The Tata Nano is rear wheel drive and its engine has been fitted also rear. The rear tyres of the car are fatter than front so and the car pulls whole power from rear wheels. so according to its dimension and manufacturing technology its different radius.
New Tata Nano with EasyShift is available at Rs. 2 lac (Ex-showroom, Delhi), and the final cost depends on the valuation of your car. So, first get the valuation of your car from authorized Tata dealer, so he can give you a final quote for the new car.