A basic deduction of 24 cents a mile will be permitted for automobiles used in business in 1988, the Internal Revenue Service announced. That is up from the 22.5-cent basic rate allowed for 1987. The basic rate applies to the first 15,000 miles driven in a car that is not fully depreciated. The rate for miles above 15,000 and for all business use of a fully depreciated vehicle is 11 cents a mile, the same as last year. The IRS said the mileage deduction for charitable activities remains at 12 cents a mile. A rate of 9 cents a mile remains in effect for deductible moving and medical expenses.