On a longish trip today I was interested in whether I'd need to refuel before getting home, so I was looking at the computer's range estimate. I know this is only approximate, but even so I was really surprised at the variability of the numbers. At one point just after fuelling the estimate was over 500 miles, then it dropped suddenly to 438 miles, and a bit later still it was back up to 495 miles, before dropping back to around 460. In between those step-changes there seemed no pattern to how it changed, sometimes it didn't change at all for several miles but sometimes it gradually reduced as you might expect it to mile-by-mile. All in the space of around 30-40 miles actual distance covered. I expected this to be a fairly simple calculation for the computer involving the fuel quantity in the tank and the average MPG at the time, but it's clearly more complex than that, it was almost as if the range was recalculated from scratch periodically, with "dead reckoning" between those times. I can understand the 500 figure being incorrect straight after fill-up with the MPG reset and the fuel gauge being misled by the fuel being brimmed to the top of the filler neck; you'd expect it to change significantly as the MPG sorted itself out and the fuel gauge started to register the actual level in the tank, but I don't understand why it then increased back up to nearly the start value. I guess there're so many possible sources of error in both the measurement of the tank contents and the MPG calculation that you really can't expect anything reliable out of it.