As someone who teaches public economics at the graduate level, I am astounded that many on this board are more thoughtful about the nuances of our income tax system than top-level graduate student who go on to design parts of this system.

I will make one point and that is the US income tax code is based on something called the Haig-Simons definition of income, which essentially says that anything that increases our ability to consume is taxable as income. We then exempt some forms if H-S income because either we want to socially engineer a certain type of behavior (e.g. deductibility of home mortgage interest payments and charitable donations) or because it is too complicated to measure or monitor (the exemption of the value of the imputed rent that homeowners pay themselves for living in their own home).

My guess is the both brp and gemac are correct, there should be a consistency between taxable income and deductible donations, but it's probably too complicated to bother because assigning a value to miles would prove very complicated.

This topic really interests me. I think I'll put a post on my blog about it.