Hello PlumPlumb, Thanks for posting on the Community page,
If you aren't using the simplified expenses method/pence per mile/tracking mileage and you want to account for all your expenses relating to your vehicle separately then you would have to use Travel expenses category for this transaction. More information on simplified expenses can be found here and also more detail on allowable and disallowable expenses in QuickBooks Self Employed can be found here.
Any questions at all let us know we'd be happy to help further.