IN my opinion, the company does not reserves right to do so. If they were to complaint about the high consumption of petrol, they could set a rule as may be to fix the petrol allowance instead of claim by mileage, and ask the relevants staffs to acknowledge on the new rule.
Besides, if the employee manage to provide the detailed and truthful daily itinerary report for visiting which customers, on when and what time, the company has no right to complaint.
Overall, i can say the company shall take corrective action on themself for not prefix the petrol claim rule or clearly stated their company policy on it early on, instead of suspend their employee. They shall clean up their front door rubbish, before blamming other parties.
The company is absolutely wrong!