It is definitely worth being guided by savings, but in these times where environmental pollution is serious, switching to ECO LPG fuel is really important.

Before you calculate savings on LPG, have you ever count how much you can earn keeping 1000 euro in the bank? 

Try.
ZERO, minus 10 euro, maybe plus 10?

 If you have to drive, you also need to use fuel. If you could pay half, how much will you save? Everything depends on how much you drive.  

There is no ideal calculator, as the driving costs vary from car to car. LPG consumption also depends on many things like settings the speed of switching between petrol and gas in the cold and warm engine, generation of LPG system, mixture of propane and butane, price of petrol and LPG etc. We could calculate your savings like this:
Multiply the price of LPG you pay at pump by 1.2 ( add 20% for a bit higher consumption, driving on petrol on cold engine etc ) and then you will get a price of petrol you would „normally pay”. There are many countries where LPG price is only 35-45% of petrol price. That’s why traveling to continent would be much cheaper on LPG. The LPG / autogas price in Ireland is usually half of petrol.
Example 1:
Petrol price – €1.40 per L
LPG price – €0.699 per L
0.699 x 1.2 = 0.8388 per L
In this case it’s like to pay 56 cents less per L, which is about 40% savings

Example 2:
Petrol price – €1.50 per L
LPG price – €0.599 per L
0.599 x 1.2 = 0.7188 per L
In this case it’s like to pay 78 cents less per L, which is about 52% savings