I’m glad that @kezo has responded as I knew that his reply would cover off the technical aspects in a way that I couldn’t.
It is indeed the case that most of my mileage in HEV mode (when the battery is depleted) is on motorways, although I try to stick between 60 and 70mph due to the way that it drinks fuel when in this mode.
Kezo also makes a very important point about accurate calculations when it comes to fuel economy in a PHEV. I alluded to this in my last post but will expand a bit more. The vast majority of PHEV users on this forum quote their mpg as what shows on the cars fuel computer, often posting photos to illustrate the point. However, these are fantasy figures that bear no relationship whatsoever to reality. The only time that the fuel computer in the car would be accurate is when the computer is reset to zero when the car has a fully depleted battery (other than the residual charge that it always holds) and then never charged.
The reason for this is because the computer in the car takes no account whatsoever of the cost of charging, so would only ever be accurate if all charging was completely free which, of course, it never is.
As to how big the difference is between what is stated and reality depends on a number of things, primarily what proportion of your driving is done locally after a full charge (in EV mode) versus longer distance runs where the engine is used (HEV mode). The other major factor is your charge costs. The only way to work out the true mpg is to work out how much each charge costs (using your kWh cost) and keep a log of them all, add this total cost to the cost of petrol and then divide that total cost by the miles driven to give a cost per mile. You can then divide that cost per mile into the cost of a gallon of petrol to give an ICE equivalent mpg.
To take my situation as an example, I don’t have a smart meter or discounted overnight tariff, so I pay 23p per kWh. The Tucson PHEV battery is 13.8 kWh but when discharged keeps 15% in reserve. Therefore to fully charge I am putting in 85% of 13.8 kWh which is 11.73 kWh. 11.73 x 23p = £2.70. I keep a small notebook in the car and keep a log of the cost of every charge (£2.70 if a full charge from 15% or the % charge needed x 3p if the car has more than 15% remaining). When I eventually refuel with petrol, I add the total cost of all charges since the last refuel to the cost of topping up the petrol to a full tank. I then divide that grand total cost by the mileage driven since the last time I put petrol in to get a pence per mile total cost. This is the only way to get completely accurate running costs. What I then do is divide this pence per mile into the current cost of a gallon of petrol (cost per litre x 4.546) to get a ‘true’ mpg equivalent had all my running costs (petrol plus all charges) been spent on petrol. This enables me to compare against normal ICE cars.
As a PHEV requires extra work over a petrol or diesel car (as you have to charge it after every use, rather than just a 5 minute fuel stop every few weeks) I want to make sure that the savings in running costs are worth the extra effort. I am currently averaging around 50mpg using this calculation compared to 42.2mpg in the petrol BMW that we had before, so the answer for us is ‘possibly’ 😂
However, the ‘true’ mpg will vary massively depending on how you use the car. With a full charge (cost to me £2.70) I can get about 35 miles in EV mode (the car says 39 when fully charged but the first mile seems to eat up at least 3 miles of that range). That works out at 7.7p per mile. Based on the current price per gallon where we live (£1.32 per litre x 4.546 = £6.00 per gallon) that’s a ‘true’ mpg equivalent of 77.9 mpg if 100% of my mileage is in EV mode and the engine is never used. To illustrate how inaccurate the fuel computer in the car is, it will show as 999 mpg (or a figure still in the high hundreds if the engine has come on at some time – perhaps to heat the car). So if all my driving was local in EV mode (which it isn’t) then the savings would be very large.
On longer journeys, where the engine is running more, true mpg is probably around 40 mpg (big difference) so that’s why my average (combining both) is working out at a true mpg of around 50 mpg (versus the fuel computers claimed 80 mpg).
So, in summary, those who have low electric costs and do almost all their journeys in EV mode will be achieving a true mpg equivalent of over 100 mpg. However, those who never charge and do a combination of short and longer distances are likely to achieve a true mpg that is around one third of that. Hopefully this illustrates why if choosing a PHEV you really need to be able to charge at home after each use and if you can’t do that a self charging hybrid, or an efficient ICE car, would be better.