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Here’s a travel tip you don’t hear every day: Can you guess how I upgraded all my ticketed airplane seats from basic to premium economy for free? I lost 20 pounds. With two inches taken off my waist, the spacing between me and the arms of my seat has increased to a level I could normally only get by paying for an upgrade. My personal journey of weight loss inspired some research that can benefit any cost-conscious flier, particular those who, like myself, have experienced being “gravitationally challenged.”

Passengers frequently rage at the airlines for restricting seat sizes, and with good reason. Data gathered by former Consumer Reports Travel Letter editor Bill McGee, supplemented by some research I did using Seatguru.com while wedged into seat 47Q, shows that the smallest seat width in coach class across American, Delta, and United Airlines declined around 15% over the past three decades.

But examining airline seat size decline in isolation understates the march of rear-end pinch. Even if seat size had stayed constant, flying would still feel more uncomfortable because the proportion of Americans who are overweight swelled from 55% in 1989 to about 73% today. Clearly, the nation needs a new statistic to assess the combined impact of smaller seats and bigger (cough) seats. I therefore charted the ratio of minimum coach seat width to the proportion of U.S. adults who are overweight.  At the risk of being cheeky, I label this variable the Airline Seat Size to Passenger Heftiness Index Tracker (ASSPHIT), which reveals a startling 36% decline in posterior comfort over the past three decades.

The extraordinary ASSPHIT of the first class section is mainly for the corporate traveler; what’s the best coach class choice for the corpulent traveler? Delta Airlines, with minimum ASSPHIT of .23 (and even better ASSPHIT on most of its airplanes) is your best bet among legacy carriers. In contrast, one of the seat configurations of a particular United Airlines narrow body jet offers a minimum coach width of just 16 inches. Even if you have a narrow body yourself, that’s one tight ASSPHIT.

As for my recent success at improving my personal ASSPHIT, the hard fact is that most people who lose weight gain it back again. Given how often I fly, I am hoping to beat those odds. I know I have your good wishes, particularly if you are crammed into the seat next to me.

[Cross-posted at The Reality-Based Community]

Keith Humphreys

Keith Humphreys is the Esther Ting Memorial Professor at Stanford University. @KeithNHumphreys