In another post I said that I usually have less than a half tank of gas in my car b/c I figure less gas = less weight = better gas mileage. Someone else however, felt that my logic was incorrect, but they did not go on to explain why. I've never tried to prove or disprove this theory, I just assumed it to be true. Does anyone really know if less gas = better gas mileage?

Note:
1 gallon of gasoline = 6.25 lbs (roughly)

My car has a 32 gallon gas tank, so with a full tank I'm hauling an extra 200 lbs around. Conversely, my car weighs 5400 lbs (give or take), so an extra 200 lbs only increases the weight by 3.7% (but I can still feel the difference).

Please negate the continuously fluctuating gas prices, I simple want to know if less gas = better gas mileage

3/2/2006 10:18:00 AM

wow, negiliable for you, and if your worred about gas milage dont drive a 5400lb vehicle...but seriously i wouldnt worry about it, its prob like .1 MPG change

3/2/2006 10:30:21 AM

What car weighs 5400#?

3/2/2006 10:53:36 AM

Yukon XL

3/2/2006 10:55:41 AM

if its the 5.3, then it makes no difference whether you are empty or hauling around 1000#, you'll get the same damn mileage. with a 4.8 you may see a very, very slight difference. if you are that worried about mileage, a high flow exhaust will pick up about 1.4 mpg on the 5.3, and lose 30# from the stock muffler' thats about the size of a 55 gallon drum, and a cold air intake, or even just a cotton/gauze filter in the stock box will give you another .6mpg. so for about $250-300 you can easily pick up 2 mpg on your yukon. at $3.00 a gallon, you'll save $.006 a mile with an increase from 12-14 mpg, so thats a 50,000 mile payback, for $300 spent on 2 mpg. is it really worth it?

[Edited on March 2, 2006 at 11:14 AM. Reason : ]

3/2/2006 11:13:28 AM

any full size car or truck these days is going to weigh a lot more than 2 tons

i have a compact truck (1988 mazda b2200) that is definately not heavy, and it weights in at 3000 lbs. longer/wider frame, bigger bodies, creature comforts...they all add up on larger vehicles


my take on less gas in the tank is this...when you keep your tank at half full, that means you have half the tank for gas to become vapors in. if you fill it up, you'll get better gas milage at the beginning because not as much gas can evaporate. keeping it at half full means that when you open your tank to fill up, you vent all those vapors out and leave fresh air for new vapors to form.

this is also probably flawed logic.

you could easily test though....run several full tanks and calculate gas milage. then run several half tanks and calculate gas milage. the only problem is calculating gas milage on half tanks because you can't be sure that you've put in the same amount that you've used. you'd have to do this over many many tanks to average out the miscalculations, but you could get an idea as to wether there was a huge difference or if it was negligable.

[Edited on March 2, 2006 at 11:14 AM. Reason : weigh, not way.]

3/2/2006 11:13:29 AM

i've always heard you get better gas mileage with the tank 3/4 full or more - but i've never cared enough to investigate because the inconvenience of worrying about it can't be worth it

3/2/2006 11:15:51 AM

^ for ease of calculation, in a 18" cube tank, ~25.2 gal, half full at 70degrees f, you would lose .762# of fuel in the airspace, assuming a sealed tank. empty, you would have 1.439# of fuel in the vapor. so you would lose .23 gallons at each fillup, so thats 75 cents every time you fill the tank from empty to full, 31 cents from empty to half, and 37 cents from half to full.

so you save 12 cents at every tankful by filling from empty to half twice, and almost nothing filling from half to full twice.

[Edited on March 2, 2006 at 11:21 AM. Reason : conclusion]

3/2/2006 11:19:48 AM

stop with all this mathematics bs

less weigh = more mileage

but why waste your time worrying about such stupid things

3/2/2006 11:33:40 AM

god you're gay

3/2/2006 12:39:20 PM

HAHAHAHAHAHA

so a guy with a Yukon XL is concerned with gas mileage?

Freaking hilarious

3/2/2006 1:10:05 PM

how about this:

cruising at 3100rpm versus cruising at 3400rpm for 2+ hr trips

negligible? (that's 70mph vs 75mph for me, if you wonder)

3/2/2006 1:18:12 PM

Quote :

"so a guy with a Yukon XL is concerned with gas mileage?"

I'm not as much concerned as I am just curious. I accepted the fact that I get shit gas mileage a long time ago, but at the same time every little bit helps. Wasn't it Benjamin Franklin that said, "a penny saved is a penny earned?" Besides, if what 69 said is right, than 12 cents per every full tank begins to add up (assuming gas prices stay constant). But saving gas isn't the only reason I usually have less than half a tank. I find it rather disheartening to watch the meter tick past $70 for a full tank of gas, so by only filling half a tank it lessens the blow. Thus, the point of this thread was to determine which half to fill.

3/2/2006 2:12:16 PM

You will put alot less wear on the pump with more gas in the tank. Those cost money to replace.

Benjiman Franklin was a communist fucker.

3/2/2006 2:33:36 PM

you will get a couple percent more gas mileage.

gradually (slowly) accelerate from a stop, put half a tank of gas in the car, check air filter, etc...

you will notice a difference.

just don't run the tank to empty and you won't overheat the fuel pump.

3/2/2006 2:38:49 PM

if you have to worry about 12 cents per fillup - then buy a bike or trade the yukon in

3/2/2006 2:42:24 PM

Quote :

"negiliable"

3/2/2006 2:46:17 PM

filling the tank half full means more trips to the gas station and therefore you will have to calculate the gas spent on these trips. That's not to say anything about the time that is wasted. If for you time=money, then you have to take the time you waste for the extra trips to the gas station into consideration also.

3/2/2006 5:10:06 PM

^thank you!

I never understood those idiots that would only put $5 in the tank (well unless that was all they could afford at the time

3/2/2006 6:50:07 PM

Quote :

"how about this: cruising at 3100rpm versus cruising at 3400rpm for 2+ hr trips negligible? (that's 70mph vs 75mph for me, if you wonder)"

I don't know if the rpm difference matters or not. But air resistance increases with the square of velocity, so at 70+ mph even a small speed increase will make the air resistance go way up. So by going 70 you'll be saving gas, esepecially for a 2 hour trip. Not sure how much gas, whether it will be worth it or not.

3/2/2006 7:31:50 PM

Quote :

"I never understood those idiots that would only put $5 in the tank (well unless that was all they could afford at the time "

Hell, I've put two bucks in before...back in the day, when I spent all of my money on alcohol and never thought to save any for anything else.

3/2/2006 7:44:06 PM

hey I just found some numbers in my thermo book

Quote :

"In highway driving, over 50 percent of the power produced by the engine is used to overcome aerodynamic drag. Aerodynamic drag and thus fuel consumption increase rapidly at speeds above 55mph. On average, a car uses about 15 percent more fuel at 65mph and 25 percent more fuel at 70mph than it does it 55mph."

There's also a chart that shows mileage verses speed, with mileage peaking at 55mph, with about 32mpg. The speed on the chart only goes up to 75mph, with the gas mileage there being 25mpg.

Also, fueleconomy.gov says

Quote :

"As a rule of thumb, you can assume that each 5 mph you drive over 60 mph is like paying an additional $0.21 per gallon for gas."

http://www.fueleconomy.gov/feg/driveHabits.shtml

3/2/2006 7:53:30 PM

it's good to fill your tank to prevent moisture build up

3/2/2006 8:11:37 PM

Better make sure to pee pee and poo poo before road trips to ensure you get the best mileage.

Oh, and you'll need to drive naked as well.

3/2/2006 9:23:19 PM

Quote :

"an extra 200 lbs only increases the weight by 3.7% (but I can still feel the difference)."

omg is that you dale?

OH MY GOD EVERYBODY LOOK NUMBER THREE IS REBORN!! ALL HAIL THE SECOND COMING OF THE INTIMIDATORrrrrrrrrrrrrrrrrrrrrrrrrrrrr






wtf

3/2/2006 9:41:05 PM

yea all those EPA highway estimates (aka the best possible mileage you are gonna get) are for doing 55mph. (or were, shit will hopefully be changed in the new standards).

[Edited on March 2, 2006 at 10:06 PM. Reason : .]

3/2/2006 10:05:46 PM

Quote :

"any full size car or truck these days is going to weigh a lot more than 2 tons"

Have they gotten a lot heavier in the last 10 years? My '96 Lincoln Mark VIII, while a coupe, is definitely a full-size car. Weighs around 3800. The parents' 97 Crown Vic is even bigger, and weighs ~3900... I know SUVs have gone way off the deep end as far as weight goes, but I still think you're going to be hard pressed to find a car that goes much over 4500. (Excluding things like the Maybach and Rolls Phantom...)

3/2/2006 10:34:24 PM

i love driving a car with a ~2500 lb curb weight

3/2/2006 10:39:22 PM

...this entire thread is stupid.

3/2/2006 10:41:28 PM

my GVWR is 4600.

3/2/2006 10:42:51 PM

^ you do realize that the GVWR is the gross vehicle weight...meaning the vehicle's weight plus the maximum allowed payload.

3/2/2006 10:45:56 PM

here is about this one: what kind of increase in mileage do you get when you wax your vehicle?

3/3/2006 11:51:33 AM

^actually, I glued sand all over mine and put a trip wire across the front of the hood to instill turbulence and minimize boundary layer thickness.

I hope it gives me about a 400% increase in mileage.

3/3/2006 11:55:17 AM

haha

so in other words, its dirty as hell, your too lazy to wash it, and some random wire is hanging out the front. that about right?

[Edited on March 3, 2006 at 12:12 PM. Reason : ]

3/3/2006 12:11:02 PM

^frightening how close to home you hit...

3/3/2006 12:13:05 PM

Quote :

"it's good to fill your tank to prevent moisture build up "

put some desiccant in it
--------

going faster nets me better gas mileage.

Quote :

"filling the tank half full means more trips to the gas station and therefore you will have to calculate the gas spent on these trips."

if your gas station is on the way to work like mine is, then it's really no problem at all. it's better really. plus almost everywhere i shop (wal-mart, costco, etc...) has a discount gas station, so the gas is CHEAPER and still convenient. but yeah, i realize that i'm an anomaly.

3/4/2006 3:32:18 PM

that sucks my car weighs like 2800 pounds and i get like 330 miles per 12 gallon tank

3/8/2006 1:18:14 PM

i wish there were a way i could set this thread on fire

3/8/2006 2:47:38 PM

i love that he says he can tell the difference in how his car feels when he has a full tank vs a half tank

like the 100 pounds really makes a difference

3/8/2006 2:48:06 PM

Never thought of a Yukon as a car.

3/8/2006 2:54:58 PM

100 pounds to an suv is like a fat guy drinking a glass of water....does it really matter

3/15/2006 8:17:34 AM

Quote :

"yea all those EPA highway estimates (aka the best possible mileage you are gonna get) are for doing 55mph. (or were, shit will hopefully be changed in the new standards)."

I've beaten my car's EPA highway estimates while averaging 75mph, and thats on a car with over 230k miles. Oh, and my final drive is 4.40:1 (or around there)

[Edited on March 15, 2006 at 1:42 PM. Reason : then again I have an aftermarket I/H/E and lighter than stock wheels.]

3/15/2006 1:41:26 PM

Quote :

"know SUVs have gone way off the deep end as far as weight goes, but I still think you're going to be hard pressed to find a car that goes much over 4500. (Excluding things like the Maybach and Rolls Phantom...) "

so fullsize cars are only 2 tons, but the trucks are over 2.5 tons:

GMC Yukon: Curb Weight 5273lbs.s
http://www.autoweb.com/content/research/vir/index.cfm/vehicle_number_int/1018829/Action/StandardFeatures

k1500: 2403kg (5297 lbs)
http://www.internetautoguide.com/car-specifications/09-int/1999/gmc/k1500/index.html

F150 supercrew with 5.4L: 5,110 lbs
http://www.fordf150.net/specs/02f150.php

3/15/2006 4:19:10 PM

damn you people make some stupid threads.

Quote :

"100 pounds to an suv is like a fat guy drinking a glass of water....does it really matter"

ahaha, gg.

Quote :

"I've put two bucks in before"

i've put less than a dollar in more than a few times with change scrounged around the car.

3/15/2006 4:34:25 PM