Automakers around the globe spend millions of dollars on r&d each year to drive extra pounds from their vehicles.
Some have gone so far as to eliminate the spare tire from a number of new models, such as the hot-selling Chevy Cruze and Hyundai Elantra.
But while engineers are removing one spare tire, their customers and passengers have each been adding one of their own.
Just as the average Buick or Nissan or Dodge is shedding pounds quicker than the recently divorced to squeeze out an extra mile per gallon, the average American is packing an extra 225/60R-17 around the waistline, canceling out those weight savings in the real world.
According to the U.S. Centers for Disease Control and Prevention, the average American male between ages 20 and 74 now weighs 194.7 pounds, up 28.4 pounds since 1960 -- nearly the same weight as one of the four BF Goodrich Radial T/A tires one might find on a new Ford Mustang.
Meanwhile, the average woman between ages 20 and 74 has packed on an extra 24.5 pounds since 1960, or about the weight of a Michelin 215/65R-16 tire on a minivan.
Ford spent millions developing the six-speed PowerShift automatic transmission that debuted last year in its Fiesta subcompact, and cited the slushbox in March as one of the key components that helped the Fiesta and later the 2012 Focus achieve their coveted 40-mpg ratings.
Not only does the new transmission have more efficient gearing, but even with two more gears, it weighed 30 pounds less than the four-speed automatic that it replaced in the Focus.
Yet that remarkable engineering achievement and myriad others among the thousands of automotive engineers around the world are all but negated when Mr. and Mrs. Average American sit their fat butts in the front seats.
According to technicians at the EPA's National Vehicle and Fuel Emissions Laboratory in Ann Arbor, Mich., the extra 53 pounds Mr. and Mrs. Average American have packed on over the last 50 years might reduce vehicle fuel economy by as much as 1 percent, depending on the vehicle.
But while that 1 percent might mean pennies at the pump, the costs nationwide rack up like calories in an extra value meal.
Based on the total retail gasoline deliveries by refiners in the most recent 12 months available from the U.S. Energy Information Administration, 1 percent of the refined gasoline delivered in the United States over the past year is 3.6 million barrels, or 153 million gallons.
And at a nationwide average price of $3.29 per gallon, that's $503 million in additional fuel costs, despite a long starvation diet on the part of the automotive industry as their customers pack on the pounds.
While saving 30 pounds from a transmission in a Fiesta or even 137 pounds from an aluminum chassis in a Z06 Corvette are wonderful feats of automotive engineering, the industry's technological prowess is getting offset by Americans' lack of focus on their beltlines.
Not the one with door handles and window glass -- the one with the straining loops, the buckle and stretched-out holes.