Again..... you would be wasting your time putting on a larger tire and expecting better mileage. If that was correct, you could have 5.13's and 40" boggers and not effect economy.
There is a reason why we all seem to continually get worse and worse mileage out of our K5's compared to when they were stock on 31" tires.... we keep adding more lift, bigger tires, more weight, and extra power.
If you actually want to do something towards saving fuel....dump the 4.56's for something much better. 3.73's are going to be about perfect with a set of 31" STREET tires.
I swapped out 31" tires for a set of 33x12.50's on my Dodge Cummins....and lost 1mpg on the freeway. If that slight extra rotational mass can make that much difference on a diesel that doesn't sweat making 800lbs of torque, you know a small block is going to not like more weight it has to get moving and keep rolling down the road.
Also, RPM isn't the sole factor that effects highway economy, like many people think. On a gasser, especially a small block, they get best mileage with good vaccuum. If 2000 rpm yields much better vaccuum then 1800, then better economy will be achieved at 2K RPM. The heavier the vehicle is vs. it's power output, the more vaccuum and torque curves come into effect. Where RPM is important to keep to a minimum is when you have achieved best vaccuum and are spinning more RPM than required to achieve that.