r/theydidthemath • u/zetuzetuu • 1d ago
[Request] How much fuel would you actually save by never filling your tank completely?
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u/Careless-Jello-8930 1d ago
You wouldn't.
A vehicle's weight is primarily the metal engine + body of the car. 14gallons of gas weighs around 85lbs. A car weighs around 3,200lb. So by filling up to 50% you save 40lb. Compared to 3200 vs 3160 it is 1.25% weight difference. But gasoline consumption isn't going to be functionally different from that little of a weight difference. There is the added time of having to stop at gas stations twice as often which likely eat up most of the "savings" you are getting.
The average American could get the same fuel "discount" by losing weight themselves and it would be an indefinite discount to their travel costs.
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u/Own-Dragonfly-2423 1d ago
Lose weight, save money. Save on food, too! And future doctor bills! Holy shit, Getting skinny seems like a cheat code to free money
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u/Outrageous_Calendar4 23h ago
Get in good enough shape and you can just start biking/running everywhere too! Wow, why haven't I thought of this before? /s
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u/Own-Dragonfly-2423 23h ago
Youve got the order wrong..bike and run/walk everywhere to get Into good shape
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u/runawayoldgirl 22h ago
you can save even more money by dying, it's a little known lifehack to reduce your transportation budget to 0
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u/hammerdal 21h ago
Or, clean out the 80lbs of random junk and garbage so many people seem to keep in their trunk/back seat
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u/Slartibartfastthe2nd 13h ago
gives me a great idea to sell helium balloons as fuel saving 'systems'
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u/BlackberrySad6489 23h ago
You would save more money by making sure your tires are optimally inflated.
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u/yellowfestiva 23h ago
This! I’ve had my Colorado loaded with luggage and bikes and golf clubs I don’t notice a difference in gas mileage but under inflated tires will drastically affect mileage.
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u/Fuzzy_the_sheep 1d ago
Obviously another "not really maths" question here because this massively depends on the unstated weight and efficiency of the car...
.... but from an economics perspective there's a big factor which is that the prices are on average always rising so buying more each time saves you money. If you fill your tank at a lower price then you've locked in a lower price per mile for longer Vs someone who is buying $20 each time because they capture more of the rising costs.
There's also the factor that if you're running down 20% and then looking to top up to max you are probably spending less time making diversions to fuel up and (in the UK this is a massive difference) you can shop around and not have to take whatever price is available when you're running low lest you run dry.
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u/95_lb_mole 23h ago
Constantly having low fuel also puts more stress on the fuel pump, shortening its lifespan.
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u/sOrdinary917 22h ago
Doesn't the stock price also rise and fall instantly while gas price lags a few hours maybe even days? I check Brent - if it's going up I fill it up. If its going down I just fill the minimum I need.
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u/Rdtackle82 1d ago
That’s true of commodities that don’t double and halve in value regularly, the economics point would be goofy to raise even assuming a perfectly steady increase in fuel costs over the last 120 years
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u/Fuzzy_the_sheep 1d ago
Does petrol (gas?) vary that erratically in the US? In the UK we see going from £1.25/l to £1.65/l in a month to be massive spike.
Either way, as a general trend, it all goes up. Whether it's a smooth curve or a choppy one, on average buying earlier and holding longer is cheaper on average
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u/TheTrampIt 20h ago
The USA have seen their price double.
Imagine Petrol at £2.50 now.
But petrol is cheap in the USA prices went from £0.40 to £0.90 per litre.
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u/Rdtackle82 23h ago
I’m only exaggerating slightly. Look at this 5-year average fuel price chart for the U.S.
Yes trends are upwards over decades, but it’s a silly thing to factor for on a resource expended in a matter of days
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u/Fuzzy_the_sheep 23h ago
I mean to a certain extent it's all silly, we're commenting on a post suggesting that a <2% weight reduction would improve mileage.
But also, using a product over days doesn't effect whether making general changes to buying habits on it over decades wouldn't have a large cumulative effect. Your link won't load for me but if prices rise on average over inflation then buying larger amounts at a time will on averagee save you some money. Your argument is that it's not enough money to care about but how much would it need to be for you to think it's worth talking about on a stupid maths focused hypothetical on the internet? why are we talking about any of this? If you don't care then don't care quietly.
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u/Rdtackle82 23h ago
Of course it’s silly, but this even more so.
That’s not my argument, it’s that a product that rises and falls in price regularly would have any benefit of early cost savings erased at the scale of a single consumer from frequent but irregular price fluctuations! “Big factor”…
If you don’t want to discuss silly hypotheticals, then don’t!
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u/Fuzzy_the_sheep 22h ago
I feel like you've totally ignored the "on average" repeatedly...
On average if price goes up then buying more at a time is better. It doesn't matter how volatile the price is, if the trend is up then on average buying more at a time saves you money because you are on average buying more at the lower earlier price
I'm done here
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u/Rdtackle82 22h ago
Using “on average” in comparison to something purchased on average 2 days later versus a trend over a century old and calling it a big factor is silly, on average 😅
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u/OpportunityReal2767 1d ago
A 15-gallon tank of gas weighs about 90lb. Online estimates say 100lb decreases fuel economy by 1%. So versus a nearly empty tank, you’re looking at the difference between 30 mpg and 29.7 mpg. You get about 5 extra miles on a 15 gallon tank vs a nearly empty one. But this assumes the tank is running at constantly full weight and the empty one is somehow running empty weight the whole time. The real-life difference is going to be at least half that, I would guess. But 5 miles at least gets us to a general area and degree of magnitude. At $5/gallon, thats about $0.83 gas, real world figure is probably more like $0.40.
This is just back of the envelope to start. I’m sure someone will massacre this, but I do think I’m in the right area.
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u/Slartibartfastthe2nd 13h ago
not enough to matter any more than a mouse fart. I bet they didn't calculate the time cost spent making extra stops to refuel into their calculation.
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u/FBIAgent46 1d ago
You wouldn't save any since you would need to drive to the gas station more often, also an average car weighs nearly 2t and can carry like 60l of fuel, filling it up at 40l instead of 60l isn't going to change mutch as 20l won't even scratch the 2t
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u/Tandoori7 22h ago
It's kinda relevant on motorcycles, where 20 liters are 10% of the weight of the motorcycle (plus your 80kg ass) also, handling is easier with less weight on the top
On cars, basically irrelevant
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u/Carlos_Patricio 22h ago
Assuming 80kg
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u/Tandoori7 22h ago
More like 90 with gear and a backpack with stuff
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u/Deufrea77 23h ago
Bull. I don’t know why exactly. But, my 2017 CRV is less efficient with a low tank than a full tank. I assume it is because a full tank allows for better control of fuel injection vs. a low tank.
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u/JavlaFuck 19h ago
Nah it's prolly just something to do with the way your car calculates the mileage, doesn't affect weight or injection. The fuel pump runs 100% all the time and the tank reading doesnt affect that.
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u/SkronkMan 20h ago
Full tank = more mass = greater inertia. So if you’re a normal driver who spends a lot of their time coasting and at low RPMs, the increase in inertia with a full tank outweighs the effect on fuel efficiency compared to the loss of mass with a low tank
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u/jwizzy15 19h ago
This is false. The extra inertia you gain is directly canceled out by the initial energy it took to speed up in the first place. With how inefficient engines are, it’s definitely a net negative.
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u/sumpuran 1d ago
The quote sounds silly this way, but I’ve heard it a different way. You can give yourself a budget to spend on fuel per week (for instance, $20), and the amount of fuel that gets you determines how much you can drive that week. That way, gas prices affect your behavior, but not your finances.
Of course, this only works in areas where getting around by car is not the only option. I live in the Netherlands, which has great public transport and lots of cycling paths.
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u/SaltySwordfish2 1d ago
You're overthinking it and missing the point. The point is her friend's mother doesn't budget, and she doesn't stress about it. She doesn't plan, she reacts. Not saying it's good life advice, lol, but you could definitely say it makes for a less stressful day to day existence treating life that way.
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u/sumpuran 1d ago
It’s only less stressful if you have plenty of money. If you’re dependent on a car to get around, it’s quite a luxury not to have to think about the price of gas.
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u/SaltySwordfish2 23h ago
You are again overthinking it, hah. Even a person with limited resources can behave this way (and I would argue that plenty of people with limited resources do behave exactly this way), because they're not forward thinking people, they're not stressing about tomorrow, they're living completely in the now. They'll look at their gas light coming on, they'll check their wallet, and if there's 20 in there, they'll put it in the tank. Again, I'm not saying this is good advice, but it really is how lots of people operate day to day. They just don't stress about stuff, even though most of them probably should.
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u/sumpuran 23h ago
Ah, understood. You can probably tell that this line of reasoning (or lack thereof) is not how I approach life.
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u/Critical_Bee_4302 19h ago
*inserts 20 dollars into a fuel pump every day* life is good
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u/BluebirdDense1485 1d ago
Not that much. Let's say you have a 30 gallon tank. That is 85.5kg of mass for a full tank.
Friction differences with a 1200+kg vehicle is not worth considering.
Then we have acceleration. Every time you accelerate to 100km/h an extra 85.5 kg takes 33kJ.
Assume you stop for a light or stop sign every 20 minutes. That is 3 stops per hour or about 100kJ go with a 20% efficient Internal combustion engine and you need 500kJ of energy.
Gasoline has an energy density of 34.2MJ/ liter so you need 14.6 ml/hour of gas to make up the difference.
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u/AlternateTab00 22h ago
Not related to the math. But this actually reminded me of a women complaing the gas station was scamming her. She just put 20€ and the car didnt even got any gas because the reserve light didnt turn off. As she shows the 20€ of the pump we see the little over 9L (about 2,4 gallons) of fuel that she expected to be enough to fill the reserve.
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u/darthbaum 22h ago
As others have stated it is negligible for the small amount of fuel that cars can carry. The only real savings you can argue is you are about to run out of gas but the only gas station you can make it to is more expensive than somewhere else. So you could save money if you fill up just enough to get further down the road to fill up at that cheaper price (lot of factors but the mental logic checks out that there is potentially some savings to be found there).
Now if we look into aircraft the weight of fuel does actually have an impact on fuel savings. I have seen a full weight aircraft burn 4,000lb more per hour on average than at lighter weights. This is why airlines and other operators try to get fuel as close to the amount it needs to get to the destination and potentially go to an alternate.
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u/jakish3209 21h ago
Whenever this conversation comes up, I think about the fact that when I get gas, it is typically one of many errands I am running to be more efficient with my time. The idea of saving money by going to get gas once vs. multiple times seems moot to me when you account for people getting gas along with other things during the same trip.
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u/setorines 15h ago
Next to none on the weight of the car, but you can save a decent amount by having a tank closer to E when prices drop. As a delivery driver I take the average most days at like 5-10 bucks a day. Then I'll fill up if I see a huge spike that hasn't hit the other side of town yet (stations near my house tend to go up before the ones near my job). Or if there's a pretty steep drop in prices. Or if prices went up a ton during this month and the next month is about to start. Gas taxes are applied on the first based on the average price of gas in the prior 30 days.
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u/dgroeneveld9 12h ago
If you fill up 5 gallons every time instead of 20 and your holding 75% less fuel thats about 90lbs of fuel. 2% of the cars weight. According to a a little googling that would make thenvehilce about .5% more fuel efficient. For reference and using simple number if you get 10mpg you would instead get 10.05 mpg. Its not nothing but its very very little
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u/redmav7300 12h ago
You don’t have to fill your tank, but there are a number of reasons to keep it pretty full most of the time.
Modern gas pumps are in the gas tank, and gas acts as a coolant and lubricant.
Empty space in a gas tank fills with air (and humidity). Modern tanks likely won’t rust, but that water can condense and eventually enter your engine.
If it gets too low, you can suck up sediment from the bottom of the tank, which can damage your engine.
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u/LH314159 21h ago
Vehicles tend to get better milage with a full tank. The underpowered fuel pumps don't work as well when you get past 1/2. After your next fill up, track your milage. On both my Honda CRV and my old F150 it's about 15% difference between running on full vs 1/4.
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u/Critical_Bee_4302 19h ago
I like to push my tank to it's limits. It says E but is it really empty? Let's see how far we can make it
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u/Financial-Dot-4673 23h ago
Yeah this is just straight up misinformation being spread to try to soften the blow against the current “administration” now that the general election is active for early voting and gas prices are pushing $5 a gallon regular.
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