[HypermilerAX]

tIf you want to increase the diameter, it's better to do it with higher sidewalls rather than wider tires and larger rims.

You could go with small truck tires like 195/75 R16, they don't have the best class but still a very good B class. It's 10.2% bigger, though. Did you measure how much room you had left for bigger tires? On the AX, the limit was about 10% bigger.

To know how much rpm loss will affect efficiency, you can compare max rpm and and rpm at max speed in final gear.