MPG vs gallons of fuel in tank.

To get the best mileage is it better to:

  1. Fill tank when it reaches half full mark. (Less fumes)

or

  1. 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.

Reply to
Bailey B
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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.

Reply to
Mortimer

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:
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Reply to
Mike Romain

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

Reply to
ratatouillerat

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

Reply to
ray

America could fix its fuel consumption problem simply by banning fat people from driving and riding in automobiles.

Reply to
.

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

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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.

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

Reply to
Ad absurdum per aspera

To get the best possible milage you should to the following:

  1. Combine trips where possible and drive as little as possible.
  2. Accelerate quickly and hold aa constant a speed as possible.
  3. Shift up as soon as possible.
  4. Don't speed.
  5. Use engine braking where safe to gradually slow down to stop signs and red lights.
  6. Look ahead and anticipate slowing traffic.
  7. Keep the car in good tune with tire properly inflated.
  8. Buy a car with high mpg ratings. . . . . . . .
  9. Worry about how much gasoline you should carry.
Reply to
John S.

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...)

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.
Reply to
Ad absurdum per aspera

Five years ago I noticed the increasing number of huge vehicles on the road. Being old enough to have lived through the shock of 1973 when OPEC turned off the taps to raise oil from $2 per barrel to $40, and seeing what happened to the owners of huge cars, and knowing that many large vehicles on the road means more demand, and that escalating demand usually means rising prices, we bought a small car. Good thing we did. I can understand the young buying big stuff but can't understand the short memories of folks my age. Not only are those big pigs hard on fuel, they depreciate enormously in times of high fuel prices. Think back to Chrysler's decline when they failed to start making smaller cars in the early '70s and persisted in building New Yorkers and Imperials, cars that got 10 mpg. It nearly broke them.

Dan

Reply to
Dan_Thomas_nospam

This should ruin your dinner:

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and for dessert, m'sieur, the
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is very sour tonight. If you're in a hurry, look at Fig. 1 of the former and page 10 of the latter.

Superimposing an oil price curve like this one

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is cute, though pretty obvious. In the mid 80s, instead of a third wave of the energy crisis, we got a long if lumpy trough in oil prices. That of course is also when the SUV craze (folded into "light trucks") in a lot of these discussions) took off, and in a more nebulous aspect and after a few years of delay, I also perceive that improvement in passenger cars started tending to be used to get greater size and power rather than in a continuation of the trend toward greater fuel economy. The result was a lost generation when it comes to improving the economy of the overall US passenger fleet, after ~15 years of notable improvement.

We are now at the far end of that period, and getting spanked by the Invisible Hand.

--Joe

Reply to
Ad absurdum per aspera

Oh, he knows, he just doesn't want to face reality. It is his fun car - he spent 2 grand last year on pretty much a new set of body panels (door skins, quarters, fenders, etc...)

The problem is he wants more MPG and more HP and that flat out aint gonna happen. He's "jealous" because my TA gets better mileage and is faster, but he hasn't spent nearly what I did buying my car... and his heads are from the 70's, his cam is from the 60's, he's got enough ground clearance and frontal area to challenge my truck, I've got 6 speeds and he has three. Also, the front end probably could be aligned etc... plus if he kept his damn foot off the go pedal and didn't have to light 'em up every light he might get better mileage.

Ray

Reply to
ray

I knew this day was coming. Consider that a 2008 Honda Accord has like

250 hp and doesn't even get 30mpg on the highway anymore...

I guess collectively people are pretty dumb. I don't complain about mileage... but I choose to drive high HP cars and know that's the cost of doing business. My truck is almost 20 years old, of course it drinks gas - but you can't put a fridge in the back of a Civic.

I laugh at people who complain about the mileage their SUV's don't get.

Ray

Reply to
ray

My 1978 Dodge long body van is thirty years old.318 engine, automatic transmission.I can (and have before) put fourteen feet long boards in my van and I could close the rear doors.I have no idea of how many miles per gallon it gets.Nowadays, I only average putting about 245 miles on the van each year. cuhulin

Reply to
cuhulin

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