The fronts will always wear faster (especially along the outside) so you have two choices. I prefer the later. 1. Don't rotate the tyres, allow the fronts to wear out first, when they wear out, replace the fronts only and continue with the original rears until they wear out too. 2. Rotate the tyres front to back every 10,000km's also have them re-balanced every rotation, then all four tyres will be wearing at the same rate and would be replaced at the same time.