Science Questions

How much petrol should I put in my car?

Sat, 21st May 2011

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Paul Young asked:

Is there an optimum level to which you should fill your car's petrol tank so it doesn't just use the excess energy simply to carry the petrol around? Do vehicle manufacturers make the capacity of the petrol tank the perfect size with this in mind?


Thanks for the great show




We put this to James Painter, from Bloodhound SSC...

James -   Hello, I'm James Painter.  I'm one of the engineering leads on the Bloodhound land speed record project.  We’re at the moment designing and developing a vehicle to set a new land speed record and we’re targeting 1,000 miles an hour.  

Petrol pumps in a petrol station in Switzerland.Well that's an interesting question.  I mean really, once you're carrying around more fuel than you actually need to get from A to B or to your next petrol station, you're carrying around more fuel, essentially, than you need.  So, I suppose essentially non optimum, but certainly, it does depend on how far you want to go, how quickly you want to get there and how much you enjoy sweating when the low fuel light comes on.

Certainly in terms of vehicles and automotive manufacturers are a lot of family size fuel tanks tend to 40 to 60 litres and really, that's driven by giving vehicles a reasonable range.  And also, in terms of the packaging space that the manufacturer has got to play with in terms of getting the fuel tank located.  So when you are carrying around this additional fuel, certainly starting and stopping, accelerating and breaking that fuel is a bit more of a penalty within town driving, rather than when you're on the way to way and you're actually doing at a constant speed on the motor way.

Diana -   So it’s best to only fill up what you need but this could lead to hundreds of visits to the petrol station which might be out of the way and therefore add extra mileage.


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if warm humid air finds its way into the colder petrol, condensation can occur= water in the pet roll.
emptier tank with splashing pet roll aireates the pet roll? air is mostly N with some O & water
My 88 chevy s-10 runs better when its raining out CZARCAR, Mon, 16th May 2011

Gasoline weighs about 750g per liter, so carrying 80 liters of petrol has about the same effect as hauling around an additional 60kg person.  If you spend most of your driving time on the motorway with properly inflated tyres, then this shouldn't affect your mileage in a noticeable way -- your car's efficiency is set mostly by drag, which isn't greatly affected in that case by how much weight is in the car.  If you spend most of your driving time in town, you will spend some extra petrol accelerating the petrol in the tank -- but if you have a very small, light car that (including you) has a mass of ~600kg and a 40 liter petrol tank, you are looking at (at most) a 5% effect on mileage when your tank is full compared to when it is empty.

drzowie, Tue, 17th May 2011

Tanks tend to be sized so that a person can get about 200 to 400 miles per tankful.  My little Fiat has a 5 gallon tank. 

While you can  calculate the maximum weight of the tank, it is probably best to calculate the average amount of gas.  So, if you are one that drives from Full to Empty, then you would average about half a tank.  If you drove from half to one quarter tank, then you would average 3/8 tank.

How much extra driving does it take for the average fill-up?  If you are driving on a freeway, it is probably an extra mile, and a couple of extra start, stops, and accelerations.  In town, do you go to the nearest station?

Around here, the nearest E85/Biodiesel station is several miles out of the way, and they shut down in the early evening.  I try to stop whenever I am in the area, but that is not always possible.

Anyway, while there may be some effect on the fuel efficiency.  It might be best to only keep 1 gallon in the tank.  While that might be ok for the Fiat, it isn't too practical for many larger vehicles that might take a gallon or so for a round-trip from town to home and back to a filling station.  And it isn't worth it to me to go to the gas station on every trip to town.

One of the other issues is that there is often a problem with the low-end accuracy of many fuel gauges.  So, in most cases it is difficult to judge from the last gallon down to the last pint of fuel. CliffordK, Wed, 18th May 2011

Properly inflated tyres will do more for fuel consumption, along with driving as if there is a raw egg between your foot and the pedal. Driving and anticipating conditions ahead to minimise harsh acceleration or braking will do more than what having a variable amount of fuel in the tank.

That being said, I generally drive to between the 1/4 and 1/8 points before refilling, as then I know I have around 10l left in the tank at least, and will be able to do around 100km if needed. SeanB, Wed, 18th May 2011

wont an overinflated tire give better gas mileage? at the expense of uneven wear CZARCAR, Thu, 19th May 2011

That's a good question. Even if it did improve the mileage, it's probably not a good idea from a safety perspective, and it may end up costing more than the gas saving to replace the tires, although that would be quite difficult to determine.

I see Michelin is currently promoting tires that are supposed to significantly reduce fuel consumption. The only problem is that Michelins tend to be a bit more expensive than the competition, so I'm not sure the economics really work. Geezer, Thu, 19th May 2011

It all depends on the fuel efficiency of the vehicle to start out with, and the actual improvement.

Say you have 50,000 mile tires on a vehicle that gets about 20 mpg.

Then over the lifetime of the tires, you use:

50,000 miles / 20 mpg =  2,500 gallons of fuel.

If you get a 10% efficiency increase, that would save you about 250 gallons of fuel.

At $4 / gallon.

One ends up with about a $1000 savings over the life of the tires. CliffordK, Thu, 19th May 2011

That's pretty good. Are they claiming that sort of improvement? Geezer, Fri, 20th May 2011

Basically they reduce rolling resistance without significantly affecting the tire's grip when braking.  The problem is that according to consumer reports, rolling resistance is a relatively small factor in fuel costs, and clearly these tires reduce it, but don't eliminate it. 

So it sounds like you're saving a fraction of between 4%-7% of your fuel costs, so maybe on the order of $300 over the life of the tires? jpetruccelli, Fri, 20th May 2011

So it sounds like you're saving a fraction of between 4%-7% of your fuel costs, so maybe on the order of $300 over the life of the tires?
Is that 4-7% under theoretical ideal conditions?  or a difference between typical standard tires and low rolling resistance tires?

Wikipedia lists:
Theoretical: 5-15%
Actual savings 1.5-4.5%

So, the savings may pay for the difference in price of the tires over the entire lifetime of the tires.
But, probably won't save a lot of money.

Have we decided whether it is worth it to put in $5 worth of gas every day? CliffordK, Fri, 20th May 2011

It seems like a reasonable idea not to schlep any more gas around with you than necessary. It's probably more a question of how much we are willing to pay for the convenience of not having to fill up too often, and the cost of getting to the gas station of course.

They used to say that it was a bad idea to run around with an almost empty tank all the time because it encouraged the tank to rust, and that can lead to some really nasty problems. However, I don't think rusting is a problem these days. Geezer, Fri, 20th May 2011

I've seen some plastic fuel tanks.
However, I believe the tanks on my vehicles are all steel.

I believe that they don't coat the insides of the tanks with zinc.

And...  the alcohol that many municipalities require can promote rust. 

At least it is best to be somewhat uniform with how deep you typically drain the tank. CliffordK, Sat, 21st May 2011

I would think they are at least zinc passivated, although I've never ripped one apart to find out! Geezer, Sat, 21st May 2011

There are discussions about biodiesel and vegetable oil being able to leach out zinc from anything that is galvanized.  I'm not quite sure why an oil would be able to do that.  Perhaps they aren't 100% neutral, especially if not a commercial product.

Of course, that doesn't stop manufacturers that don't really support alternative fuels assuming it isn't a problem with the stock fuels.

Even so, the way that galvanizing works is essentially the idea of a "sacrificial anode", thus to be effective, it slowly dissolves. CliffordK, Sat, 21st May 2011

Zinc passivation and galvanizing are very different processes, but is the end result the same?
Geezer, Sat, 21st May 2011

re: Michelin tires saving fuel
When I was younger, I worked in a Michelin tire store in Calgary, Canada. We installed all brands of tire there and I learned that Michelins were overall better made than the competitors in that Michelins need less lead weight to balance the wheel, were truer (less out of round) than most other brands, and lasted longer than any other tire. Commercial users (typically trucks) would find the higher cost of Michelin tires cost less as they had fewer problems and lasted longer when used in high-mileage situations.
As for over-inflating tires to improve performance, bear in mind that the optimal inflation pressure will put the most tread onto the pavement. That is, adding much more air than recommended may cause the tread to bulge in the middle and pull the shoulders of the tread away from the road. While over inflation is slightly bad, under inflation is very bad for the life and performance of a tire. Why? First underinflating a tire makes it run more hot, leading to the tire disintegrating and failing. Second, under inflation means the tire can carry less load which means it won't hold control around corners as well as it could.
When you look at special-purpose experimental cars which achieve fuel mileage of several hundred or even one thousand miles per gallon, they ride on tires which are as skinny as bicycle tires and inflated to just below burst point in an effort to reduce rolling resistance as much as possible. Obvious such factors as cornering, long tread life, and ride comfort are not considered.
diverjohn, Wed, 25th May 2011

Biodiesel is not exactly an oil, though it looks like one and burns very similar to one. It is more like a soap, being made from long chains joined together, formed from an acid- base reaction, and being a organic salt. It is quite hygroscopic, and will tend to absorb water from the air, which causes it to break down with time, releasing the acid component.

Your fuel tank is mostly plastic, with metal tanks having an inner lining bonded to it of plastic that is resistant to long term immersion in fuel. This is to prevent any reaction if the tank was galvanised, as this would react with the alcohol in fuel, and would generate a powdery deposit that would clog filters. This happens with older cars ( mostly pre 1980) that get run on a fuel containing alcohol, where the alcohol reacts with the aluminium of the carburettor and blocks the passages with a deposit. SeanB, Wed, 25th May 2011

The Tank should never be below 25% or quarter of a tank for reason of the fuel acts as a coolant for the fuel pump. If it runs at a lower level you have chances of air being sucked up and temporary lapse of lubrication to the fuel pump, over heating of pump and possible drawing of the sediments at the bottom of the tank into the fuel system. The fuel savings from driving with a lower amount of fuel doesn't out weigh the cost of a fuel pump ($300-1000 Cnd) Ton, Sun, 29th May 2011

I have another reason for keeping as lttle petrol as possible in my car.
I don't have a garage or secure parking place, so I reckon that my car is less likely to be stolen if there is very little petrol in it.
This is especially true in the case of the so called "Joy Riders" who steal vehicles to thrash around in, and then abandon and usually set fire to them after they have crashed them or wrecked the engines.

Another reason for not leaving much fuel in the tank, is Insurance Companies do not include things like fuel when calculating how much Compensation to Pay Out on a Stolen Vehicle Claim. With the current Obscene price of fuel in the UK, that could equate to an uninsured loss of over 100 GBP. Dr Graham, Wed, 30th Nov 2011

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