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Car Forum / Honda Cars / October 2004Accuracy of Honda Gas Mileage Gauge
My 04 Civic Hybrid has a digital gauge that displays the current miles per gallon consumption as well as mpg for distance on the trip meter. Can anyone tell me how these guages work and what are the spec tolerances? I always set the trip meter at each fill-up to automatically get mpg, but I also calculate the mpg based on the gas pump fillup quantity. Those results are always different, sometimes by almost 1 mpg over 500 miles. Differences vary so neither is always higher or lower. I believe gas pumps are quite accurate for amount dispensed, but the difference probably is caused by inconsistency in gas pump cut-off nozzles so that the gas tank is filled to same level. I usually try to use the same pump but my guess is that the nozzle trip point is not accurate and is affected by the outdoor temperature. Anyway, I didn't know which was likely to be more accurate and wondered if someone knew. Thanks <snip> > I always set the trip meter at each fill-up to automatically get mpg, but I > also calculate the mpg based on the gas pump fillup quantity. Those results [quoted text clipped - 3 lines] > inconsistency in gas pump cut-off nozzles so that the gas tank is filled to > same level. <snip> I have found that the pump shutoff point varies quite a bit. I Measured over 10 fill ups where I picked pumps more or less at random (but always at the same station). Driving conditions were consistent during the entire time. The average gas mileage during the test was 32.3 MPG. Using this number I calculated the expected number off gallons needed to fill the tank and compared with the actual. I found that the actual ranged from 0.98 gallons below expected to 0.82 gallons above expected for a range of 1.8 gallons. Standard deviation was 0.64. I did a control experiment using the same pump every time 9 times in a row. During this test the average MPG was 32.9. During this test, actual was from 0.35 gallons below expected to 0.25 gallons above expected for a range of 0.6 gallons. Standard deviation was 0.20. Randolph, Thanks for the data. It confirms that the fill-up quantity is quite variable. What size tank do you have? That 0.6 gal diff on 11 gal fill-up would really throw off the mileage calculation. 0.2 gal is much better but hard to always get same pump. Would like to know specs on the digital gauge which I hope is more accurate. --V > <snip> > [quoted text clipped - 25 lines] > was from 0.35 gallons below expected to 0.25 gallons above expected for > a range of 0.6 gallons. Standard deviation was 0.20. On 10/11/2004 7:13 AM VW spake these words of knowledge: > Randolph, > Thanks for the data. It confirms that the fill-up quantity is quite > variable. What size tank do you have? That 0.6 gal diff on 11 gal fill-up > would really throw off the mileage calculation. 0.2 gal is much better but > hard to always get same pump. Would like to know specs on the digital gauge > which I hope is more accurate. --V Look, I hate to point out the obvious, but this whole thread is really f.cked up. You measure your mileage over an extended period of time, with respect to miles driven. The possible variance delivered by the how much you 'top off' becomes insignificant after a few tanks. Let me provide an example: At mile x, you fill the car. When the low fuel light comes on, you fill it again, recording how much gasoline you put in. In order to get a reasonable estimate from tank to tank, you try to fill it pretty much the same way each time (i.e. til the first click off at the pump, as full as you can possibly get it, three squeezes for luck, or whatever) but this doesn't matter over the long run. Let's say you put in 13.44 gallons. If you wish to know an approximate mpg for that tank, you also write down how many miles you've driven between fillups and divide that number by 13.44. Repeat this over 10 tanksful, just as an example. As long as at the end of the tenth tank you have the tank about as full as it was back at mile x when you first filled it up, you will have a very accurate picture of your mileage over that period. Irrespective of exactly how full you got the tank each individual time, your gas mileage is the total number of miles you've driven since mile x, divided by the total number of gallons you've put in. This is true even if several times out of the 10 fillups, you only put in $5 'cause that's all you had with you. The fact that you didn't fill the tank all the way, or even half way, makes no difference at all. You *do* want to fill the car pretty much to the same level as it started on the fillup where you're going to measure - in this example, the 10th fillup. If you put in 13.44 gals, 13.76 gals, 2.55 gals, 12.54 gals, 13.12 gals, 14.65 gals, 12.88 gals, 3.14 gals, 13.23 gals and 12.89 gals, your total gallons put in is 112.2. Let's say your miles driven (current mileage - x) is 2964; your mileage over that period of travel (2964 miles) is 26.42 miles per gallon. This does not depend on micromeasurement, you don't have to do any calibration, you don't have to buy anything but a pencil (and the gas, of course), you don't have to go to the same pump (or station), you don't have to buy all your gas during the same season - in fact, the more tanksful you get, the less possible effect any variable can have on your mileage calculation, and the accuracy of the gas gauge doesn't enter into the picture in *any* way. All it requires is a very basic knowledge of 5th grade math. RFT!!! Dave Kelsen  Signature... I didn't say it was your fault. I said I was going to blame you. You can't make it any simpler than that---People want to make it Rocket Science. The only basic variable(s) are the accuracy of the gas pump metering system and the writing down of the numbers. To view the variability of the calculations all one has to do is to compare individual gas fillups against the long term, high volume/long miles. It becomes very obvious the longer between calculations the more the small stuff washes out and accuracy increases. MLD > On 10/11/2004 7:13 AM VW spake these words of knowledge: > [quoted text clipped - 53 lines] > RFT!!! > Dave Kelsen Sorry if this didn't come across clearly in my original posting, so maybe this will clarify: I was trying to get data to compare the accuracy of the computer/electronic gauge compared to the traditional fillup/divide method which everyone knows has variability. I agree that averages over more fillups will give a good average long term mpg, however, at least I personally sometimes want to know mpg for specific shorter trips. What I really hoped is that someone had the specs on the digital gauge or had other experience/knowledge with them as they're getting more common in cars. If the computer is reasonably accurate I won't bother to do the fillup recording and calculations (which I've done for years to monitor my car performance). However, my experience with electronics is that these new gauges might not be very accurate, say 5% over total range, to keep their cost down (as with fuel tank gauges). And quite often there are web postings from people unhappy with "low" gas mileage which can be difficult to assess if it is not measured accurately or the different operating conditions are invalid for comparison. Even the EPA ratings on new cars are subject to debate. Thanks for the the responses anyway. > You can't make it any simpler than that---People want to make it Rocket > Science. The only basic variable(s) are the accuracy of the gas pump [quoted text clipped - 66 lines] >> RFT!!! >> Dave Kelsen On 10/12/2004 6:02 PM VW spake these words of knowledge: > Sorry if this didn't come across clearly in my original posting, so maybe > this will clarify: I was trying to get data to compare the accuracy of the [quoted text clipped - 17 lines] > > Thanks for the the responses anyway. I would say that your best bet with respect to measuring specific shorter trips would be to do the same as I outlined before, but additionally to reduce the variables as much as possible. Go to the same station/pump when possible, and fill up the same way if possible, for example stop when the pump first clicks off automatically. My (2001) Odyssey's gauge and low fuel light are famously inaccurate; Odyssey owners have found that in general, when the gauge reads empty, you have *at least* three gallons left. Some owners report nearer to five gallons! I don't know of any existing generalized data with respect to the accuracy of modern gauges. RFT!!! Dave Kelsen Signature... I've reached that age in life when I surreptitiously ogle my co-worker -- a smokin'-hot blonde Russian chick with legs that go on for days -- and all I can think is, "Man, I wish I could get her to say, 'Boris! Is Moose and Squirrel!'" -- Allen Lindsey Dave, Thanks for your response. I should explain that the new digital computer gauge in the Civic Hybrid provides both a instantaneous reading of mpg and it also provides the mpg for trips set by the 2 trip odometers. By reseting the trip odometer at fill up, it will automatically displays mpg for that trip until I fill up again, or anyother distance or trip. This is easier than recording fillup quantity and doing the calculation (even if not hard). My interest (as a techie type and being gas conscious) is how accurate is the gauge? I assume it uses a flow metering device and sensor to measure gas consumption but I don't know if it is more accurate than the gas pump method. Hopefully it is, but thats why I was asking. I am also going to post on Cadillac newsgroup since they've had these type of computer gauges for quite some time. Cadillac computer/f"uel data center" even provides an estimate of the miles you can drive before running out of gas (which I heard is not very accurate). Probably bored everyone so sorry. -V > On 10/12/2004 6:02 PM VW spake these words of knowledge: > [quoted text clipped - 34 lines] > RFT!!! > Dave Kelsen On 10/15/2004 8:55 AM VW spake these words of knowledge: > Dave, > Thanks for your response. I should explain that the new digital computer [quoted text clipped - 11 lines] > estimate of the miles you can drive before running out of gas (which I heard > is not very accurate). Probably bored everyone so sorry. -V Gotcha; I had one of these in my 96 Chrysler LHS; they have been in many cars for many years now - my brother-in-law has one in his Trailblazer, my mother-in-law had one in her Lincoln. My experience was that it was a useful guide, but nothing delivers the accuracy of my own measurements. They may have gotten to the point where they *do* though, now. It's been a few years. Good luck. RFT!!! Dave Kelsen Signature... Yes, I am an agent of Satan, but my duties are largely ceremonial. > My 04 Civic Hybrid has a digital gauge that displays the current miles per > gallon consumption as well as mpg for distance on the trip meter. Can [quoted text clipped - 12 lines] > Anyway, I didn't know which was likely to be more accurate and wondered if > someone knew. Thanks Fuel delivery at a gas station is set to automatically adjust the volume to account for variations in temperature. In this part of the world it's adjusted to 15*C. If it's warmer than that then you will actually receive more fuel than the pump will read. This could possibly explain part of the variation. Just out of curiosity what's your mileage been like in that vehicle? Peter H Peter, Thanks for your input. In the DC metro area commuting, I can get 50+ mpg with careful driving in the right lane at 55-65 mph. If I go in faster lanes over 60, the mpg goes down to about 43. --V >> My 04 Civic Hybrid has a digital gauge that displays the current miles >> per gallon consumption as well as mpg for distance on the trip meter. [quoted text clipped - 23 lines] > > Peter H If worrying about 1 mpg over a distance of 500 miles is your only worry in life then you are doing pretty good. Additionally you have too much damn time on your hands to micro manage your mpg. > My 04 Civic Hybrid has a digital gauge that displays the current miles per > gallon consumption as well as mpg for distance on the trip meter. Can [quoted text clipped - 12 lines] > Anyway, I didn't know which was likely to be more accurate and wondered if > someone knew. Thanks |
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