CO2 emissions. The level of business mileage is not relevant. The greener (environmentally!) the car, the lower the charge.Check your position to confirm that the company car is still a worthwhile benefit. It may be better to receive a tax-free mileage allowance that could be up to 40p per mile for business travel in your own vehicle.
Where private fuel is provided, the charge is also based on CO2 emissions. You should review the arrangements to ensure no unnecessary tax charge arises. If you have opted out of free fuel during the year, the charge will be proportionally reduced. However where you opted in during the year a full charge is applied.