25p per mile = 5p saved in corporation tax (20% of 25p)
45p per mile = 9p saved in corporation tax (20% of 45p)
Therefore, spending an extra 20p on mileage to save an extra 4p in tax means that the business is 16p (20p less additional 4p tax saving) worse off in terms of actual cash spent.
Of course, if you’d like more of an explanation, please get in touch with your Client Account Manager.