First: Appeal your case to the dealership management. Seek out the service manager or service director, and suggest that the two of you drive the car to demonstrate the concern.
Second: If the car has had aftermarket modifications, be prepared to detail those changes, with receipts for parts and service work. Give the dealership a chance to understand the mods, and explain how, in your opinion, they have affected (or not affected) the car.
Third: If the dealer still denies your claim, ask to meet with the factory representative. All vehicle owners should have that opportunity if requested. The factory rep may travel to the dealership only once/twice a month. Be patient but firm, and insist that an appointment be made. If the dealership refuses, call the vehicle manufacturer’s customer service department. The number is in your owner’s manual.
Fourth: When meeting the rep, again request a test drive, and be prepared to explain your case. Again, have any and all receipts available. Ask that a repair order be generated to document the visit. Take notes to make your own documentation.