mistery

A little while ago, i fitted an S3 lower brace, and had problems with one of the nut inserts stripping the thread, having to drill it out, a satilyboy kindly sent me a couple he didn't need of the bigger type with the in built washer. Any hoo, i fitted these a few weeks ago, and i've gone to...

I am currently struggling to get above 41mpg when cruising at 80 on motorway for around 150 miles. I usually get above 50. Any ideas. Thanks