Sudden drop in mpg?

[MankyManning]

I have an X-reg 115bhp A4 TDi and I have noticed in the past week and a half that the average mpg has dropped significantly, it used to be always just below 47mpg for months but now it is dropping below 42 and still heading downwards. This is the long term average mpg using the onboard computer as opposed to the spot measure.

My driving patterns haven't changed but I did get a new set of tyres a couple of weeks ago plus it has got colder.

Is there anything I can check? I am a bit stumped as to what has caused it.

[joz119]

I have an X-reg 115bhp A4 TDi and I have noticed in the past week and a half that the average mpg has dropped significantly, it used to be always just below 47mpg for months but now it is dropping below 42 and still heading downwards. This is the long term average mpg using the onboard computer as opposed to the spot measure.

My driving patterns haven't changed but I did get a new set of tyres a couple of weeks ago plus it has got colder.

Is there anything I can check? I am a bit stumped as to what has caused it.

Hi maybe its the cold weather like you said using heaters more ect

[MankyManning]

I suppose it is always possible but I would have thought that heating was "free" as it were given the engine generates heat anyway afte about 10 minutes or so of driving. Just seems like a lot to drop.

[philla4tdi]

someone on here mentioned the petrolstations use different fuels in the winter to aid cold starting, but mpg drop is a side effect... not sure myself. you serviced it recently? changing filters always helps.

[spartacus 68]

Got to be either fuel, service or tyre related.

Either that or you've left your Thule roof box open!

Check tyre pressures. A much over-looked area. As suggested - if the car is due a service - then air and fuel filters would be my first port of call.

Could be a long-shot, but if your emissions have changed recently (smoking on start-up), then a can of Forte diesel conditioner straight into the fuel filter will do the trick. Drive hard in second and third gear to clear any soot or carbon build up. You'll need to replace fuel filter afterwards.

Also check engine breather hoses - in case one has come adrift, or there's a split somewhere.

Lastly - if all else fails - run a tank of high quality premium diesel through the car. See what happens.