Fuel Ecomony Myths

Discussion in 'Fuel Economy' started by Homeboymi, Apr 28, 2014.

  1. Homeboymi

    Homeboymi Member

    Read the top Fuel Economy Myths at MSN.com and wondered if you folks agree with this:

    Topping off the tank helps gas mileage

    Do you continue to add gas to your car even after the gas-station pump automatically shuts off, indicating that the tank is full? Many people think that by topping off their tanks they're getting as much fuel as possible into the car and thus can go just a bit farther between fill-ups. The reality is that after your tank is full and your gas nozzle shuts off, any additional gas is drawn into a gas station's vapor recovery system — and back into its storage tanks. And according to AAA, you could even damage your car's evaporative emissions system by topping off your tank.
  2. RedylC94

    RedylC94 Well-Known Member

    At stations that don't even have vapor recovery nozzles?
  3. Ophbalance

    Ophbalance Administrator Staff Member

    I'm not sure reality matches mine? Perhaps if you leave the nozzle embedded in the gas stalk, but untrue if you pull the nozzle out far enough that it's no longer going to catch any of the gas in the return. I've found that the Prius is highly irksome in that it cuts off at least a gallon (and typically it's more) early at pretty much any service station I've ever been to. That, to me, is an additional 60-100 miles of travel.

    Yep. It can. I've never had an issue across 3 different vehicles and 6 years of doing so, but it's within the realm of possibility. Two out of three have been within my control for the last 5 years, the third I sold.
  4. 08EscapeHybrid

    08EscapeHybrid Moderator

    I used to work at a service station, and after we had upgraded to vapor recovery the complaints started. One lady "pumped" almost 30 gallons into a compact car by "topping it off". It is entirely possible for the vapor recovery system to siphon off the extra fuel. Also, liquid fuel in the evaporative emissions system on the vehicle can cause damage to that system. I fill my vehicles, and when the pump clicks off, I'm done.
  5. RedylC94

    RedylC94 Well-Known Member

    I haven't seen one of those vapor-recovery nozzles in a while (Aren't they becoming obsolete?), but as I recall, it's theoretically impossible to pull the nozzle out that far and still get fuel into your car, if the recovery contraption is in good condition.
  6. fishnrib

    fishnrib Christian

    After the first click pull out the nozzle and slowly pump to get that extra 4 gallons of gas.

  7. ksstathead

    ksstathead Moderator

    Not fully understanding EVAP systems, nor how I would know if I damaged it, I am a second click filler. Sometimes the first click is too soon, but usually two clicks gives me a sense of more consistency.

    I am not one who prides himself on tank distance. I will grant that a few less fill ups gives a small benefit in mpg, but at the cost of more time per fill, and risking damage to EVAP.

    We are close to having an EV and a Prius for our fleet, so since I only fill one car and since it goes 500 miles per tank anyway, I am not missing too much fuel economy.
  8. Prozac

    Prozac Well-Known Member

    Considering that I am not trying to run for any records, I am a click and remove type of guy. I figure over the lifetime of fill-ups it will all average out.
  9. RedylC94

    RedylC94 Well-Known Member

    That won't work so well if the nozzle comes out of the restrictor opening before the rubbery vapor-sealing thing leaves the outer end of the filler. Of course, you can cheat by holding the sealing thing back, or otherwise preventing a good seal. (That's why I used the word "theoretically.")

    Some pumps can be made to pump slowly, but some can't, which causes the fill-to-click method to give inconsistent fill on some cars.
  10. some_other_dave

    some_other_dave Well-Known Member

    Not in my area. They're still in pretty much all of the stations that I can think of around here.

    That is exactly what is shown in the photo of the technique above.

    Oh, and I personally am a "second-click" guy. I don't feel it's worth my time and effort to fill up to the brim, because I am not testing radically different techniques or using partial fillups to judge my driving.

  11. rfruth

    rfruth Well-Known Member

    I'm a one-clicker (no use trying to top off a late model Ford, the excess gas just leaks out onto the ground unless you keep an eye on things)
  12. RedylC94

    RedylC94 Well-Known Member

    Betweens the extremes of absolute fill-it-to-the-brim and stopping at the first click-off, I think the best compromise is top off until you must fill very slowly, then stop. Any inconsistency of that method vs. to-the-brim is likely to be less than inconsistencies due to variations in the car's orientation on non-level fill areas.

    It would be good to know which models are vulnerable to flooding of the evaporative control system by fill-to-brim.
  13. Ophbalance

    Ophbalance Administrator Staff Member

    Then maybe I've never seen a nozzle with a proper recovery? None of the pumps between here (NC) and PA along I95 have had a boot like that. They're all pretty much just plastic housing and metal tube with no rubber boot.
  14. RedylC94

    RedylC94 Well-Known Member

    I don't remember the details (Someone else here might.), but generally such vapor-recovery systems were required in urban areas the EPA considered in danger of exceeding safe HC levels. I think we used to see them in Richmond. Not in Raleigh?
  15. 08EscapeHybrid

    08EscapeHybrid Moderator

    I know in the 80's everything had vapor systems on it. I know for a fact that my 1980 Pontiac, 1981 Buick, and my 86 Chevy all had them.
  16. RedylC94

    RedylC94 Well-Known Member

    That's correct about the 80's cars. Cars have had fuel vapor control systems since about 1970, more effective systems now than back then, of course. Before then, vapors (and sometimes unfortunately liquid gasoline with a full tank on a hot day from some vehicles) vented directly to the atmosphere.

    The recovery systems required in some areas on gasoline pumps are much more recent. There was a fight over whether the pumps or cars would be required to contain refueling fumes.
  17. tribosessive

    tribosessive Well-Known Member

    I am getting a P0441 code-EVAP purge valve stuck in the open position. This has got to be due to improper technique in topping off. I have learned my lesson well.
    Last edited: Apr 30, 2014
  18. some_other_dave

    some_other_dave Well-Known Member

    For imports, at least, the vapor recovery systems started in the late 60s or early 70s. I know that rules were different for imports and domestics in some ways, so domestics may have started later.

    They've been required in every gasoline-powered passenger car in the US for quite a few decades, at any rate. Current ones are much more sophisticated than older ones, which mostly had an air chamber which was plumbed to a charcoal canister which was plumbed to the engine air intake.

  19. RedylC94

    RedylC94 Well-Known Member

    My parents' '71 domestic Dodge had a recovery system with charcoal canister, etc. I'm not sure whether that was the first year they were required. My '72 Subaru somehow got by with a simpler, canister-free vapor vent to the air cleaner housing. That was at least a big improvement over some mid-60s vehicles. My parent's '66 F-100 was a particularly egregious offender (and still is; my brother has it now), with a nasty habit of dribbling gasoline out the filler cap if parked leaning toward the left in warm weather.
  20. Mountain Driver

    Mountain Driver Active Member

    Older Honda's like mine end up with excess in the evap system. Honda states this IS harmful.

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