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View Full Version : Fuel Efficiency/Consumption Question - How Does Drivers Measure Gas Mileage?



KhalilWilliams
02-15-2012, 02:01 PM
I was reading a thread started by fellow FIAT fan/forum member talindsay about city based gas mileage last night, under the impression that everybody who posted in that thread crucially scrutinises their fuel economy/consumption with a discerning brain. Prior to that, I watched a review by mpgomatic (one of my favourite reviewers) on the 2012 Hyundai Veloster and noticed how that car didn't achieve the maximum EPA highway estimate of 40 MPG, or (approx) 5.8l/100km and made a comment on it pointing out how in the advertisements it would state in fine print "Your actual mileage may vary."

With that, as well as how I know the Cinquecento for being such a surprise in fuel economy and the fact that I have only been licensed for less than an entire month now, I couldn't help but ask the following question: How do drivers measure their fuel economy/consumption in their cars? Do you simply rely on the readings from your car's EVIC or do you use any particular mathematical formulas to calculate your gas mileage?

PFVA63
02-15-2012, 02:39 PM
Hi,

I typically go by what the EVIC says, but for the 1st couple of tank fulls I double checked that by dividing the mileage I got on the last tank by the amount of fuel I just put into the car, assuming that if I am more or less filling up the tank each time then the amount of fuel that I just put in should be about the same as what I had burnt. Of course there are probably a lot of little variables in there such as time of day, temperature, the speed of the pump, and how sensitive the pump cut-off is, that might lead to some differences but I figured it was the best I could do at the time.

I don't have all the info on me right now, but my recollection was that the mpg that I calculated this was was typically very close (within a fraction of a gallon) to what the EVIC said so now I just go by what the EVIC says. :)

Pat

FiatPhil
02-15-2012, 03:48 PM
I use the EVIC. Since I reset the information every tank, I figure comparing EVIC numbers to EVIC numbers is a fair comparison.

talindsay
02-15-2012, 05:24 PM
The EVIC will give you a hint, but trip computers are always inaccurate, some cars better than others. I've only run four tankfuls of gas through my car so far, so I don't have good data to confirm this yet, but so far it looks like the EVIC is a touch optimistic - about 1 mpg high or so for my driving patterns.

First off, there *are* a couple right ways to measure fuel consumption, there are a lot of wrong ways, and there's no magical way to get a perfectly accurate reading within about ten tankfuls.

So, how do you do it? Well, if you know for a fact that the factory filled the tank completely full and that nobody added fuel since then, then you can start from 0 miles and start tracking, but most likely that's a bad assumption so your tracking can start at the mileage that you first fill the tank. If you want to get any kind of sense of gas mileage on a per-tank basis, you have to decide what you call "full" as you're pumping gas, and always honor that. For me, I'm careful to seat the filler the same way every time, and I fill until it auto-shutoffs, and don't top off at all. Now obviously, different pumps shut off at slightly different times which is one of many reasons why you'll never get an accurate measurement from just one tankful, but if you're consistent then at least you've done all you can to get an accurate single-tank reading. Filling at the same pump at the same station might also help, but it's still not likely to clear the issue entirely.

Anyway, the math is fairly simple - every time you fill the tank, you're replacing the gas you've burned since you last filled up. So at a single tankful level, you know the amount you put in is approximately equal to the amount you burned since you last filled up, IF you always fill the tank. To get the economy, you just subtract the two odometer readings to get the miles traveled. You have the number of gallons it took to fill the tank on the pump, so simply divide the mileage by the gas put in. This gives you an approximate gas mileage for that tank of gas.

If you don't fill the tank at a given fill-up, that's fine - it just means you can't get a reading for that specific tankful. You can only get a reading from filled tank to filled tank, but that's okay, you'll just add up the total distance from filled tank to filled tank, and add up the total fuel used from filled tank to filled tank, and divide. This is actually more accurate anyway since it's more tankfuls.

To get meaningfully accurate numbers, you want to add up all the gas you used over a long period of time and divide the total mileage over that period of time. Since the minor inaccuracies of tank fillup point are NOT cumulative (it's only the difference between the starting point measurement's "full" and the final measurement's "full" that introduces error), that possibly quarter-gallon difference, which really messes up a reading over 8 gallons of usage, has almost no effect on an average calculated over 800 gallons of usage. So track the gas you're putting in as accurately as you can - I keep three decimal places - because while the error is huge on a single tankful, it ends up being very small over the life of the vehicle.

I enter my logbook into a spreadsheet a few times a year and update the numbers. As long as you always record odometer reading and exact amount of gas, you'll be able to calculate mileage over any distance later. I actually make a set of entries in my log: odometer reading, date, location, type of driving, other noteworthy details if applicable, octane rating, amount pumped, price per gallon (rounded to the nearest penny), and EVIC mileage estimate. I used to calculate separate gas mileage readings for the different driving types, but I realized that if you start parsing out the fuel readings into different categories, the error *does* potentially become additive, and hence the accuracy of the readings dramatically decreases. I continue to record driving type for my own record though, as it can at least be indicative at the per-tank level.

In my professional life I work with social science data and statistics, does it show?

talindsay
02-15-2012, 05:33 PM
One quick clarification: it might be obvious that on a single fill-to-fill run you ignore the amount you put in at the first fill-up, only using the fuel added at the second fill-up to calculate. But equally true, and less obvious, is that you can NEVER add up the amount of gas it took to fill the tank the first time into the equation, and your distance equation will always leave out the miles until that first fill-up since you don't know anything about how much gas you were replacing on the first fill-up. It's a minor point since after 50,000 miles the couple of gallons' difference won't amount to a tenth of an MPG, but it's still worth noting - you can only use measurements where you know how much you put in and how far you'd traveled *before* you put it in.

So, if your first log entry is

54 miles, 2.300 gallons

And your next is
354 miles, 10.000 gallons

(see how I made that easy for myself?)

Then your equation would be 300/10.000, or 30 MPG.

Similarly, if when the odometer reads 30,054 miles and you've put in a total of 1002.300 gallons of gas since you bought the car, then your equation would be 30000/1000, NOT 30054/1002.300 because you have to exclude that initial fill-up, since you have no idea what its starting point was. It probably wouldn't make much difference at that point, of course.

mneuman916
02-15-2012, 09:05 PM
Using the fill up and divide by method I find EVIC to always be roughly 1 to 1.5 MPG higher than actual calculated.

PFVA63
02-15-2012, 09:05 PM
Hi,

Since I had to fill up my car today I double checked the EVIC vs how much gas I put in. According to the EVIC I got 38.1 mpg @ an average speed of 26 mph over 12hrs & 27 minutes. According to the fuel pump I put in 8.911 gallons.

Dividing 333.9 miles by 8.911 gals gives an average of about 37.5 mpg. So if I assume that the amount of fuel that I put in was equal to the amount that I burned the EVIC is off by about 0.6mpg.

However, if instead the EVIC reading is correct, then that would mean that the difference in the fuel that I burned and the amount of fuel that I just put in was about 0.15 gallons (or just under 19 ounces which is just a little smaller than this http://www.sun-rise.com/water/5_bottles_w_sizes.gif ).

Overall, since I didn't use the same gas station both times that I filled the car, and I believe that I filled at different times of the day, with different temperatures and such, and since I don't brim the tank (to avoid spillage) etc, it wouldn't at all surprise me if the amount that I put into the tank might be a bit different than what I burned, so I wouldn't be surprised if my actual mpg may be somewhere between the above numbers.

Pat

KhalilWilliams
02-16-2012, 04:07 PM
Thank you all very much!

I find this information to be very useful, especially since I would like to keep track of my gas mileage in the oncoming months. :)

Have Trumpet Will Travel
02-20-2012, 01:04 AM
As Pat (PFVA63), and talindsay have said, the more fill-ups / miles are tallied, the more accurate the (cumulative) figures become, as little as 0.1 m.p.g. My 2012 "Pop" 5-speed shows 43.5 m.p.g. per EVIC and "doing the math" bears that out. I use the "A" tally on the EVIC for individual fill-ups and the "B" for overall. Of course, the "B" record will reset itself every 99hrs. 59mins., so I'll be starting-over at around 4,500 miles total since new. My "A" trip-meter has shown as high as 47.2 m.p.g., but generally settles back to around 43.8 m.p.g., so the EVIC seems a little optimistic, once again by only a couple tenths of a mile-to-gallon. All in all a VERY fuel-efficient car, at least in a rural envireonment. H.T.W.T.






"

pxpaulx
05-31-2012, 10:45 AM
Easiest way to keep track of mileage for me is using fuelly.com (in my sig). I'm only 2 tanks into the Fiat, and just started using the site in January when we got our new 2012 subaru. Since I'm guessing you're young (1 month licensed as you stated...) I think you'll find fuelly ridiculously easy to keep updated - the site has a relatively unique ability to be updated via text message - you simply text your miles, price, and gallons (acronym is MPG making it easy to remember!) and after those 3 details you can also add notes such as average mph, city/highway mix details and anything else you can think of.

As has been suggested several times above, the most accurate gauge for MPG is the tried and true miles/gallons when you fill-up - it isn't completely accurate on a tank by tank basis, but the more tanks you track the more accurate the overall number becomes.

Wolfie
05-31-2012, 12:55 PM
Fuel consumption calculator in Excel:
http://office.microsoft.com/en-us/templates/fuel-consumption-calculator-TC030006314.aspx

Or: http://www.vertex42.com/ExcelTemplates/mileage-calculator.html

ewflyer
05-31-2012, 01:38 PM
I've simply kept every single gas station receipt from every fillup since I picked up the car at the dealership (with 4 miles on it). I thought I'd wait a few months or even a year then add up all the fillup gallon amounts and divide by the car's odometer total. This should give me a good picture of the car's fuel economy based on my normal driving habits.

Doodles
05-31-2012, 06:20 PM
A much simpler way is to start with an EMPTY
tank, not a full one.

When the yellow light comes on, signifying loW fuel, Put a set #of gallons in as soon as you can.
I always put in 5 gallons.
Reset trip mileage to 0
Drive until yellow/ low fuel light comes on again and there you go.
The number of miles driven divided by 5 ,equals your mileage.
I go exactly 150 miles on 5 gallons, equalling 30 mpg.

This is also cheaper, because you don't fill up,
And You don't have to wAit as long to find out your MPG.

panther76
06-01-2012, 12:57 AM
A much simpler way is to start with an EMPTY
tank, not a full one.

When the yellow light comes on, signifying loW fuel, Put a set #of gallons in as soon as you can.
I always put in 5 gallons.
Reset trip mileage to 0
Drive until yellow/ low fuel light comes on again and there you go.
The number of miles driven divided by 5 ,equals your mileage.
I go exactly 150 miles on 5 gallons, equalling 30 mpg.

This is also cheaper, because you don't fill up,
And You don't have to wAit as long to find out your MPG.

ive had the yellow light come on and needed 8.9 gallons to fill. ive had it come on and needed 9.6 gallons. im certain it wont always come on at exactly the same spot. the larger fill info done over several fill ups is the way to get as accurate as possible, imo.

pxpaulx
06-01-2012, 08:39 AM
This is also cheaper, because you don't fill up,
And You don't have to wAit as long to find out your MPG.

That is purely psychological, if you would like to tell yourself that the gas is cheaper because you only put 5 gallons in, be my guest!

Regardless of the volume of gas pumped, it will all get burned a gallon at a time.

Doodles
06-01-2012, 12:03 PM
ive had the yellow light come on and needed 8.9 gallons to fill. ive had it come on and needed 9.6 gallons. im certain it wont always come on at exactly the same spot. the larger fill info done over several fill ups is the way to get as accurate as possible, imo.

The perceived difference is because you have to note the mileage
as soon as the yellow light Comes on; if you driving 15 more miles , that's about a half gallon more ,there's your discrepancy. I didn't mean that gas is cheaper by only putting in 5 gallons , its just that you don't have to come up with 40.00 to fill tank.

My mileage calculated this way has been very consistent .

Doodles
06-01-2012, 12:19 PM
That is purely psychological, if you would like to tell yourself that the gas is cheaper because you only put 5 gallons in, be my guest!

Regardless of the volume of gas pumped, it will all get burned a gallon at a time.

I didn't say gas was cheaper ,just that 5gallons will cost you less than 10 gallons .

pxpaulx
06-01-2012, 02:00 PM
I didn't say gas was cheaper ,just that 5gallons will cost you less than 10 gallons .

I know I was just kidding around...I know, I've only posted twice so I'm supposed to keep it all straight faced and tight lipped!

I guess if all you've got is $20 to put in the tank that is fine, but if you put $20 twice a week rather than $40 once, that is two trips to the gas station, which is actually less economical from a consumption standpoint!

the only time I'm not filling it up is on the final trip...to trade it in (I promised my wife that will be a looooonnnnnggggg time away though!).

panther76
06-01-2012, 07:00 PM
The perceived difference is because you have to note the mileage
as soon as the yellow light Comes on; if you driving 15 more miles , that's about a half gallon more ,there's your discrepancy. .


nope. as a rule, i pull in for gas when the light comes on. ive never had to drive more than about a mile for this where i drive.

it happens because that light isnt always coming on at exactly the same point. is it relatively accurate? yes, but it is not perfectly linear.

i still say use up at least 3/4 of a tank, divide gallons needed into total miles, do it over several tanks, and try to use the same pump each time.

now, id expect the pump click off point to be no less consistent than the low fuel light, but at least it allows both a larger dividend and divisor.