That friend was an intelligent man to suggest that.
A car has 4 wheels at a time. To travel 7,500 km forward and 7,500 km back, you need each wheel to travel 15,000 km and that means a total of 60,000 km for all wheels.
Each new wheel can run 10,000 km. So you need a minimum of 60,000/10,000 = 6 new wheels.
First all the present wheels on the car are replaced with the new wheels (W1, W2, W3 and W4). Two additional wheels W5 and W6 are kept inside the car.
After traveling 5,000 km, replace W1 by W5 and W2 by W6. Keep W1 and W2 inside the car.
After 10,000 km, the life of W3 and W4 is over. So replace them with W1 and W3. Discard W3 and W4.
After 15,000 km, you return back to my place. Now replace my tires back on the car. Discard W1, W2, W5 and W6 as they ran their life time of 10k km.