was also 2k. So we paid 43k for the top end hybrid. Pretty pricy. It said 31 and 27 mileage EPA. I already knew from our 2 other vehicles that EPA is full of it as a rule. We only have 100 miles on it and the computer says 22mpg. Most has been small trips of about 2 miles. I took it out on the highway last night just to help break the engine in. From what i have read here, we could be doing better, but it isnt that important to alter our life to squeeze a few more mpg. My jeep grand cherokee limited is 'supposed' to get 16 and 20 according to the EPA but in reality gets about 12 mpg around town. The 20 is fairly accurate on the open road under optimal driving conditions. The Jeep has an inline 6 , 4.0 litre and torques about 195hp. we also have a little ford escort which is a total miser. She gets about 20mpg in the city real world, and at least 35mpg on the highway. Love that sucker. We traded a beautiful and powerfull but economical disaster Chevrolet Trailblazer LTZ because of the high miles on it and the horrible 10mpg real world city driving. My wife loved her 'machine' and was sorry to give up her 'kick azz' monster truck. So we got her the Highlander hybrid to get the best of both worlds and appease her needs. This highlander has better pickup and power than both of the suv's, and of course the gas mileage is much much better. We didnt buy the thing to squeeze every drop out of the EPA , or we would have bought a pirus. Better yet, bought a bicycle. To think we can drive a 268 hp fully loaded SUV and get 22mpg real world out of the sucker in the city is a miracle in its own right. As much as i detest the bait and switch of the EPA of all vehicles, i already knew to expect 80% real world. We are delighted that we are helping the enviroment and putting a whole pile of $$$ back into our pockets, instead of giving it to oil companies who artificially inflate gas prices whenever the hell they think they can get away with it , using excuses that are pathetic and intelligence insulting. |
Re: Discrepancies between Energy Monitor and actual MPG
CONGRATULATIONS!!! You'll love it more each day. I can say don't worry about your mileage till you hit about 5K miles, that seems to be the break in point on the Hybrids. Mine jumped considerably at 5K and when we got our warm weather back. Still fighting cold mornings which isn't helping, but averaging high 20's (26-29mpg) pretty regularly now.
PS: It will easily beat them Chevy's and a lot of others if you want to...;)
Originally Posted by tomdavie
Hi Gang.
We just bought our Toyota Highlander Limited 4x4 about a week ago. The whole thing we wanted was better environment, gas mileage, and of course the high resale value of toyota, amongst other pluses in quality. We paid $43k which included : 42000 MRSP -3k off = 39000. We got gap and extended warranty = 2k . We paid taxes title insu which was also 2k. So we paid 43k for the top end hybrid. Pretty pricy. It said 31 and 27 mileage EPA. I already knew from our 2 other vehicles that EPA is full of it as a rule. We only have 100 miles on it and the computer says 22mpg. Most has been small trips of about 2 miles. I took it out on the highway last night just to help break the engine in. From what i have read here, we could be doing better, but it isnt that important to alter our life to squeeze a few more mpg. My jeep grand cherokee limited is 'supposed' to get 16 and 20 according to the EPA but in reality gets about 12 mpg around town. The 20 is fairly accurate on the open road under optimal driving conditions. The Jeep has an inline 6 , 4.0 litre and torques about 195hp. we also have a little ford escort which is a total miser. She gets about 20mpg in the city real world, and at least 35mpg on the highway. Love that sucker. We traded a beautiful and powerfull but economical disaster Chevrolet Trailblazer LTZ because of the high miles on it and the horrible 10mpg real world city driving. My wife loved her 'machine' and was sorry to give up her 'kick azz' monster truck. So we got her the Highlander hybrid to get the best of both worlds and appease her needs. This highlander has better pickup and power than both of the suv's, and of course the gas mileage is much much better. We didnt buy the thing to squeeze every drop out of the EPA , or we would have bought a pirus. Better yet, bought a bicycle. To think we can drive a 268 hp fully loaded SUV and get 22mpg real world out of the sucker in the city is a miracle in its own right. As much as i detest the bait and switch of the EPA of all vehicles, i already knew to expect 80% real world. We are delighted that we are helping the enviroment and putting a whole pile of $$$ back into our pockets, instead of giving it to oil companies who artificially inflate gas prices whenever the hell they think they can get away with it , using excuses that are pathetic and intelligence insulting. |
Re: Discrepancies between Energy Monitor and actual MPG
Bob:
thankyou. I have a question for you. What do you guys think of the cali companies hacking the prius with extra plug in batteries to get 100mpg? Do you think toyota will catch on and offer us plug in refits for the highlander? |
Re: Discrepancies between Energy Monitor and actual MPG
Originally Posted by tomdavie
Bob:
thankyou. I have a question for you. What do you guys think of the cali companies hacking the prius with extra plug in batteries to get 100mpg? Do you think toyota will catch on and offer us plug in refits for the highlander? |
| All times are GMT -7. The time now is 08:17 PM. |
© 2024 MH Sub I, LLC dba Internet Brands