Does it seem like your gas gauge stays on "Full" for a long time before lollygagging down to the halfway mark? Then does it skip almost immediately from half to one-quarter? Have you driven seemingly forever with the fuel warning light on?
A retired auto company engineer, who requested anonymity, confirmed what many have long suspected and observed: Fuel gauges are, by design, pathological liars.
You wouldn't have it any other way.
And here's proof: When digital gauges hit the scene, this engineer programmed the fuel gauge to say "Full" when it was at its maximum 18-gallon capacity. When the fuel level dropped to 17 or so gallons it said (what else?) "17." Immediately, the company was deluged with complaints that the car was producing poor fuel mileage. The company tested the "problem" cars: All made the advertised miles per gallon. After talking to consumers, this engineer figured out the problem: The drivers weren't complaining about miles per gallon, they were irritated that the gauge didn't stay on "Full" for very long. (This car was sold largely to retired people who have time to worry about such things.)
The engineer came up with a solution: He recalibrated the gauge to skip "17" and go straight from Full to 16. The complaints evaporated.
The same engineer revealed that his company required its cars to be able restart after being parked on the super-steep Lombard Street in San Francisco with the gauge showing "E." Since there had to be enough gas to cover the fuel pickup on Lombard, on a flat road there were two usable gallons left in the tank.
Running on Empty.
I put this to the test since I'm one of the few people who gets irritated when his car doesn't run out of gas soon after the low-fuel warning light illuminates. Several years ago I ran a Dodge Grand Caravan more than 110 miles after the low-fuel light came on. With a 20-gallon tank on a vehicle that gets less than 25 mpg, that means the warning came on with at least four gallons of gas in the tank. This is not true for every vehicle every time: I've been surprised a couple of times, but that's what AAA is for.
However, if you're a long-distance driver, eliminating a fuel stop can save useful time. Or maybe you'd like to wait another 20 miles until you find a station that fits your needs. On an older car it may not a good idea to suck the last few ounces out of the tank as the dregs may contain contaminants or water. But that's not what we're suggesting: We want you to know you can go another 20 miles and still have a gallon or more of good fuel remaining.
Marketing concerns such as the one with the digital gauge means almost every gas gauge is non-linear. The gauge stays at "Full" even after you've burned off a couple of gallons. Also, if you have a 20-gallon tank, there are nine or fewer gallons in the tank when the gauge reads half. And, as I also proved with my test in the Grand Caravan, you probably can drive for 50 or more miles with the gauge saying "E." It went 30 miles with the fuel needle below "E," resting on its stop peg. The first time you try this, carry along a can with a couple of gallons of gas.
True Story of a Lie.
The first time I suspected gas gauges were serious prevaricators was during a drive from Los Angeles to Denver. As I approached Salina, Utah on Interstate 70 a sign warned "Last Services For 104 Miles." The Lincoln Town Car's fuel gauge said its 20-gallon tank was about 3/8th full. Assuming the gauge was accurate, my calculations said I'd have at least a 50-mile cushion. Importantly for me, if I could refuel in Green River rather than Salina I could make my final destination without another stop. (As a race driver, I'm always trying eliminate pit stops.) But about 20 miles down the road the "low fuel" light came on and the gauge was almost at "E."
This stretch, I later discovered, was the longest without a gas station on the entire 48,000 or so miles of the Dwight D. Eisenhower National System of Interstate and Defense Highways. Did I mention that it was just past midnight? I'm offended if you think that I spent more than 0.01 second pondering turning around. Instead I wondered if I would save a useful amount of fuel by switching off the engine as I crested the many Utah mountains and coasting as far as I could, even though I knew this was a risky maneuver that I wouldn't recommend to others, and the Town Car would surpass 110 mph (double the speed limit at the time) in Soapbox Derby mode. Or should I put just put it in neutral?
The warnings from the Lincoln's dash got progressively dire. For the last 20 miles before Green River the digital dash was quoting the Book of Revelations to describe what would happen if I didn't stop for gas, now. The way I looked at it, every mile I made was two I didn't have to walk. When I made it to Green River, I figured the tank would accept at least 19.9 gallons. But, nope. It wouldn't hold any more than 18.8. Liar.