My fuel economy sucks as I barely get above 40mph on my commute - the average speed since I filled up on Monday and reset the trip is 19mph (over around 50 miles). I am sure I'd get 40+ mpg if I could drive everywhere at 50-60 but I'd have no licence!
I plan to have the belts and a full service done in November before the MOT
The -10's are twice as expensive but have a pure copper electrode with a groove for the electrode to sit perfectly and the -n's have solid brass with no groove.