Across four days, my colleague Andrea and I tested prices on 80 different routes around San Francisco. Using both test and personal accounts, we entered the same start and end points into Uber and Lyft within moments of each other.

Their base fares differed by 14 percent on our test accounts — a gap of $4.15 per trip.

The thing is, neither app was consistently cheaper. They flip-flopped. So you really do have to check both.

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In my tests, the bottom-line prices — after promotional discounts — diverged even more dramatically for Uber. Its discounted price varied by 25 percent between my test and personal accounts.

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The same economists studied how people actually use the Uber and Lyft apps. They found only 16 percent of riders who open one app also open the other to check prices.

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A regular New York City rider who takes 100 trips a year could save about $177 annually by comparison shopping (though actual savings depend on trip lengths).

That's a lot of rides.