IRS announced new standard mileage rates for 2012 in Notice 2012-1:
55.5 cents per mile for all miles of business use (business standard mileage rate).
23 cents per mile for use of an automobile …for medical care
23 cents per mile for use …as part of a move for which the expenses are deductible
14 cents per mile for use of an automobile in rendering gratuitous services to a charitable organization
For medical and moving, this is a small decrease from 23.5 cents in the second half o 2011, which was up from 19 cents in first half.
Most ministries use the official IRS rates for reimbursing staff for personal use of their vehicle on ministry activity. If you go over that $0.555 rate, reporting and taxability issues get complicated.