Is this normal

[Nasir]

£10 fuel has given me 68 miles. Car computer reading out avg mpg 53.2
I work it out to be around 43 mpg
My car computer is out by around 10 mpg

Cost of fuel was 141.9 shell v power +

All this time I really thought I was getting 50/55 mpg. I was happy at that figure. But I'm actually only getting 40mpg

I'm So annoyed.
Should I not be getting more. It's a bluemotion ?
I drive the thing really slow always to the limit. I have 9 points for speeding.

I bought the car as I thought it was good on fuel. But is it really ????

Also when I fill up at BP I find I get less miles than shell. Maybe I should use supermarket fuel. It's cheaper.
Really pee'd off.