Calculator: Drive your Car - or Rent a Car?
Most people drive their own vehicle on road trips, but on a long road trip, it could be less expensive to rent a car. It's not only the difference in the fuel economy of a more efficient rental car, but also the cost of maintenance (oil changes, brakes, tires) but also depreciation in the value of the car itself.
How to compare:
CarMax has a good calculator to compare the cost savings of fuel economy, comparing two vehicles. Compare your car against something you would rent, and then against the cost of renting the car.
You can rent a car at most car rental companies through Costco's Car Rental and get a better rate.
If you don't know your car's maintenance costs, assume about $75 for an oil change / tire rotation.