How much petrol should I put in my car?

22 May 2011

Question

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
 
Paul

Answer

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.

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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