Raid
You cannot assume the economy trip is accurate - none of my last 5 cars had an accurate trip recorder.
You also cannot assume that your odometer is accurate - my current car is 1.95% slow and my previous 4 cars were all inaccurate between 1.2% and 2.6% slow.
I check mine when on at least a 50 mile motorway journey by comparing the odo with the kilometer distance boards. As you pass a kilometer board get your passenger to note the distance displayed and the odometer reading at the same time. After travelling a long distance do the same for another distance board. It is then simple maths to determine the distance travelled according to the motorway boards (convert Kilometers to miles) and the distance given by the odometer. Express this as a percentage.
You new need to brim the car - and I do mean brim it until petrol is about to overspill the filler neck. This will take several minutes of patience. Write down the odometer reading and reset the economy trip to zero
When you next need petrol - brim the tank again right to the top. Again note the odometer reading, the amount of fuel added and the economy trip recorder reading.
Take the original odo reading from the second one - this gives the miles travelled on the last tank of fuel. Multiply this distance by the percentage inaccuracy of the odometer to get the actual miles travelled.
Convert the litres of fuel you have added to gallons. Now divide this into the actual distance travelled to get a dead accurate mpg.
You can now compare this with the economy trip reading to establish how accurate it is.
My previous car was a 1.6 litre petrol manual box KIA Venga - which is an aerodynamic brick compared with an i30. I averaged 42.17 mpg over a distance of 17,000 miles before I sold it.
My new car is a 1.6 litre petrol auto Venga which did 503 miles on its first tank of fuel and averaged 42.12 mpg.
Having said all that I should explain that I now drive for comfort and safety rather than speed.
I always accelerate gently and try to anticipate the traffic conditions at all times so I minimise the use of brakes. Whenever you use brakes - you have just wasted fuel - you should have taken your foot off the loud pedal earlier.
I cruise on A roads and motorways at between 55 and 65 mph - depending on mood and how busy the road is.
I don't carry any junk in the boot - extra weight hits fuel consumption.
My tyres are set at 36 psi - even though the book recommends 32 psi - another economy aid.
In a manual car I always use the highest gear the car is comfortable with. In an auto this is taken care of for you.
On a downhill run I usually let the car go as fast as gravity takes it (but watch out for speed cameras). I try not to use the brakes to kill the speed - the next hill climb will do that.
I don't buy supermarket fuel - Shell or BP standard 95 octane is my preference. I have tried 97 octane several times - but on every occasion the economy was the same or even worse than 95 octane. High octane fuel has never given me better mileage.
Have fun.....................