The other day, I made a long trip north to the hot weather for two reasons - to deliver a set of 13" non-original alloy wheels and tyres from my FIAT 850 project (sold for a good price) and to pick up my BOC TIG welder (bought for a good price).
I drove gently on the trip to Auckland, getting overtaken quite a lot on the motorway where I stuck to the speed limit. Average was 5.6L/100km. Then I drove to Whangarei and it was all rather beautiful, I forget what the fuel economy was. Temperature about 28 degrees outside, strong sun and heavy rain, so the air conditioning was on all the time. There were some long jams in the small towns and some great cruising on the new toll road (see the bizarre curved bridge).
The Trip B counter auto-resets, so the readings shown here are for the trip from Whangarei all the way home - more than three hours in a mix of conditions (have to drive through Auckland). I saw nine police cars (parked up with radar guns) and five stationary cameras - the 100km/h speed limit was being enforced with great vigilance.
The problem with this is that the 159 QTronic can't get into top gear below 110km/h indicated (104km/h actual), so it's not really possible to use top gear for best economy!
Anyway, despite not aiming for economy, the result was the same - 5.5L/100km which is over 51mpg. I brim-filled the tank twice to check, and the trip computer was actually pessimistic by 0.05L/100km. Note the range of well over 1200km (745 miles) on a tank. I was very happy with all this, particularly as I found a service station selling diesel at $1.47/litre ($0.10/litre cheaper than usual) - and the previous owner paid my road user charges for 12,000km, so it's a big saving over petrol at $2.31/litre.