Firstly indica Xeta is petrol variant in Indica series. Indica is divided into 4 catogories based on their engines. Xeta, e v2, v2 and Indicab. Xeta is petrol variant, E v2 is diesel variant with cr 4 engine, v2 is diesel variant with normal indica engine, Indicab is v2 designed as a taxi.
To get the best mileage from your vehicle, we will advise you to get your car serviced on time, regular check for tyre pressure and follow speed limits. Please visit link this link for more such tips- http://www.cardekho.com/advisory-stories/driving/tips-to-ensure-good-fuel-economy.htm
ECU (Engine Control Unit) is a electronic control unit, which control engine operations ensuring optimum running. If the ECU of your car has been damaged, please visit authorized Tata workshop to know the cost of the ECU and to get it replaced.