True of the speedometer, but NOT of the odometer. The odometer repeatabilty is as close to 100% as you will get even with a cable driven odometer. (it is a directly geared measuring device with ZERO vatiability - X number of cable turns per mile from the day it's made till the day it is scrapped ( generally 1000 turns per mile, but some older cars were 600 turns per mile, some motorcylses 1450, etc - but they never change) With electronic speedos and odos (virtually all cars today less than 15 years old) repeatability is almost 100%. Accuracy CAN be very close to 100% too, as on most cars under 10 years old today, the speedometer can be accurately reprogrammed to the tire diameter so repeatability is only affected by tire wear (mabee 3/8 inch in 24 over the life of the tire)
Directly related? but not necessarily 100% linearly related High manifold pressure (low vacuum) means heavy load which means poor mileage. The reverse is also true - but calibrating vacuum to MPG is virtually impossible with any level of accuracy. It WILL give you a good, better, worse indication though. Keep the vacuum up and you will get better mileage.
Nearly all my driving is on secondary highways so I pretty much am driving at pretty optimal speeds for mileage although there are some traffic lights, they tend to be miles between stops. I have developed fuel efficient habits so I nearly always squeeze every last MPG on my trips. I have a manual, so I slip it out of gear and coast to lights and nearly always accelerate gently. I leave a lot of room to the car in front so I can ease up on the gas rather than hit the brakes. I think I am doing about as well as can be expected all the time, so my mileage seldom varies unless I do more city driving. High 19 or low 20 MPG, very consistent.
I don't know what you mean. I have checked my odometer against the markers on the highway as well as against my GPS (I think the highway markers are more accurate than the GPS). It is spot on with the current tires to 1% or better. I had some larger tires at one point and it made the odometer read a bit low, also the speedometer.
BTW, someone said something about one being accurate meant the other was accurate and that is not necessarily true. My speedometer is mechanical and so has a separate calibration factor. With the present tires it reads a bit high, about 1 to 1.5 MPH at highway speeds. That one is harder to calibrate than the odometer (which is pretty much on point) because it is hard to maintain a constant speed for long enough to get an accurate reading even with the cruise control. But with lots of readings I am pretty confident these numbers are right.
So my odometer is accurate and precise.
Of course it is. States inspect them at some point.
I don't agree. I let the pump click off and then continue to pump for a number of more clicks until it cuts off immediately. I always need to run at least another fifteen miles before I am home so that is better part of a gallon burned so I don't need to worry about the gas warming up and running out of the tank. I believe this makes for very consistent fill ups.
My MPG results pretty well show the consistency of my measures.
You know what happens when you assume... ;)
I think one time in nearly 20 years I got 22 MPG. I think I can count on my fingers the times I got 21 MPG. These days with nearly all my driving on the highway it is much less than 1 in 20 fills that I see less than 19 or even 19.5 MPG. It is nearly always just under or just over 20 MPG, more just under :-( If I were the dancing type I would have a little happy dance when it actually is over 20 MPG, lol. It makes my day.
I think the consistency of my MPG readings show how well each of these can be measured. As you say, the pump is going to be dead on. Other than scale error which can be calibrated out the odometer will be very good. Filling your tank can be good as well. It's not like they design gas tanks to have air pockets.
You don't need to know any of this specifically. You just need to measure your fuel mileage and measure the accuracy and precision of the results. Why do you care which of the three has what specific degrees of accuracy and precision? You care about the accuracy and precision in the result and you can measure that. Remember there are other factors as well that actually impact your MPG from tank to tank. They will show up when trying to measure any one influence so might as well calibrate them in too.
When you consider how the old speedometers worked it's amazing they came anywhere close to reality. I had a '60 Plymouth where the speedometer looked like a red bar progressing across a horizontal display rather than the usual needle. The guts were a tube about a foot long and an inch and a half in diameter suspended in bearings and loaded with a spiral spring. The mechanical cable from the tailshaft of the transmission tweaked the tube with each revolution via a magnetic link. It was an analog integrator with the spring controlling the tube's rotation.
The standard dial type was the same principal but the Chrysler engineers went out of their way to be weird. That was also the era of the pushbutton Torqueflite tranny and left handed lugnuts on one side.
A lot of modern speedometers are just as bizarre converting a perfectly good digital pulse train to an analog voltage to drive a dial rather than going straight digital.
I'm a fairly economical driver but on longish trips I'm more concerned with getting there. 80 mph guarantees the fuel economy is going into the dumpster.
Once again: This horse is dead, skinned, flensed, tanned, jerked and dried. And whatever life it has clung to for these many repetitive, redundant and often ridiculous posts is based on the essential confusion between and con flation of "Accuracy" and "Precision". These do not mean the same thing, ne ver have the same application, and seldom are on even parallel tracks when used properly.
When dealing with furlongs per bale, sufficient accuracy may be had using n o more than 20% of the fingers of one normal human hand and the track marke rs. Miles per gallon, and kilometers per liter are quite similar, requiring only second-grade arithmetic to solve within repeatable limits. Full stop.
I forgot, I can tell the difference in fuel economy by driving 65 MPH rather than 60. Driving at 65 very much (only about 1/3 of my trip allows that) will assure that I only get 19 mpg rather than pushing 20.
There is a 10 mile stretch with only one traffic light and a posted speed limit of 45 MPH. If I can get up to 50 so I'm solid in fifth gear my mileage rocks.
I tried that one day on a flat stretch so there would be little variance. This was on my regular trip to work. Speed limit is 65. One day I did 70, the next 65, then at 60 is was dicey, the next day I tried
55 for about 30 seconds and decided not to risk my life.
I forget the details, but 60 was better than 70 by a couple of mpg. Problem is, I prefer driving 75. If I could get away with it I'd go 85+ but don't want to pay the fines.
I understand you because you're exactly the type of person that I had in mind when I asked the question in the first place.
Does your tripmeter have a decimal place and digits after that decimal place?
The speedometer example was only brought in to point out that the vain hope that averages result in better "accuracy" is patently false.
Mom-and-pop type of people actually believe that a speedometer reads even close to accurately - and worse - some here propose the vain notion that the more readings they take, somehow (magically?) the more accurate the results will be.
A speedometer that reads high isn't going to result in more accurate calculations even if you do a billion test runs.
You don't seem to understand what accuracy and precision even mean. Haven't you taken even one science lab course?
I'm not at all surprised about your concept of the fuel-level estimation, and, in fact, you're exactly the mom-and-pop type person I was talking about when I opened the thread.
I understand you.
I'm sure you do believe that.
I'm sure your MPG results support any theory you want them to support. I believe you.
You don't know how funny that statement was to me when I just read it now.
I bet you see that decimal place even though it's not in the tripmeter estimation nor in the filllevel estimation.
You see, I understand you because you're the type of person I had in mind when I asked the question.
I'm sure you do.
Whoa! I never said the pump was "dead on" and anyone reading this thread who thinks I think the pump is "dead on" would have completely misunderstood everything else I said.
All I said was that the inaccuracies and imprecisions in the pump reading are likely better than the otherwise astoundingly huge imprecision in the fuel-fill level estimation and in the lesser inaccuracy of the tripmeter estimation.
Define "very good" please.
I'm sure you believe that filling the tank is "accurate" since you calculate 19.5 miles per gallon and not something like 19.5 rounded up to
20 and then the error taken into account such that it's more likely anywhere between 19 and 21 mpg than it is 19.5 mpg.
Actually, they do have air pockets. Those air pockets change in size based on temperature & pressure & fill level.
Even the fuel changes in density based on those parameters.
Of course I don't. 19.5 mpg is all I need to know. And if I change "something" which results in 19.7mpg, then of course, that something was the cause. I understand. I really do.
I care because when I do a calculation, my assumption is that 19.5mpg is actually something closer to 19 to 21 mpg than it is to 19.5.
If the "change" I'm measuring is within that margin of error, then I can't say anything about what that "change" was.
And, more importantly, neither can you. Which is the entire point after all.
I got curious myself on what the numbers revealed and looked at the NIST numbers again.
I computed an empirical cdf and compared it to normal...statistics from the 20,036 observations are below:
I then compared to normal on the same plot and as outlined above N(mean,std) is too long-tailed on both ends in comparison. It turns out that N(mean,std/1.5) is pretty close on both tails to about the +/- 6 point.
Anyway, from the above it's simple enough to get some pretty good estimates of what pump volume errors one might expect...the table below is from the empirical cdf NIST data...
The man is right You are wrong. You ASS U ME too much - and at the risk of insulting the few GOOD engineers on the list, you OBVIOUISLY are an "engineer", but not one I'd hire for a job. The job would come in WAY over budget, WAY late, and would need to be completely redone by techitians and technologists at great cost, or to save time and money, completely decommissioned and scrapped - starting over with someone who knew what thet were doing, and how to do it - engineer or not.
I love that you are the only one quoting actual numbers and not pulling them out of your butt to answer the question!
But your numbers confuse me because they seem to be in cubic inches. You also mentioned that metric pumps are more accurate, but that's impossible, simply because the pump is as accurate as the pump can get, which, we can assume, is a mechanical thing (and not a metric thing).
All you're saying is that a liter is four times smaller than a gallon so the error is four times less for a given liter versus a given gallon but that's not saying it's more accurate. It's just saying the volume is less so the resulting error is less.
Anyways, can you just summarize what the error is for a typical USA pump in gallons?
For a typical 20-gallon fill, how many gallons off can reality be, plus or minus from the indicated reading on the pumpmeter?
At under 70 my car usually is in the 35 mpg + range; at 80, it is more like 32. I get even better mileage in Oregon with its 55 mph speed limit. I also get bored out of my mind. There isn't a whole lot of anything between Ontario and Bend but I figure as soon as I get up to a decent speed a OSP cruiser will materialize from the sagebrush.
That stupid speed limit is the least of Oregon's problems.
The article talks about Washington but most states have a similar protocol. Pump 5 gallons of gas. 1 gallon is 231 cubic inches, so that is 1155 cubic inches. The volume must be within 6 cubic inches or roughly 0.5%. I'll let you do the math for 20 gallons.
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Montana uses the same test. Note that he estimates 2 to 3% of the pumps fail and have to be repaired and also says with normal wear the pumps tend to dispense more than stated but some may dispense less. That's where averaging over a number of tanks comes in unless you fill up at the same pump at the same station every time. I certainly don't.
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