I've given up chasing mpg to save £3 on a tank of fuel.
Today, on a trip down to Hartlepool (37 miles each way) I got 57mpg indicated (probably 52.5mpg actual) going down and 59mpg (54.5 actual) back. On a 100 mile trip (we were thinking about going down to York today, but didn't), i'd have probably cracked 56/57mpg actual. On that basis, sitting at 65mph would probably have netted me about 60mpg actual (equivalent of 62mpg indicated on a stock car). Pretty much the same as the Scirocco The 17C ambient temp helps.