10 the fuel avg u will get for witin city drive not matter summer or winter AC or no AC, driving really aggressively in PWR mode can reduce the fuel milage otherwise more or less it remains constant.
however on highways you will get 15 if driven upto 120kph.
you can use regular ELM327 ver 1.5 and above with torque application, many other apps also work easily with this OBD scanner.
Be very careful if you are going to swap transmission with a kabuli unit, most of them are not compatible with stock ECU and wiring and TCC lock up wont be triggerring no matter wat u do, it will not only affect fule consumption but also overheat the transmission on highway speeds.
better stick with stock transmission also try and replace the ATF, it takes TOYOTA T IV atf, toyota WS ATF also works.