What time is it? Check the fuel gauge.

We visited the CERN during the science summer camp. We had been driving for an hour when the radio and the clock in my old car stopped working.
Sarah got worried we would not arrive before the center closes to the visitors at 5pm. She asked: "What time is it?". Turns out neither of us was wearing a watch. I answered: "Check the fuel gauge". Sarah looked at me as if I were not making any sense at all. An explanation was required.
"The fuel gauge was at that position when we entered the highway at 1pm. The car's clock stopped working when the gauge was down to that level, an hour later. We have been driving at constant speed so we can assume the gauge is decreasing at a constant pace. We can use the fuel gauge to estimate the current time."
"It is 3 o'clock and 45 minutes." estimated Mike.

Sarah kept silent for a few moments. If one burns fuel at a constant rate then the fuel gauge is like a clock. There are water clocks, atomic clocks, radio clocks, mechanical clocks, sun dials, star clocks, etc. Alex's clock is based on his fuel tank so it is some sort of water clock.

"How much time before our planet runs out of fuel?" Sarah then asked. A very good question indeed.