Bro - sorry for the late response. I havnt checked the average yet as its not out of workshop. heard that it give 8/9 in city and 10/12 on highways.
You'll have alot of choices as far as engine is concerned. depending upon your usage and choice. first you will have to decide that either you prefer diesel or petrol. I like diesel engines for better torque and power, however usually they are slow as compared to petrol.
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However if gear change is required then SSR gear is top choice, no matter you put in deisel or petrol.
If i would have given a choice in petrol, i would first keep the stock 2F or change it to 1UZFE.
For diesel, 13B / T as top priority and 5L as second.
Regards