Firstly the rate you use is at your own discretion. To have a reimbursive travel allowance would be a good idea. This means you pay employees per km. traveled. Should you pay at a rate of less than R3.05 per km and the employee travels less than 8000km for business during the year, that would be a tax free reimbursement.
Should you pay more than R3.05 per km or should the employee travel more than 8000km business during the year, you will be able to claim against the allowance on the same basis as with a regular travel allowance. Logbook required etc. There is no limit that you can claim, as long as you can prove it and have a detail logbook.