I am looking at getting a small car to get to and from work.

Some run on 95 and others 91 octane petrol.

One car is rated at 4.8ltr/100km using 95.
Another car 5.5ltr/100km using 91.

With the different prices of fuel how can you make an effective comparison?
Is the pricing of 91 and 95 based on the the amount of energy that can be extracted?
No doubt there are manufacturing, usage and tax issues.

Cheers