MPG vs gallons of fuel in tank.

"Bailey B" <[email protected]> wrote in message
news:29506-4804A8FD-1034@storefull-3232.bay.webtv.net...
> To get the best mileage is it better to:
> 1. Fill tank when it reaches half full mark. (Less fumes)
>
> or
>
> 2. Fill the tank only half full for less weight? But more fumes.
>
> I guess its a weight of extra gasoline vs loss of fumes issue.

And is the difference noticeable compared with that between having extra
passengers?

My car has a 60 litre tank. Assume to a rough approximation that fuel (as
for water) has a density of 1 kg/litre. So the difference between a full
tank and an empty one is 60 kg - the weight of a small person. And that's as
a proportion of the total weight of the car which might be 1200 kg or more.

I've heard it suggested that car should not be run with less than about 1/4
tank of fuel in case dirt in the bottom of the tank gets into the injectors
or carburettor. Is there any truth in this or is it OK to run a car right
down to the red low-fuel warning and beyond? Subject to not running out of
fuel, especially in a diesel where it's sometimes hard to get the fuel
flowing again even when the tank is filled up.

On Tue, 15 Apr 2008 14:42:08 +0100, "Mortimer" <[email protected]> wrote:

>"Bailey B" <[email protected]> wrote in message
>news:29506-4804A8FD-1034@storefull-3232.bay.webtv.net...
>> To get the best mileage is it better to:
>> 1. Fill tank when it reaches half full mark. (Less fumes)
>>
>> or
>>
>> 2. Fill the tank only half full for less weight? But more fumes.
>>
>> I guess its a weight of extra gasoline vs loss of fumes issue.
>
>And is the difference noticeable compared with that between having extra
>passengers?
>
>My car has a 60 litre tank. Assume to a rough approximation that fuel (as
>for water) has a density of 1 kg/litre. So the difference between a full
>tank and an empty one is 60 kg - the weight of a small person. And that's as
>a proportion of the total weight of the car which might be 1200 kg or more.
>
>I've heard it suggested that car should not be run with less than about 1/4
>tank of fuel in case dirt in the bottom of the tank gets into the injectors
>or carburettor. Is there any truth in this or is it OK to run a car right
>down to the red low-fuel warning and beyond? Subject to not running out of
>fuel, especially in a diesel where it's sometimes hard to get the fuel
>flowing again even when the tank is filled up.
>
>

I don't think it's a dirt problem; the dirt and the pickup are on the
bottom anyway, full or empty. It would have to be floating dirt...

However, it's good advice. Injection systems require high fuel
pressures, so the fuel pump is usually in the tank, electrically
powered, pushing fuel. The pump relies on some amount of residual
fuel to provide cooling. Running tank dry will likely not lead to
immediate pump failure, but making a habit of it may lead to that.

$400 repair bill for dropping tank to replace pump...

I taught my kids to treat the 1/4 mark as time to refuel; both to
spare the pump and to avoid running out of fuel in wrong neighborhood.

Pete

> better mileage

If you were trying to "game" an MPG number in a road test, you'd want
to run as light as possible in every way. I think the US EPA doesn't
even bother controlling for this one, though, when testing cars for
their gas mileage -- probably because it is too small a concern
compared to the many other factor. There are a host of these -- just
some of them would include vehicle weight, aerodynamics, driving
cycle, cold vs. warm start, ambient conditions, engine efficiency,
driveline efficiency, and even the surprisingly varying composition of
the gasoline itself. (A car's "curb weight" assumes a full tank of
gas and might be an unmentioned additional condition for the start of
tests. )

Somewhere on http://www.epa.gov/fueleconomy/regulations.htm are lists
and discussion of the things they *are* concerned with.

Certainly the weight of the gasoline wouldn't be one of the big things
(and again, weight isn't the only thing). Say a 15 US gallon tank
holds a hundred pounds of gasoline -- a lot if you're draining it out
so as to drop the tank to replace a fuel pump (speak of the devil...)
but a thirtieth to a fortieth the curb weight of the typical American
passenger car.

I don't even see too much discussion of it among "hypermilers" though
there is some -- probably among those who are extreme enough to also,
say, leave the jack and the spare tire at home to shave a few tens of
pounds from the all-up weight of the car.

Another bit for any calculations you want to do -- the effect of
having to gas up more or less often, which requires not only stopping,
but broaching what is, to first order, a sealed system these days, and
exposing the vapors to the outside air at least briefly.

> I've heard it suggested that car should not be run with less than about 1/4
> tank of fuel in case dirt in the bottom of the tank gets into the injectors
> or carburettor. Is there any truth in this or is it OK to run a car right
> down to the red low-fuel warning and beyond?

There is often an amazing amount of ook down there, especially in an
older car. Newer cars in general, and newer ones with polymer rather
than metal tanks in particular, would be less vulnerable -- however,
they also tend to be fuel injected, so anything that *does* happen can
injure or kill a pump that costs a few hundreds of dollars and is a
pain to get at.

--Joe

Bailey B wrote:
> To get the best mileage is it better to:
> 1. Fill tank when it reaches half full mark. (Less fumes)
>
> or
>
> 2. Fill the tank only half full for less weight? But more fumes.
>
> I guess its a weight of extra gasoline vs loss of fumes issue.
>

I have an old Jeep and even back in 1986 it has an 'evaporation' system
on it to collect the fumes in activated charcoal for later re-burning so
nothing is 'lost' to the atmosphere.

All the new cars have this also. It 'has' to work, even on my old beast
to pass emissions. My gas cap even gets tested to make sure it doesn't
leak fumes.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
'New' frame in the works for '08. Some Canadian Bush Trip and Build
Photos: http://mikeromainjeeptrips.shutterfly.com

Bailey B wrote:
> To get the best mileage is it better to:
> 1. Fill tank when it reaches half full mark. (Less fumes)
>
> or
>
> 2. Fill the tank only half full for less weight? But more fumes.
>
> I guess its a weight of extra gasoline vs loss of fumes issue.
>

I can tell you this much - my Trans Am is faster at the drag strip by
about .05 with less gas in it. I've ran my fastest times with the
"check gauges" light on for low fuel.

In the real world, you'd probably get a teeny bit better gas mileage if
you drove your car almost empty - a gallon of gas is around 7 pounds, so
20 gallons is 140 pounds.

That said, most people I know seem to have 140 pounds of extra crap
travelling with them in their car and/or around their waist, so I don't
think it's worth worrying about.

Also, if you only fill it up to halfway, you're gonna have to stop more
often, and if you're driving on the highway, that probably means you'll
burn MORE gas stopping and looking for gas stations.

If you want to burn less gas, it's really really simple.

1. Drive less.
2. Drive slower.
3. Drive a smaller car.
4. With a smaller engine.
5. At the lowest engine speeds possible.

I laugh, because my buddy has a 72 Nova and he's wishing he could get
better than the 12-13mpg it gets right now. And he's also wishing it
would go faster. And he's too cheap to use premium. There's no free
lunch. My co-worker sold his Cougar for a pickup and complains the gas
mileage is poor. Duh.

Ray

> I can tell you this much - my Trans Am is faster at the drag strip by
> about .05 with less gas in it. I've ran my fastest times with the
> "check gauges" light on for low fuel.

Indeed; kinda like my comment that you might want with the minimum gas
if trying to "game" the gas mileage of a single run. (Your reason is
probably a lot more exciting for the spectators, though!) But
getting the best MPG per se (never mind other factors in prudent and
pleasant driving) on a long term average is different than making the
car as light as possible for a single trip's MPG or a pass at the
standing quarter mile.


Another poster mentioned that

|> I taught my kids to treat the 1/4 mark as time to refuel; both to
|> spare the [car's fuel] pump and to avoid running out of fuel in
|> wrong neighborhood.

Absolutely. The little red light, or even a needle in the orange
zone, doesn't mean "this is your normal hint to fill up soon" -- it
means "you have made a mistake in planning or attentiveness, but the
right actions taken promptly might still allow you to avoid the
consequences."

It's also good to know the accuracy and linearity and reproducibility
of the gas gauge indications. Simple concepts underneath those fancy
words, easily determined (knowing the tank capacity) just by
keeping an eye on how much gas is required in order to fill up from
various points. In some cars I've had, filling up, say, a 20 gallon
tank that reads 1/4 full takes 15 gallons, every time. In other
cars, the gauge really should be labeled "Amusement device only. Not
intended for gambling" -- either because an indicated eighth of a tank
translates into a couple thimblefuls of accessible gas, or because the
gauge (and/or its sender) has a different version of the truth at
different times.

The "wrong neighborhood" can also be the lack of a neighborhood -- and
anything else -- for miles and miles! Additional good bits of
knowledge to have in your head on a road trip are how far to the
previous and next towns; how much fuel you've got; how far that fuel
will get you (which can vary with seasonal gas formulation, weight in
the vehicle, and the outside environment); and what speed would get
the most out of it if you didn't like the answers to the earlier
questions.

(Not liking the answers suggests that you might uncover a 6 P's
problem* as you analyze the trip later on, but for now, the mistake is
in the past and the point is to figure out how to recoup...)



> I laugh, because my buddy has a 72 Nova and he's wishing he could get
> better than the 12-13 mpg it gets right now. And he's also wishing it
> would go faster.

If wishes were horses, then beggars would ride, and the guy with the
shovel would have fertilize'.

As you know and I know and most of the people here know, but he
doesn't seem to know, he might be able to play around the margins of
the Nova's equipment and state of tune and his driving style and milk
another couple mpg out of it, but the only way he's going to get big
improvements is to relegate it to weekend fun and pick up something
quite a bit different as his daily...

Cheers,
--Joe

* "Six P's: Proper Planning Prevents Piss-Poor Performance.
Favorite saying of a well organized friend.