26-06-2013, 09:15 AM
(26-06-2013, 08:25 AM)Poodle Wrote: Assuming all other conditions are the same, you'll get more mpg if you've only got a maximum of £30 in the tank rather than filling up, purely because of weight. Keeping it down below 1/3 of a tank means the car weighs 40kg less than it would do with a full tank, so when accelerating that's 40kg less work for the engine to do. You notice it as you're doing a lot of local driving - start/stop and short journeys, etc - if all you did was motorway commutes there would be barely any difference, as it's only really during acceleration it'll make a difference.
Actually 2/3 of a tank wont be 40kg. Assuming that your tank is actually 60l (I've had 70l in a 306 in the past) 2/3 is only 40l but diesel is significantly lighter than 1kg/l (this is why it float on water) its only about 800g/l depending on temperature so 2/3 of a tank actually only weighs 32kg
Petrol is lighter still though at about 750g/l
Still 32kg is a significant weight extra to lug around all the time, itll be about 3% more weight in the car assuming about 1200kg for the car
Actually this puts me on to a thought, when I've stripped about 100kg out of a 306 its added about 5% to my mpg figure so allowing for driving like more of a hooligan with a stripped car then it would probably be even more of an improvement