[sallen]

There has been many a thread discussing this.

I think the theory is that the air has nowhere to go so becomes a squashed "cushion" of air that then deflects incoming air much like an "over the grille block" would do. I'm sure somebody will straighten my story here if need be...

I might try a clear block on the outside just to see if there is any better gain. My personal 2 pence is that covering the top grille makes little or no difference as the euro Golf has a huge gaping bottom grille. Unless I tackle that I don't think I will see any significant gain.

To date I have seen only a tiny improvement with slower driving, grille block and high tyre pressures. If I drive everywhere at under 2000rpm I get about 45mpg (imperial). If I drive like a lunatic on the turbo all the time with diesel smoke pouring out the back I get about 45mpg so I can't win.

Also if I go on a motorway journey and drive at 70 I get 45mpg, if I drive at 50-60 I seem to only get a tiny bit more...

Nevermind though, Bottom grille block, DTM mirrors, undertray and intercooler should fix that.