Expenses for business trips using one’s own personal car can be claimed in the tax return. Either the actual costs are deducted as business expenses or a fixed rate per kilometer traveled is applied. The following standard rates per kilometer have been set by the Federal Ministry of Finance: 0.30 euros for cars, 0.13 euros for motorcycles and scooters, and 0.08 euros for mopeds. The rate increases by 0.02 euros for cars and 0.01 euros for motorcycles and scooters for each person accompanying the driver on the trip for professional reasons.
Source: Wikipedia, edited (only available in German)