Hi Don 1973 & Welcome...
Assuming you dont have a petrol leak which you should be able to detect with the Mk1 human nose...
Your car computer is basing the remaining fuel on your current driving style & technique, that will change if you adapt the way you drive... Been driving long? You can never expect great economy in a city environment though.
Your engine will still be tight up until it has covered a few thousand miles, economy will improve.
Dont labour the engine, let it rev & run, find its Bhp & Torque sweet spot, usually around 2,500 to 3,500 RPM.
Using that formula I have found my current car is good for 42MPG knocking about even though it is a 1.6 petrol.
The very best fuel economy device is the drivers brain & left foot.
I also found running down hills in top gear (not out of gear) foot off the throttle, helps a lot
!