for best fuel average you can use 185/75/15 on genuine rims ..but for this size you have to search more ..usually 185/65 and 185/70 available ..with 185/75 (you will get more ground clearance (as 195/65 ).
Section height data ;; Tyre size section height (ground clearance )
195/65-15 127
185/65-15 120 (fuel efficient )
185/70-15 129 (More fuel efficient )
185/75-15 139
175/65-15 113 (Genuine Size)