[Josh49]

Originally Posted by ages944

About performance / efficiency:

First of all, gasoline has a density of ~702kg/m^3. Converted over, this comes out to 5.86lbs / gallon.
Our E90s have a 60L gas tank. In US gallons, this is 15.85gal.

Therefore, a full tank of gas should weigh right around (15.85 * 5.86) = 92.88lbs
Half a tank of gas weighs 46.44lbs.

The difference in gasoline weight is really not all that much if you think about it, especially when compared to the total amount of weight that the engine has to move.
Our E90s (325i auto for this example) weigh 3351lbs empty.

Compared, the cars then weigh 3443.88lbs fully tanked versus 3397.44 with a half tank.

a quick calculation of the power to weight ratio shows us that having a half tank of gas in the car should yield a 1.35% better power to weight ratio.
Full: 215hp / 3443.88lb = 0.062429 hp / lb
Half: 215hp / 3397.44lb = 0.063282 hp / lb

0.063282-0.062429 = 0.000853 hp / lb
(0.000853 / 0.063282) * 100 = 1.35%

-or-

(45.44lb / 3443.88) * 100 = 1.35%

by comparison, the 330i auto has a 13.2% better power to weight ratio. 1.35% isn't something to get too excited about.


... I know, I'm a geek...

If I wasn't tired of writing already, I'd point out that the Toyota Corolla (a car which has nearly the same drag coefficient as our cars) gets 9mpg better on the highway.
It's not because it's engine is light years better than ours or anything (nowhere close), but that the car weighs 700 lbs less. So, how much of an effect will 46.44lbs have?

Nice explanation. Exactly what I said just your explanation "weighs" a lot more!!

[arcrox]

Thanks for crunching the numbers. I've wondered as well about the merits of tank filling, but not enough to break out Excel.

Don't the quoted weight numbers from BMW include a certain amount of gas? I think I recall reading the quoted numbers take into account half a tank of gas.

[ages944]

Originally Posted by arcrox

Don't the quoted weight numbers from BMW include a certain amount of gas? I think I recall reading the quoted numbers take into account half a tank of gas.

yeah, I thought about that too. However, even if they did, the ratios would still work out the same.

Originally Posted by arcrox

Thanks for crunching the numbers. I've wondered as well about the merits of tank filling, but not enough to break out Excel.

lol, yeah. but I didn't actually use excel. Just calculator. I'm such a masochist, heh heh

[Hunter399]

I only put 20 dollars in each time because with a full tank the car feels much heavier and slower

[dgoodson]

i always top it off... too much of a hassle to keep goin to the station to save 2-3 bucks a tank. heck, i usually drive till the computer says i only have 5-10 miles left...

[e90rocks]

Originally Posted by hunter399

I only put 20 dollars in each time because with a full tank the car feels much heavier and slower

I can't imagine that one could really "feel" a 46 lb difference in a 3400 lb car. That being said, if the 46 lbs were strategically placed (eccentrically) then I suppose it could change maneuverability characteristics slightly.

I suppose I should fill up partial tanks until my getgo card (groceries adding up to a single discounted fill up) is ready for use...but I think my time is more valuable than the few bucks that could be saved. (That excuse works for cutting the grass, too....).

If I really wanted to save on gas, I suppose I'd gently ease onto the highway with smooth acceleration, and never open my sunroof. Now where's the fun in that?

[E91 Dude]

Originally Posted by dgoodson

heck, i usually drive till the computer says i only have 5-10 miles left...

I freak out even when the low fuel light comes on. 5-10 miles left is pretty brave. Not only because you might run out of fuel, but the owner's manual says, "Refuel as soon as possible once your cruising range falls below 30 miles/50 km, as otherwise engine functions are not ensured and damage can occur." I wonder what kind of damage can happen if you repeatedly run low?

[DanielleS]

Originally Posted by roppongicorolla

The specific gravity of gasoline is between 0.72 to 0.77 (Diesel slightly heavier). That means 10 gallons of gas weighs about 60 to 64 lbs. That extra weight is more noticeable with a lighter car. If you compare against someone with a near empty tank, the difference is the equivalent of a small girlfriend.

That made me laugh!

[JamieA]

Originally Posted by David325Australia

Run it down to fuel reserve then fill it to the top with Shell Optimax 98RON for the best drive. Beautiful!!!

I don't know David, since Shell shacked up with Coles Supermarkets, I don't think their fuel is as good as it was, Mobil has gone to the dogs, so I am a BP Ultimte 98 man now...

What is your (or any other Aussie's) experience?

[firstkill]

I usually only do a partial fill up because I use the touchless wash often.

The 8 gallon mark gives me the discount. So I just stop at around 8 gallons. This way i get my car washed cheaper and more often.



~fk

[chonko]

I always fill it up. It's the only way I can monitor my driving habits- I always target exceeding 400miles on a full tank combined driving.

[NaTuReB0Y]

After it's filled up..........I squeezed another $1.00 into it.....sometimes it overfilled!

[strykr]

It all depends on what car I plan to drive that week. If I know i'll be driving the car for the whole week, i'll fill her up. If not i'll put like 20-25 dollars. I have a civic or is300 as my commuter car.

[Flyingfart]

I just put 5 dollars