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How much do I have *really* left to drive?

Discussion in 'Gen 3 Prius Fuel Economy' started by pakitt, Apr 2, 2012.

  1. pakitt

    pakitt Senior Member

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    "How much do I really have left to drive before really needing to find a gas station?"

    I am pretty sure you asked your question several times, on different cars. And I am asking that myself each time since fuel prices do change very often and very fast in Germany. I need to fuel up when the price is the lowest possible within the past week, otherwise I need to pay a delta of even up to 10€c/13$c...

    Moreover, we drive a car all geared up to high FE. Hence we check continuously how much we are consuming (especially those here on PC...) and we are checking the MFD continuously...

    But we all know that:
    1) the MFD is at least (for MY2010) 5% optimistic
    2) the distance-to-empty is way too conservative
    3) when you hear the beep, the car has well more than 2 gal/10L of gas left - at least on my Prius

    My experience is that when I hear the beep, I check the miles-to-fuel and I see 30km left. After 30km, I go to fuel up and I am putting in 35L/9gal only!....
    With the average fuel consumption of this car, 10L/2.65gal left in the tank means driving in the worst case at least another 200km/125mi @ 5L/100km-47MPG...

    Essentially I personally find the fuel gauge and distance-to-empty frustrating and useless. Coupled with the MFD average error, the overall "fuel reporting" of the Prius MFD leaves a lot to be desired in terms of accuracy. I don't use at all the distance-to-empty, for example...

    Therefore I made a "tool" to estimate more accurately how many kilometres I still have to drive before really needing to look for a gas station and start checking prices.

    Essentially it is a spreadsheet that I print on a half sheet of paper and I keep it in the upper glove compartment ready for use.
    It links together the MFD error, the km driven so far, the current MFD reported tank fuel consumption average, the litres I want to have as a reserve and how many km I want to drive with that reserve. The tool will indicate me how many km I really have left before hitting the reserve and start "consuming" my "range to the gas station".

    I have made a version for L/100km and MPGs both in .ods (that I cannot upload...) format and .xls for Excel.

    Cells B2 to B8 are to customise the table to one's personal needs, average distance to gas stations, lifetime MFD error (either personal or estimated, say 5% as per average reported by PC users), lifetime average fuel consumption to estimate reserve range (I use that, you could use whatever average mpg you feel confident with).
    Moreover you can set the "starting check distance" (I don't know what else to call it) and the granularity for cells A14 to A31. You can customise the MFD fuel consumption on row 12 based on your own personal experience (e.g. your min/max MPG over the lifetime of the vehicle).

    So how does it work? well, let's assume that you are happily driving and you get the dreaded "beep".
    If you were to follow the manual and the "distance to empty" notification you would be stranded "soon" (typically in 30-40km/15-20mi) - well the reality based on my experience is a bit different.

    If you have driven so far 690km/430mi, and the MFD reports 4.3L/100km-54MPG, you have another 177km/105mi before you hit a reserve of e.g. 4.9L/1.3gal, which at the average lifetime fuel consumption of 4.7L/100km-50mpg means 105km/65mi to reach a gas station.
    When I am in the yellow area, define by cell B4 value "Range to reserve", it means that I should start considering fuelling up. In town I can push this better, in country-wide trips I really have to start taking care on figuring out where the next gas station might be.

    So you have a total of 282km/170mi *at least* before you run dry.

    According to the Prius user's manual when you hear the beep, you have 6L/1.6gal left, or in the above case 130km/80mi. My experience tells me you have at least 10L/2.6gal when that happens (210km/130mi) and that does not take into account all the variables mentioned above. Quite a difference...

    You can set the table so that it has additional error buffers to avoid remaining really dry.

    *I assume no responsibility if you remain stranded in the middle on nowhere based on assumptions on range provided with this table*.


    I have used it for the past 6 months so far successfully and based on the filled up quantities, the estimations provided are accurate, at least in my case, for the data I have used. Now I can fuel up 40L or more, instead of 35L or less. And with the prices of the fuel we have here (around 7-8$/gal) it makes a difference if you are fuelling the right day. And 5L means at least for me 3-5 days of commuting in town while keeping track of fuel prices changes to spot a minimum.

    Please let me know what you think and if you spot any errors in the formulas (especially the MPG version) or elsewhere in the file.
    I can make an imperial gallon and km/L version if so desired.
     

    Attached Files:

    #1 pakitt, Apr 2, 2012
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  2. ny_rob

    ny_rob Senior Member

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    pakitt...

    You put a lot of effort into this post, so I'm almost embarrassed to give such a short reply...

    I've filled my tank twice so far since owning the car- both times have maybe driven 10-12 miles after the beep/pip started flashing. This last fillup showed "0" mi to empty, don't remember if it was at "0" on first fillup but it probably was. Both times it took 10.1 gal to fill the tank which means I had 1.8gal remaining at time of fillup. I'm getting around 50mpg overall, so in theory if I bled her dry I could go another 90mi after the MFD shows "0" mi to empty.

    From my research here on PC- it looks like my 1.8gal remaining after "0 to empty" shows in very common for the 2012 Prius.

    Good thing is- that once you learn your cars particular idiosyncrasy it's repeatable and easy to figure your "0 to empty" figure from there.

    In my case- 90mi reserve is very generous- my one way commute to work is 28 miles with easy to reach gas stations at either end of the commute.
     
    #2 ny_rob, Apr 2, 2012
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  3. pakitt

    pakitt Senior Member

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    Thanks! It seems then that the MY2012 has a more accurate distance to empty, since 1.8gal is more reasonable than my 10L/2.6gal when distance-to-empy show 0. When I get the beep, the distance-to-empty is about 20-30km/15-18mi...

    I wonder what would happen if I disconnected the 12V battery to reset the computer...maybe I would get more accuracy?...
     
    #3 pakitt, Apr 2, 2012
  4. ny_rob

    ny_rob Senior Member

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    .... well, there's certainly some funny business going on with the "miles to empty" warning in any case. As some have mentioned here it may indicate "6 miles to empty" when you pull in the driveway coming back from work- only to find it indicating "0 miles to empty" upon start-up the next morning. This has already happened to me once in just two low fuel situations. Others have posted that it goes from 20 miles left to 0 miles left very quickly or almost instantly in some cases.
    I think I'm just going to ignore the "miles to empty" indication in the future once I've zeroed in on exactly when the last pip starts flashing- at least that tells me how much fuel is actually in the tank.
     
    #4 ny_rob, Apr 2, 2012
  5. ghosteh

    ghosteh Member

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    I'm not at all suprised that a German would be irritated that something wasn't accurate enough! haha!

    Let us know if that works, because I've noticed the difference too. For a while, I thought I only had a 9 gallon fuel tank.
     
    #5 ghosteh, Apr 2, 2012
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  6. SoCalBPrius

    SoCalBPrius Active Member

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    From my experience & others I have read, 10 gallons or just under seems to be the norm that the computer indicates "0" miles left til we need a fillup. IMO, the prius is pretty consistent in telling us when it beeps or goes to 0 that it's going into reserve (gas) w/ almost 2 gallons left:).
     
    #6 SoCalBPrius, Apr 2, 2012
  7. Tideland Prius

    Tideland Prius Moderator of the North
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    Toyota typically keeps 10 litres as a reserve so most, if not all recent Toyotas (even as far back as the late 90s. I can't confirm for older Toyotas) will have 10 litres left when the low fuel light goes on.


    I usually just take note of the kms and L/100km rating when the light comes on. I use a conservative 5L/100km (47mpg) and assume 200km before I run dry (typical in the summer since 4.3L/100km or 55mpg is usually close to 4.5 or 4.6L/100km or about 51-52mpg). I've driven close to 130km on the low fuel light before and still have 4 litres left in summer driving.
     
    #7 Tideland Prius, Apr 3, 2012
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  8. ebenke

    ebenke Eco Member

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    My other car is an '07 Tundra. With 0 miles left on the display, I still have 6 gallons ! Even with just 14.3 mpg's, I can still drive almost another 70 miles. Or syphon it into the wife's Prius and drive for days !


    iPhone ? (sucks)
     
    #8 ebenke, Apr 3, 2012
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  9. walter Lee

    walter Lee Hypermiling Padawan

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    When I get a beep and the last bar start blinking, my 2010 Toyota Prius 3 will accept about 9.0 to 9.1 U.S. gallons of gas (first automatic click off when doing a fillup). When I hear the beep, there is about 32 miles left on the odometer range as well. It's pretty consistent so I am sure that I have about 2 gallons left at the beep. I've driven past the 0 miles range left several times but sofar the Prius has never run out of fuel entirely. My max range being 727 miles/10.125 gallons/ 70.9 mpg.

    Just refilled my tank... the beep occured at trip A odometer=594 miles but i didn't refill it until the trip A odometer=674 miles and I needed 10.587 gallons (63.66 mpg calculated) and the miles left was 0 miles ( the MFD=66.6 mpg). Unfortunately, a Prius mileage can vary greatly depending on how you drive it so how far that last gallon of gas will get me can vary greatly.
     
    #9 walter Lee, Apr 3, 2012
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  10. Sergio-PL

    Sergio-PL Member

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    Pakitt - lot of effort in this calculation.
    I was using (much simplier) excel file to track payments to my leasing company (I've paid for every km over the contract) and also had something like "max possible km's on this tank".

    According to fuel level indicator I preffer analogue style that I had in Alfa Romeo 159, where "beep" meant 7.5 l in the tank and arrow on the last bar, slowly going down, where 0 meant - engine dead :)

    Back to Toyota... I have experience with 2003 Yaris, 2008 & 2010 Auris (petrol / diesel resp.) and now 2011 Prius. All of them have digital gauges with more or less bars. Prius is different among them - last bar in Prius stays solid and start blinking where going empty. Yaris & Auris achieve the same level with 2 bars. Last bar starts blinking immediately with beep (no beep in Yaris - you can easly miss the point.....).
    No matter which Toyota we take as Tideland said if you go straight to the fuel station just after beep there will be about 10 litres in the tank.

    In Yaris (petrol) ~ 130 km (city)
    In Auris (petrol) ~ 105 km (city)
    In Auris (diesel) ~ 130 km (city), ~ 200 km (@110 kmh)

    All cars above send beep signal with distance to empty between 30 - 50 km. In Yaris it is repeatable for 2 years now, in Auris for 3 years.
    In Alfa since it was heavy diesel with > 10 l/100km in worst case city scenario (ie. DPF filter clean up, with consumption over 25l/100km for 15 minutes) I allways refilled earlier. But this beast had 70 l tank!

    In Prius I keep my tank stats on TRIP A. Trip B could be used to keep stats "after beep". But to be safe I'm assuming that in city (in normal, not below freezing conditions) Prius will eat ~ 5.5 l/100km (worst case scenario) and ~ 6.5 l/100km in HWY conditions.
    I don't want to dry it. I don't trust fuel stations operators. :cool:

    Pakitt as far as I remember in Germany you don't have to think where next fuel station is. I remember signs showing "next station 500m, after that next one in ie. 61km". We have (temporarly) part of A2 highway near Frankfurt (Oder) where no fuel stations are available for over 120 km without any notice before. It's not funny hear "beep" then. I did... and suddenly my speed dropped from 130 to 90 kmh... :) And what? I put 38 l to the tank @Orlen at the border...
     
    #10 Sergio-PL, Apr 3, 2012
  11. lambdacore

    lambdacore New Member

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    Great, that is exactly what I do.

    My ScanGauge tells me I have about 10 L left when the fuel indicator starts flashing and my fillups have confirmed that.

    I also use 5 L/100 km to estimate how long I can go on.

    So I check the ScanGauge, see about how many litres I have left and just multiply by 20.

    Everything went better than expected.
     
    #11 lambdacore, Apr 4, 2012
  12. pakitt

    pakitt Senior Member

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    In any case it is clear that the Owner's Manual information is widely incorrect. It states the beep comes when you have 6L/1.6gal (page 499 US manual, similarly in the German one), that is almost 50% less than what in reality happens!

    The reason for the table is essentially to avoid doing the calculations in my head, and bet updated as I approach the "reserve" how much I have still left. I don't want to remember each time when the beep came up or things like this.

    I use trip A for the tank and B for the current trip - and since I am profiling the home-work fuel consumption, this means B is daily used for this trip.
     
    #12 pakitt, Apr 5, 2012
  13. 8Nguy500

    8Nguy500 Junior Member

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    Thanks for the post very informative. I would support all you findings with my experiences with a 2012 Prius, 2017 Prius and my current 2019 car. Thanks again
     
    #13 8Nguy500, Aug 2, 2021
  14. ALS

    ALS Active Member

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    I have found in my 2010 that when the last PIP starts to flash I have around 2.5-2.8 gallons left. The car takes around 9.1-9.2 American gallons to fill her up. BTW the 2010 has a 11.9 U.S. Gallon tank. I never trust the computer in the car.
     
    #14 ALS, Aug 3, 2021
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