People keep asking me how I consistently get such good fuel figures. The method I found returns the best results is by using Deceleration Fuel Cut Off (or DFCO as it is usually referred to). This is a system, built into all modern petrol and diesel cars, with a view to cutting emissions. My 2006 Jazz has it and I am sure it has been a function of many Fuel Injected engines before that.
We all know that you can take your foot off the accelerator, while travelling in gear, and the car continues forward, slowing gently. This is when the engine designer makes use of DFCO. Provided the forward momentum is sufficient to keep the engine revs up above a set speed, the fuel injectors are cut off, to prevent unnecessary fuel being burnt. So you are moving forward, in gear and using no fuel. Maximising this situation is what gives great mpg figures.
The easiest way to know when you are in DFCO mode is if you have an instantaneous mpg display on your dashboard. The later Jazz’s with their A & B mpg surely have an instantaneous display. My Mk 1 doesn’t, but I have fitted a ScanGauge E to the OBD II port, and this displays instantaneous mpg for me. If I freewheel, down a hill, with the engine running, it will display 200, 300, 400 mpg, depending on the speed you are traveling, but if you are in gear and utilising DFCO it displays 9,999 mpg. This is like freewheeling, down a hill, with the engine off, except the engine is running, you are in gear, and the car is under complete control.
If I am descending a hill at say 50 mph, foot completely off the accelerator, the car will be in DFCO. If the speed starts to drop to say 45 mph and I apply a slight amount of throttle I will come out of DFCO. I could now be getting 140 mpg, but it is not 9,999 mpg! I prefer to allow the car to travel at 45 mph and continue to get 9,999 mpg than speed up to 50 again. Even with following traffic you are hardly a nuisance.
Below 30 mph 5th gear will not give DFCO, the engine revs are too low, so I will change down to 4th or even 3rd. Sometimes you have to go down to a lower gear to start DFCO but as soon as you are in that mode you can change up and it stays there. It is a function of the hysteresis in the system.
Sometimes I mix DFCO with a bit of coasting in neutral. There is a particular stretch of road I use regularly, a 40 mph zone, a country road restricted after a girl was killed playing “chicken”.
It starts with a slight downhill, then levels out as it approaches a roundabout. I get the benefit of DFCO as I descend the hill in 5th, as the road levels and the speed starts to drop off I plop it into neutral which prevents further slowing, and for the final braking for the roundabout I use engine braking in 3rd, which resumes DFCO mode.
Making most use of DFCO is what starts to push the mpg figures up. Learn how to use it and reap the benefits of it.
Any questions, ask away. Any observations, feel free to voice them.