The formula
Petrol: km ÷ 100 × consumption × 500 ֏. Gas: the same, but consumption × 1.15 (gas burns 15 % more) and 210 ֏ a litre. The difference is the monthly saving. That is exactly what the site's calculator does.
Prius taxi: 3,000 km, 5.5 l
Petrol 82,500 ֏. Gas 39,800 ֏. Saving about 42,700 ֏ a month, 512,000 ֏ a year. A 180,000 ֏ installation pays back in 4–5 months.
Family Corolla: 1,500 km, 7.5 l
Petrol 56,250 ֏. Gas 27,200 ֏. Saving about 29,000 ֏ a month. Payback 6–7 months.
Camry V6: 2,000 km, 11 l
Petrol 110,000 ֏. Gas 53,100 ֏. Saving about 57,000 ֏ a month. A 220,000 ֏ installation in 4 months.
What the adverts leave out
Filters, about 10,000 ֏ a year. The cylinder test, 10,000 ֏ every two years. Subtract them and the numbers barely move. The saving is so much larger that they simply disappear.
Your numbers
The calculator is on the homepage and on every car page, already set to that model's consumption. Put in your mileage and prices to see your figure.