1KZ-TE 2982CC diesel gives better mileage than petrol 5VZ 3387CC V6. As per my knowledge, diesel variant is more torqy but less powerful while petrol variant is more powerful but at the expense of more fuel. As far as tax is concerned, for both will be the same but for Isb reg Token is only approx 700, but income/withholding tax is 24000. If u r efiling then income tax is 12000. For Lhr/Punjab, income tax is same as that of Isb but token tax is 15000. Above figures are for less than 10 years old reg vehicles. For more than 10 years old vehicles income tax is zero.