2004 Automobile Standard Mileage Rates  New Rates   Beginning Jan. 1, 2004, the standard mileage rates for the use of a car(including vans, pickups, or panel trucks) will be 37.5 cents a mile for all business miles driven, up from 36 cents a mile in 2003; there are different rates for medical, moving or mileage related to charitable services.   Taxpayers who use no more than four vehicles at the same time for business purposes may use the standard mileage rate, starting in 2004. Currently, those using more than one vehicle at a time cannot use the standard rate at all, leaving them to track the actual expenses for each vehicle.   "With this change, more than 800,000 businesses will become eligible to use the standard mileage rate," said IRS Commissioner Mark W. Everson. "This reflects our ongoing interest in reducing the burden for businesses to comply with the tax laws."