Service, air filter and fuel economy?

[Bloke]

Excuse the complete lack of knowledge on cars or mechanical topics but here we go.

Bought a 2002 GT Tdi 130 back at the start of feb, drove it for about 2 weeks before I could afford to put it through a full service.

Avg mpg during this time was 50 to 56mpg on a nice motorway trip, 52mpg over a tank, cc on.

In for a full service and I was told "there was a k&n panel filter fitted so we just placed that in your boot and you have a fresh stock filter installed".

I had to first google wtf a panel filter was... So I didn't have a clue what he was talking about.

Now on the same motorway journey with cc on at the same speed, avg mpg over a trip is 40-45mpg, 44mpg over a tank.

It drives the same as before but could the change in air filter explain the loss in mpg? is there something else I should be looking for, or am I just being paranoid?

[Hoggy]

Just stick a fresh k&n or pipercross filter in

They let more air through than a standard filter.

[Agreeable Slick]

There are a whole raft of things that could have effected it.

Is the new oil thicker than the stuff that was in it? (depending on service interval it could have broken down)
Ambient temperature has decreased again recently and diesels take longer to warm up thus burn more fuel initially.
MPG on the MFD (Multi function display) is generally a bit hit and miss so calculate it manually by logging amount put in the car and miles traveled on odometer.
Is the MFD set to the right display? I.E. there are two positions, one which logs live data, and one which logs averages. (1 and 2 respectively)

I highly doubt a change of filter would have that larger effect on claimed MPG, I dislike K&N filters at the best of times as they are just an oily mess which gunk up inlet paths after a while. Stock is much better.

[Bloke]

Apologies for the thread necromancy but I thought i'd post a reply with the actual answer.

(p.s. wish this forum had tapatalk)

So the actual outcome of it all. I was driving along turned a corner and BANG! SCCCCCRRRRAAAPPPPPPEEEE!.

Yup, After what must have been 3 weeks of just dangling down at the front and acting like a sail my undertray lost a screw and smashed into the floor with an almighty racket.

Bought some replacement parts on ebay, fitting kit and fixed it all up. Now back up to its usual 54mpg+.