Question about expenses

OK novice freelancer alert.. just wondering how it works with mileage claims when working as a freelancer (Marketing position). Do I keep receipts and claim petrol as my expenses, or charge the 45p per mile to my employer for them to claim it back. Only thing is it's a 160 mile trip. Any advice very welcome. It's a commission based role by the way, no basic rate (happy to do this one cheap for experience, and re negotiate once I've proved my worth but don't want it costing me)