Last week I tanked in the evening ca. 20:00 in Koeln (Huerth to be exact) . 1,174 Euro Diesel / l (not too bad - better than in Belgium - Antwerp 1,43 /l ).Taxes are probably a lot lower in Luxembourg. In Germany, you pay a fixed tax of 65.5 cents per litre and another 19% VAT for petrol/benzin (so 60-70% taxes). [for Diesel it's 47 cents per litre + 19%]
I asked the clerk about price diversity - he confirmd, apparently it is not the best option to tank on Shell in germany in the evening ( Nachmittag Tarif); he said best option is to tank 7:00-11:00.
From my experience - in germany it is best to tank away from Autobahn, best on Autohof or city next to Autobahn Ausfahrt.