The best time to replace a company vehicle is usually when its rising costs and disruption begin to outweigh the benefit of keeping it.
Age is only one clue
A newer high-mileage vehicle may be closer to replacement than an older car used lightly. Service history, condition, warranty position and expected future mileage all matter.
Look beyond the monthly payment
A vehicle that is fully paid for can still be expensive if fuel use, repairs and downtime are increasing. Compare the whole cost over the next year or two rather than focusing only on finance.
Think about business changes
A change in staff numbers, routes, clean-air requirements, payload or customer work may make a different vehicle more suitable even if the existing one remains reliable.
Plan the change rather than react
Ordered replacement is usually easier than replacing a failed vehicle at short notice. Build likely replacement dates into your fleet budget and review them at least annually.