A litre of diesel is the energy equivalent of approximately 11 kWh.

If you get 25 mpg that means it "costs" you 440 watt hours to go one mile.

Chances are, the lion's share of this is going to go on a mixture of;

1. rolling resistance
2. aerodynamic resistance
3. acceleration

#1 is affected by good maintenance, and custom options such as wide or narrow tyres.

#2 is affected by vehicle speed, not much else you can do to change it.

#3 is affected by vehicle weight, and jackrabbit starts.

Once you own a vehicle, there isn't much you can do about air drag coefficients, or vehicle weight, they are pretty much fixed.

Once you own a vehicle, you CAN make a huge difference to actual air drag by keeping the speed down, 60 mph or less, you can also make a huge difference by driving smooooooth and steady and reading the road ahead.

Once you own a vehicle you can minimise rolling resistance by maintaining the vehicle running gear well, and fitting narrow tyres.

You can boost your economy by 30% doing these things.

Making SIGNIFICANT changes is fuel economy can only be done with a NEW vehicle.

It needs to be very low drag, which means low.
It needs to be VERY light, think 750 kg kerbside weight.
It needs to be SLOW, think 0-60 of 15 second and a top speed of 70 mph

Do this and you can get a 100 mpg diesel car.

Or go from 440 watt hours per mile travelled to 150 watt hours per mile travelled.

But you absolutely CANNOT do it with a vehicle built like a modern car, steel body, glass windows, leather seats, 2,800 kg kerbside weight, fat tyres, 125 mph top speed, etc.

In the end, it is all going to come down to cost.

Here in the UK one kWh cost around 13 pence, that's around 20 cents, doesn't matter how you buy it, as electric, as diesel, as natural gas.

The amount of kWh that your weekly wage can afford is going to decide what kind of economy your vehicle runs.