If you've ever wondered how PHEVs manage to achieve remarkable MPG test results, full details of the WLTP test procedure can be Googled.  But I'd like to let PHEV newbies know what to expect when driving. Consider the 2 extremes - a conventional car sat with an idling engine delivers 0 miles per gallon, fuel is being used but you're not going anywhere. At the other extreme a PHEV can easily achieve a displayed 999 miles per gallon by using only battery power for journeys within the range of your vehicle's battery, no fuel is used to cover this distance (but you have to get into the habit of charging the battery before travelling). What happens beyond your battery's range limit, once the engine kicks in,  needs a new way of thinking about MPG.

You will see your "999" MPG begin to fall, quickly at first then gradually slowing as it approaches your vehicle's "engine only" MPG. As engine use becomes the main source of distance covered the engine's MPG will gradually dominate the maths when calculating the overall average for the trip. If the journey was only slightly further than battery range most of that distance was done on battery power "999" will dominate the maths so you will have an MPG of several hundreds. 

Using Top Gear maths, a PHEV with a 50 mile battery range would achieve 700 MPG on a journey of 55 miles but 40 MPG on a journey of 200 miles.

Remember that it always takes energy to move any vehicle, just try pushing it yourself. A conventional car gets this energy by exploding flammable fuel in an engine to produce the rotary motion needed to turn the crank/gearbox/wheels, an EV uses stored electrical energy to turn the motor/wheels. An engine converts about 30% of the fuel's chemical potential energy into motion and the rest is wasted, mostly as heat. An EV uses about 75% of the energy taken from the supply grid to charge its battery to create motion. It costs approx half as much to move your vehicle using electricity rather than fuel.

The art of PHEV driving is to select the best energy source for each part of your journey, read the handbook. The engine is at its most efficient during long stretches of motorway cruising (preferably at 56mph in lane one). You will maximise the battery's range by using it in urban stop/start traffic together with "regen braking".

Thanks for this thoughful and insightful post.  I am thinking of getting a Ford Transit Custom PHEV, as most of my journeys will be within the electric-only range, and I would expect to run it in EV mode almost all the time.  But for the occasional long journey, I would use EV to get to the motorway, then use the petrol engine, saving the rest of the EV range for the last non-motorway leg, thus hopefully keeping emissions-free in built-up areas.  I've read a lot of reviews about the Transit PHEV, one thing that concerns me is reports of the 1.0 petrol engine getting very hot on long journeys.  I don't know if this is because drivers are pushing too hard (foot to the floor all the time) or trying to go too fast on the motorway, or if Ford has fitted an engine which they have to hand but which is not really man enough.  The only other range-extender I'm aware of is the BWM i3 and that is a very much smaller and lighter so offers no comparison.  I would appreciate any feedback you have from your own experiences.

Hello Neil, sorry for the delay in replying. I'm not into social media and only dip into this forum occasionally. I've written 2 longer pieces about living with a Custom PHEV, they're in this site somewhere!

The 1 litre engine is a remarkable piece of kit (small enough to fit on a piece of A4 paper) but you're right in thinking it's having to work fairly hard in the Transit. Your proposed method of use is just what the van suits, most journeys within battery range with the engine used to avoid any range anxiety (your "get out of jail free" card) and handle longer journeys of any length. If you apply enough throttle to take the power needle above 5 (50kW) for more than a couple of seconds you'll trigger the starting of the engine (to add the extra power needed above the battery's output). Once started, the engine will not switch off until it has reached operating temperature (to avoid lots of cold starts/stops). While warming up it will continue idling even if you stay well below the 5kW trigger point. After a minute or so you'll see your MPG begin to drop from 999 (only down to several hundred, until the engine switches off). Console yourself that any electricity generated by the engine spinning the generator will go into your battery and extend your range. I deliberately trigger an engine start during an easy part of the journey (say a downhill stretch) in order to get it warmed up, ready for use on the motorway. Once up to 56mph, I set cruise control, change MODE into "EV later" and shift from L to D (it just seems to smooth things out). You are perfectly OK to do this on the move (it's in the manual), just be very careful not to go beyond D into N or R. Maintaining about 56mph requires between 20-30kW on level ground, which the engine finds easy. You can hear the engine working harder going uphill and it's not difficult to imagine it being maxed out on steep hills or if heavily laden. Even in D the cruise control will use regen braking to maintain your chosen speed on downhill stretches. The engine continues to idle while the power needle sinks past zero into the regen zone, both of which are putting juice into the battery.

Engineering is about compromises. This van suits a particular type of use - it's limited to 75mph and is zero rated for towing, but has a 1.1 ton payload. It can cover 35miles (38 in summer) without using any petrol and can go like the clappers if you use both the battery engine together. The vast majority of my journeys are within battery range (usually charged partially/fully by the solar panels on my house) and long journeys at 56mph using only fuel have resulted in a displayed economy of 40mpg.

I've not had any hint of a hot engine despite driving in last year's heat wave and imposing the extra load the aircon. The PHEV has no temperature gauge (replaced by the battery charge indicator) but there must be some sort of warning lamp/message instead. You hear the cooling fan kicking in quite often, but remember that the van has "thermal management of the battery pack" to keep the EV battery at the correct temperature. This is why the radiator fan is active during charging even though the engine isn't running.