News

My 328 Fuel Log After One Year of Ownership

Discussion in '308/328' started by Mike328, May 19, 2005.

  1. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    2,655
    Boulder, CO
    Full Name:
    Mike
    #1 Mike328, May 19, 2005
    Last edited by a moderator: Sep 7, 2017
    Fellow F-Chatters,

    Today is the one-year anniversary of my first fill-up in the 328!

    I thought I would take a moment and post for the community my fuel log over the the past year.

    Most people see me do this and think that I'm ridiculously "detail oriented" (they use a different word), but to each their own, ya know? I like to do it because it is a tremendous way to monitor one health characteristic of the engine and car in general. Of course, there probably is no better way to document the mileage of a car. I've seen my share of shady-mileage 308s/328s. Besides, when I was a kid, I would watch my grandfather do the same for his Buick. Wisdom, I think you call it! :)

    Anyway, here it is.

    I've uploaded a spreadsheet, but here's a general analysis.

    This data is for 15 "fill ups" over the past calendar year.

    Also note that all fill-ups, except for the last three, occured in Albuquerque, NM, at elevation=5,000 feet. The last three occured in Boulder, CO at elevation=5,340 feet. In all cases, the octane gas used was the highest available, 91, which is intentionally "scaled back" due to the altitude. This is premium fuel and is "equivalent" to 93/94 octane (US rating method) at sea level.

    The spike in mileage in December corresponds to a 450-mile trip where I drove the 328 from Albuquerque to Boulder (greater Denver area)... Highway miles. Actual highway mileage I would estimate to be more, say 22mpg, because part of each of these tanks was consumed "in city".

    Other comments: reduced mileage in winter I attribute in part due to longer warm-up periods, and in general the tendancy for shorter trips (less long exploratory trips in the mountains, etc.).

    So... Here we go. In about one year of ownership:

    Total Miles Traveled: 3120 miles
    Total Fuel Cost: $418.54
    Total Fuel Consumed: 202.26 gallons
    Total Average Fuel Cost per Mile: 13.4 cents per mile
    Average cost per gallon of fuel: $2.08
    Total Average Mileage: 15.426 miles per gallon


    Now, what I've found is, my tripometer and odometer read approximately 10% high. This means that more miles accumulate on the car according to the odomoter than the car has actually travelled. This also means that our fuel figures above are optimistic in terms of mileage and cost per mile.

    Corrected for 10% mileage overread:

    (Actual) Total Miles Traveled: 2836 miles
    (Actual) Total Average Fuel Cost per Mile: 14.8 cents per mile
    (Actual) Total Average Mileage: 14.021 miles per gallon

    Microsoft Excel 2003 spreadsheet attached to this post.

    Enjoy!

    --Mike
    Image Unavailable, Please Login
     
  2. To remove this ad click here.

  3. Air_Cooled_Nut

    Air_Cooled_Nut Formula Junior

    Nov 25, 2004
    952
    Portland, Oregon
    Full Name:
    Toby Erkson
    You have inconsistent formulas. For your TOTAL FUEL CONSUMED you don't include the very first row, however, you do consider using that row for your other calculations and that's wrong. The formula should be =SUM(E7:E21)
     
  4. amslb182

    amslb182 Formula Junior

    Oct 3, 2004
    582
    Ct
    Full Name:
    Andrew LeGrant
    Wow you guys both have way to much time on your hands. :)
     
  5. Chiaro_Slag

    Chiaro_Slag F1 Veteran

    Oct 31, 2003
    7,731
    CA
    Full Name:
    Jerry
    Total Fuel cost is also missing a Fill up also. :)
     
  6. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    2,655
    Boulder, CO
    Full Name:
    Mike
    Guys,

    I spent a lot of time thinking about this. I know both formulas are missing a row. That is intentional.

    If you want to be accurate, you have to be very careful with the formulas.

    The idea is calculate the fuel cost based not on "replacement cost". It would be INACCURATE to use to entire fuel cost column--because the first fill-up was replacing fuel that I did NOT have mileage data for.

    So, if I filled up tomorrow, then I have a full tank of gas. That fuel costs what it cost TOMORROW. Two weeks later, when I fill up, only then will I know how much I travelled, that is, how much fuel I consumed (assuming I fill up every time). How much fuel of today's fill-up's fuel I consumed, at today's cost, I will know after two weeks from now.

    Similarly, the very last time I filled up, while I used the numbers for how much of the previous fill-up's fuel burned (i.e. how many gallons it took to fill up) for mileage and cost calculations, I did NOT use the total cost of the fuel for that fillup towards the total aggregate cost... because I'm still burning that fuel, I and I will to the next fill up!

    So this is a more accurate (though more difficult to follow) way of calculating the fuel cost--what I burn today gets counted based on the cost of the fuel when I filled up. The easier (and less accurate) would be to calculate it based on "replacement" cost, but I still would use every fill-up total cost figure.

    Following me at all? For something so trivial, yeah, it requires a bit of wrangling to get your mind around it. And I could still be wrong!
     
  7. To remove this ad click here.

  8. Chiaro_Slag

    Chiaro_Slag F1 Veteran

    Oct 31, 2003
    7,731
    CA
    Full Name:
    Jerry
    Ahh, I get it. :)

    I wonder how this compares to a TR??
     
  9. Fezzaphil

    Fezzaphil Formula Junior

    Sep 14, 2004
    368
    UK
    Full Name:
    Philip
    what ever the MPG thank your lucky stars your not in the UK paying our petrol prices!
     
  10. phild328

    phild328 Rookie

    Aug 2, 2004
    34
    Lebanon, PA
    How did you figure out your odometer is 10% over? Anything I would compare it to I would always worry about a margin of error in the thing you are comparing it to.
    I bring it up because my 328 seems to be 10% over. Well, I actually have no idea how much over it is, but it sure doesn't seem like I drove that far. Maybe its just so much fun to drive the miles just pass by... But really, driving point A to B with another car - it generally reads a higher number than the other car.
    So I was wondering if 328's came from factory 10% with the factory spec tires. Maybe to compensate for people who would use larger tires. Or maybe when the odometer was spec'd for the car, ferrari was planning on a larger tire/wheel and never went that way.
    If so, I don't like it.
    Say you go to sell you car and it shows 50K on it, but really it is closer to 45K. 10% can make a difference in value, maybe by 10%, which on a $30K to $40k car can be $3k.
    So because the odometer is off, you are out another chunk of change for your ferrari.
    Anybody know what the story is? Are they(328s) all high?
     
  11. Air_Cooled_Nut

    Air_Cooled_Nut Formula Junior

    Nov 25, 2004
    952
    Portland, Oregon
    Full Name:
    Toby Erkson
    I agree w/you on not including the last row for "Total Fuel Cost" which is why I didn't mention disagreement with it. Okay, I understand what you're view point but with that said then cells A7 thru F7 should be deleted.

    You do need to be careful how you do your calculations. For example, the "Average cost per gallon of fuel" should be computed from the D column (=AVERAGE(D7 : D21)) and not by the formula you used as the final result can be affected by rounding errors. Yes, with these small numbers it's not much (expand the number of decimal points out to 5) but with more numbers it can be significant. Also, with this particular example, you already have the cost of fuel per gallon ;)

    Your "Distance (mi)" shows accuracy to one decimal point but your "Mileage at time of Fillup" only shows whole numbers...inconsistency and again greater computation inaccuracies depending where the forumulas pull their data. There are other data points where decimal accuracy is off but it's not a big deal here, just remember too many errors can accumulate and give inaccurate results.

    My college computer advisor told us a story about the time he worked at a Las Vegas casino in the accounting department. One of the computer accountants was taking the rounded $0.001 cent from the calculations and depositing it in a "special" account of his. He was making several hundred dollars a day! Then a routine IRS audit came and asked too many questions one day and I'm sure you can figure out what happened to that guy!
     
  12. To remove this ad click here.

  13. Air_Cooled_Nut

    Air_Cooled_Nut Formula Junior

    Nov 25, 2004
    952
    Portland, Oregon
    Full Name:
    Toby Erkson
    I know along Interstate 5 there are odometer check sections. Also, the wrong sized tires can cause odometer offset. BTW, speedo/odo offset is not a linear error which is why it's [correctly] stated as a percentage. The faster you go the greater the error!
     
  14. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    2,655
    Boulder, CO
    Full Name:
    Mike
    Thanks for your thoughts! Responses in line--

    Glad you're on the same page with me re: the spreadsheet logic. Actually though, I can't delete A7 through F7. The Cost-per-gallon figure (column D) for this entry is used in subsequent calculations--after all, in the weeks that followed this fill-up, I'm burning fuel that cost the amount per gallon as specified by the figure in THIS row. So that data point is important, and hence the other figures around it, so no, I can't delete A7 through F7!


    What you say above is one way to do it, but you basically want to take the "unweighted" or straight average. This doesn't take into account how much gas you actually USED. In the extreme, if there were only two fillups, one fillup was 2 gallons and one was 20 gallons, the 2-gallon fillup was at $1.50 per gallon in a highway town, and the 20-gallon fillup was at $2.80 a gallon in L.A., your method would (misleadingly) calculate the "Average cost per gallon of fuel" just by taking the average of $1.50 and $2.80. This is the unweighted average, and gives you $2.15.

    That's not the figure we want. The better and more accurate way is to use the actual amount of fuel for each fillup as a weight, and calculate the weighted average. In the above example, this would be like (2gal * $1.50) + (20gal * 2.80) / 22 gal, which gives you the figure $2.61. This latter value is far more representative of the actual "average" fuel cost, right? You used more of the expensive stuff than you did of the cheap stuff. So the weighted average is how you take that into account. The formula I used in the spreadsheet is precisely this weighted average... not the straight average. I admit, though, that unless your looking for it and get your "head" around the whole thing, it can be tricky to understand exactly what it means.


    The weighted average formula over the straight average formula has no bearing on rounding errors at this precision (two/three decimal places). You'll note that excel uses IEEE 754 standard for floating point representation, which, as you point out, means therefore it is susceptible to rounding errors like all finite-representation floating point computing machines. It is not an issue here... but point well made and well taken.

    No, not inconsistency--that was the precision of the data source I was dealing with. The trip odometer in Ferrari 308s and 328s has a precision of one decimal place, with three total significant figures. The standard odometer has five significant figures, and no decimal places. I was true to the data and put it in exactly and as accurately as it was displayed. I made no attempts to "interpolate" to the hundredths place, for example, by looking at where the trip odometer's decimal point was on its "spin".


    I was very careful to round where appropriate and obeyed all significant figures. Nowhere in my results are decimal places beyond three, or total significant figures beyond five. I will remember your above point though.


    SO true! Roundoff error is the real deal, and if you work with floating point numbers to any great decision and over some number of operations, you better keep an eye out for it!

    (Nerd note: For any who are interested in implications of the finite floating point representation of computing machines, I recommend the early chapters of this text: D. Kincaid and E. W. Cheney. Numerical Analysis: Mathematics of Scientific Computing, Third Edition. Brooks/Cole, Pacific Grove, CA, 2002.)


    Thanks for your thoughts, guys!

    And just remember, the big idea here was just to give back something to the community, give an idea of fuel operating costs for these cars as well as an accurate number for fuel economy from real-world experience. Hopefully someone out there found this useful :).


    --Mike
     
  15. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    2,655
    Boulder, CO
    Full Name:
    Mike
    When I first got the car and noticed it, I would do checks along a five mile stretch of road with mile markers, and came up with about 10% high. This is obviously a very rough figure.

    I also noticed that friends' trip odometers would read higher than mine for the same trip.

    And, my speedometer reads high, even with the sending unit replaced! But, it's not a fixed amount--it seems to be a percentage. I.e. when it shows 40, I'm doing 35, when it shows 70, I'm doing 63. (I confirmed the 40/35 figures with a road "speed advisory" radar the other day, but haven't confirmed the 70/63 figure). My subjective feeling is that when the speedo shows 60, I'm actually doing 50--which is more than 10% obviously.

    As far as I know, I have stock size tires--will double check later.

    That said, I seem to recall that Rob Lay got a ticket in the 328 when he assumed the speedo was reading 10mph over (indicated 85 if I recall).
     
  16. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    2,655
    Boulder, CO
    Full Name:
    Mike
    One quick note about my previous response to Air Cooled Nut - the Ferrari 308/328 trip odometer has four digits, three before the decimal, one after, for a total of four significant figures (not three, like I said earlier).
     
  17. don_xvi

    don_xvi F1 Rookie

    Nov 1, 2003
    2,920
    Outside Detroit
    Full Name:
    Don the 16th
    Talk about revenge of the nerds!

    I keep a similar spreadsheet for my other car but heck, I just plug in the numbers and compute away. That's the difference between a mechanical engineer and a computer scientist, I suppose!
     
  18. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    2,655
    Boulder, CO
    Full Name:
    Mike
    LOL! Dude, I'm a total nerd :). But chicks dig nerds these days! ;) ;) ;)
     
  19. Air_Cooled_Nut

    Air_Cooled_Nut Formula Junior

    Nov 25, 2004
    952
    Portland, Oregon
    Full Name:
    Toby Erkson
    Thanks for the explainations, Mike, I understand where you're coming from now. Also, I didn't realize you were using the standard odometer for the "mileage at fill up" and the trip odometer for the distance traveled, my bad.

    I still see one big problem with your results...you don't drive her enough! :D
     

Share This Page