Let's assume the cost per mile to operate a vehicle is independent of who owns the vehicle. If the current revenue does not exceed that operating cost, it doesn't matter who owns the vehicle. The income it produces is insufficient.
Ok. But independence is the assumption I'm challenging. I buy 100 tires, all the same size and model, has to be cheaper - per tire - than you buying 4. With efficiency, can't I get the revenue to exceed the operating cost at some point? (I don't know if this is Uber's plan or not - just hypothesizing).
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The thing to remember, Uber is not offering something new. People have been riding around in cars for more than a century. What Uber wants to do is sell a new way to pay for it.
Yes, there are some savings to be had through mass purchasing. We can ask Wal-mart about that. Whether you can get revenue to exceed cost with these savings is the real question. Not only are you buying tires, you are buying the car, too.
Rental agencies buy a lot of cars. Enterprise Rental puts their customers in a car that's no more than 2 years old. This is because their fleet is put up for sale after 30,000 miles. It's no coincidence that the basic bumper to bumper warranty expires at 36,000 miles. This means they don't spend money on repairs for things like power window motors and air conditioner compressors. The sale of used cars recovers some of the original investment and life goes on. They certainly don't buy a lot of tires.
I don't think Uber can take advantage of this cost saving strategy. If you had a driverless car and wanted to sell it, so it could be replaced with a newer one, who is going to buy it? Other than wear and tear on the driver's seat covers, there's no cost savings in fuel and maintenance, over the driven car.
There is another limit on driverless car revenues, which they share with every service industry, and that is time. There are only 24 hours in a day. There is a finite limit of how much service can be provided in that time and the arithmetic is fairly simple. The market determines the revenue per minute and the clock determines the charge. The answer to your question, "Can revenue exceed operating cost at some point?" depends on the market.
Increasing service(longer hours, more cars, etc) works only up to a point. Too many cars means less for everyone. Starbucks learned that lesson. Raising prices makes alternative transportation more viable. Apparently Starbucks has not learned that lesson.