Ask an expert

For argument sake lets say that all other things are equal. What would you say would give the better fuel economy between a 2.0 litre 4 clynder 4 speed automatic car running on 95 octane petrol and a 1.8 litre 4 clynder 4 speed automatic car running on 91 octane. They both when travelling at 100 k/hr do about the same r/pm.

There are so many other factors at play that it's not possible to say for sure. For example, if the 2.0 litre car was an AWD Subaru Legacy and the 1.8 car a Toyota Corolla, then it would be the 1.8 litre car that's more economical. If the 2.0 litre car was a VW Golf and the 1.8 litre car a 4X4 Toyota Rav4, it would be the 2.0 litre car.
Therefore, we need more information to provide you with an answer.

ok. say a 2.0 litre Vw Golf 4 speed auto on 95 petrol vs a Toyota Corolla 1.8 litre 4 speed auto on 91 petrol. Which is likely to be more economical. ( I asked this as a few years back I had a 1.8 litre car running 91 petrol and swithed it to 95 petrol and got more kilometres out of a tank of petrol )

These two cars will generally achieve very similar fuel consumption using similar fuel, so the engine capacity has very litle effect for cars such as these.
Results from some testing we did in 2012 showed that there is no advantage in using a higher grade fuel than that which the manufacturer recommends. Sure, you will get a few more kilometres out of a tank of fuel, but the added cost of the higher octane fuel erodes the advantage.
Another factor to remember is that a car that consumes half a litre per 100km more than another will cost you very little extra in fuel (about $2 or so per week for a motorist travelling 12,000km per year), so while we encourage people to drive economical cars, would also point out that they shouldn't get too hung up in the fine detail, as there are many other important cost factors to consider with running a vehicle.
In summary, if the manufacturer recommends 91 octane fuel, that will be the most economical fuel to use.