26-06-2013, 08:25 AM
Useful input, thanks Dec.
As Anton says, we need to know price per litre before we can work out your mpg. I can explain the difference you're seeing between filling the tank and only putting in £30 a time though.
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.
As Anton says, we need to know price per litre before we can work out your mpg. I can explain the difference you're seeing between filling the tank and only putting in £30 a time though.
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.