The optional standard rates used to calculate the deductible costs of driving a vehicle for business will drop to 44.5 cents per mile in 2006, after having risen to 48.5 cents per mile during the last four months of 2005. After Hurricane Katrina, the IRS took swift steps to increase the rate from its original 2005 rate of 40.5 cents to 48.5 cents per mile to account for rising gas prices.