At the end of the day, both the hybrid and the fully electric vehicle will reduce the fossil fuel emissions of a car when compared with a similar vehicle that is completely powered through the use of a conventional combustion engine and battery combination. However, there are major differences in the degree to which this reduction takes place.
If you’re lucky, a plug in hybrid will result in more than a 50% decrease in the amount of gasoline that you need to purchase. This is governed by any changes in your driving habits, changes in the way in which the equipment works over the course of time and of course diligence on your part in keeping everything properly maintained. Driving habits in terms of acceleration curves also matter as less steep acceleration curves will tend to use more of the electric engine and sharper ones will tend to use more of the gasoline engine.
However, all things being equal, you should be able to more than halve the amount of gasoline you need to take in with a plug in hybrid.
Conventional hybrid electric vehicles should still allow you to halve the amount of gasoline you need to power your vehicle although it is also the case that the conventional hybrids only recharge through regenerative braking and that means that over the course of time they will likely end up drawing less power from the electric engine and more from the gasoline engine.
You can expect to have a higher gasoline to electricity ratio with a conventional hybrid in comparison to a plug in hybrid. Of course, both of those will be much higher than a fully electric vehicle which will use zero gasoline over the course of its life as a vehicle.