Gio mows lawns for $25 each. He already has $250 in his bank account, but is trying to save $900. How many lawns does he need to mow to reach his goal?

Gio still needs to save an additional $900 - $250 = $<<900-250=650>>650.

Therefore, Gio needs to mow a total of $650 / $25 = <<650/25=26>>26 lawns. Answer: \boxed{26}.