Hi
 

I'm the proud owner of a VW Golf Mk4 TDI, want to improve it to achieve better fuel economy.

Welcome to the site. It looks like you're already getting some pretty decent fuel economy with your Golf. What kind of goal are you looking to achieve?

Thanks.

I would like to get at least something like 4L/100km (~60MPG) at 110km/h it would be awesome. Engine ECU is already tuned, there isn't much to squeeze there. Air-conditioner compressor internal control valve is modified to slightly higher suction and lower head pressure.

Now I was thinking mostly in stealth aero mods, like dams or flat under body.. but don't know where to start.

This is a great place to start:

https://ecomodder.com/forum/fuel-eco...ifications.php

I was told it's harder to certify the compliance of some mods in Portugal than in Spain. But anyway, if you're considering some discrete aero mods, what about a grille block? Besides improving the aerodynamics on highway, it may lead to the engine reaching its optimum temperatures quicker.