MAX Update No. 65: Measuring Fuel by the Tablespoon

Reader Contribution by Staff
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Sure, your car came with a fuel gauge, but MAX didn’t.

After-market fuel gauges are tricky things, which is why nice ones cost a hundred bucks or more. They have to translate the electrical signal from a contraption inside the fuel tank into useful information about the quantity of fuel remaining. Well, OK, it’s not really all that tricky, and I don’t know why fuel gauges cost so much–the contraption in the tank is a variable resistor (think volume control knob on a radio) with a float attached to it, and all the gauge has to do is measure that resistance; but instead of reading out in ohms, it reads from E to F.

One thing store-bought fuel gauges have in common is they are remarkably vague. At least they give you fair warning that they’re not going to give you specific data about your fuel quantity; instead of reading in gallons or liters, they have a few lines on the face, showing fractions of a tank; halves, quarters, and usually eighths. We all know not to take that literally, we don’t see the needle hit the 3/8 line and say, “Yep, I’ve got a 15 gallon tank, so I have five gallons, two quarts and a pint of fuel remaining.” We don’t even believe the F and the E, we learn from experience if our own car’s fuel gauge says F for the first hour of driving after we fill it, and if it’s good for another 20 miles after the needle reads E.

There are a number of practical reasons for keeping it vague. The float has to reach the end of its downward travel while there is still some fuel in the tank–that is, once the float is resting on the bottom of the tank, it can’t read any lower than E whether it’s sitting high and dry in a completely empty tank, or if it’s half-submerged in fuel in an almost empty tank. The same goes for upward travel, the float will hit the top of the tank (and the gauge will stop at F) before all the air space is filled up with fuel. Also in practice, the fuel sender arm is set to stop a little before the float touches the inside of the tank, to keep it from rubbing (and wearing) against the tank when fuel sloshes around while driving. Also, the fuel gauge is “dumbed down” so it will respond slowly to changes in the float position, so the gauge doesn’t show different levels as the fuel sloshes; so it doesn’t show more fuel in a right turn than a left turn, for example, or go up or down when you put on the brakes.

Besides, most drivers would find it distracting if their fuel gauge notified them every time a cup of fuel went through the engine; most drivers, but maybe not you and me. Most drivers want their fuel gauge to tell them when it’s time to look for a gas station, and I want mine to tell me how much fuel I’m using. Also, I’m pinching pennies, I’m down to my last discretionary two grand in MAX’s ten grand budget, and I’m sure not going to blow a hundred of that on a gauge that isn’t going to give me the information I want; maybe I can find a cheap one at a garage sale.

So I was at a garage sale, rooting through the usual automotive bric-a-brac, thinking “There’s a variable resistor in the fuel tank which converts float level to ohms, 10 ohms at the top and 70 ohms at the bottom, and a gauge on the dash that measures ohms kinda vaguely and reads out F to E with some hash marks in between, if only there were an inexpensive way to measure ohms precisely and…hey, wait a minute!” A barely used digital ohm meter for a dollar, woohoo!

Ohmmeters of this nature are available from lots of sources, brand new for less than a ten-spot. Like most of them, this one runs off a nine-volt battery, which I removed and replaced with wiring to MAX’s 12v electrical system (hey, I was only risking a buck and most of these little electrical widgets are fairly tolerant of voltage variations) and it’s working just fine. I screwed the ohmmeter to the dashboard, right next to the water temperature gauge, and the rest is history.

Now I have a fuel gauge that reads in tenths of ohms, and even though it doesn’t translate that to fuel level for me, I can remember that 10.0 ohms;is F and 70.0 ohms;is E, and I have 600 discrete measurements in between. I don’t yet know exactly how much fuel 18.3 ohms represents, but I know it’s about five tablespoons more fuel than 18.4 ohms, and that’s close enough for MAX.

Photo by Jack McCornacBrowse through all MAX Updates.

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