Here’s one for all you math guys.
I have a class 8 vehicle (semi truck) with two 163 u.s. gal fuel tanks. Because the tanks are cylindrical, when filled, the fuel takes up 95% (154.85 u.s. gal) of the tank giving a combined total of 309.7 u.s. gal.
Now, I attempt to track my fuel mileage assiduously, but with summer here, and no source of cooling for the interior of the truck while stationary (e.g. when I’m sleeping), I’m forced to idle the truck - which consumes an industry average of around 1 u.s. gal per hour at idle.
To determine the exact consumption rate at idle, I filled the tanks to the aforementioned levels, and sat, at idle, for 5.5 hours. I then topped off, expecting to add no more than 5 or 6 gallons. I could only put in 2.14 u.s. gallons, which couldn’t be right - as I have said, the industry average is 1 u.s. gal per hour and I had only apparently burned .4 gal/hour.
The only parameter that changed between fill up and top off was fuel temperature - excess fuel in the engine is cycled back to the tanks. I filled up with fuel at 70 degrees f and by the time I had topped off 5.5 hours later, the fuel in the tanks had heated to 135 degrees f. So the fuel had expanded by an undetermined amount, thus skewing my readings.
I will conduct the test again factoring in this effect and will post the results, but I wanted to see if anyone could anticipate my findings. So the question is this…exactly how much fuel did I actually burn while I sat idling my truck?
One factor you may need to know; outside air temp was 95 degrees f the entire time.