I suspect the true reason is expense based, with a bit of old habit thrown in for good measure. Most tanks are oddly shaped (to fit the confines of the space i

I suspect the true reason is expense based, with a bit of old habit thrown in for good measure. Most tanks are oddly shaped (to fit the confines of the space into which they fit) which means that they empty non-linearly. Most fuel sensors are a float on a pivot arm, which means the sensor also operates non-linearly. While it is quite technically feasible to compensate for both non-linear functions and produce a gauge that reads to the exact tenth, or hundredth, of a gallon/liter remaining in the tank, doing so would add extra expense. And while the expense per car might be small (a few dollars more per car), over the course of a years production run, a few dollars per car times several hundred thousand cars, makes the sum total a very large amount. Plus, the vast majority of drivers very likely only really care about "when do I need to add gas" and we have a simple gauge marked in quarter increments, or a set of 6 or 8 bars that slowly disappear.