Math problems during travel

Seven tires are enough. This trip will consume 42000*4=168000 "tire kilometers", so 168000 "tire kilometers"/24000=7 The following is the tire replacement plan.

Assume that the tires are numbered from 1 to 7. Every time you travel 6,000 kilometers, you must stop and re-equip the tires according to the following plan: 1 2 3 4 2 3 4 5 3 4 5 6 4 5 6 7 5 6 7 1 6 7 1 2 7 1 2 3 Each tire has traveled 24,000 kilometers