Thank you for your question.
When it comes to expenses that could have an element of personal use, one has to be reasonable in your approach. If you have a log of private mileage, that is helpful. If you don't then I suggest you start having a diary where you log in private use mileage and this can be used to work out percentage for business and private use with better accuracy.
I suggest you allow for 10%-15% for private use and consider up to 85% of total motor expenses as business to include fuel, repairs, insurance, road tax and capital allowance.
As far as dead mileage is concerned, if you are treating 85% of expenses as business then it covers both engaged and dead mileage. Driving without customer on return journey is still business.
HMRC appear to accept dead mileage as 52-53%.
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