Many auto dealerships have financing relationships with outside lenders (i.e. Wells Fargo, Ally Financial). The outside lender has an agreed upon rate locked in with the dealership prior to your arrival. This is negotiated between the two parties prior to ever looking for a car or truck.
When you apply through the dealership for financing, the dealer forwards your financial information, gets an approval from the financing lender at 3.5%, then turns around and quotes your 4.8%. If you accept, the dealer will profit from the sale of the vehicle as well from the financing.
When your company is quoted an interest rate that seems high compared to other rates on the market, tell the dealer you will not pay this rate. Tell the dealer their rate is too high. Get another bid from your own financing. Compare the rate to your company’s interest rate on it existing line of credit. Make sure you do not automatically finance through the dealership unless you have gotten several bids and feel comfortable you are not getting cheated on the interest rate charged for the vehicle.