Mine has improved recently!
Since I got 9 points on my license my HGT has gone from 16.5 mpg to 19mpg. So proof has it that you need to go fast, get caught and it saves you money!
Realistically though I have driven about 2,000 miles in the last week or so (Midlands to Scunthorpe to Maesteg to Andover in circles!) in the most appaling weather which has meant that I have driven slower than I normally would have.
I don't pay for petrol so I don't really care but if I got 19mpg on Super Unleaded it would mean about £850 per year in my pocket over 16.5mpg! As it is my annual petrol bill is in the region of £5,900.
My range on a full 60 litre tank has gone up to 258 miles from 225.
Cheers
Nigel