The Michelin website gives you a number that is "indexed"
In other words a tire rated higher than the indexed number will be less fuel efficient.
So if the Michelin is rated at 100, and a Continental is rated at 105, the Continental is 5% worse than the Michelin.
Also if the number was 95 for instance, it would be 5% better in fuel economy.
To calculate the true difference you would need the true rolling resistance coefficient.
Check with a quality tire dealer, they can help