How the Gas Gauge works

[Gaijin]

Just got back from a 3,500 mile trip on the RX3, and that left me with a LOT of time staring at the gas gauge and its mysterious dots that appear, disappear, flash, and whatever else. What do they mean, when do they change, and -- for crying out loud, how much gas do I actually have left?


There is a pattern, and I have figured it out.


First, let me clarify a couple of things. The gas tank does NOT hold 4.2 gallons of fuel, despite what the printed specifications say. The tank itself may be able to hold 4.2 gallons if it was empty, but it is never empty, it's got a fuel pump and all the assorted plumbing and whatnot in there. So that means the real, true capacity of the fuel tank, for holding gas, is approximately 3.7 gallons.


Second, the gas gauge reports the contents based on the use of five "dots" or lights. Those dots are only really informative when they change state; i.e., when a dot disappears. Until then, you don't really know how much gas you have, you only know that you have "at least" as much gas as the next dot allows. Meaning: the top dot will stay lit, showing "full", until the first gallon is completely used up. So the tank will show as "full" when it has anywhere from 2.7 to 3.7 gallons in it. That dot won't disappear until an entire gallon has been used. So you should not think in terms of how many dots are full as being how much gas you have left, you should always be thinking in terms of the next dot down on the gas gauge. Or, put another way, when the tank shows "full", that doesn't mean you have 3.7 gallons available; instead, it means you have at least 2.7 gallons available, and could possibly have as much as 3.7 gallons.


Third - you can't just use the odometer to know when your tank will be empty, because the RX3 gets wildly different MPG figures depending on how fast it's being driven (or, more appropriately, how hard the engine is working). I've seen entire tanks as low as 39 mpg, and I've seen as high as 75 mpg when riding long distance offroad at 20-30 mph. So my advice is: don't go thinking you're getting 65+ MPG! Use the dots to determine gallons, and always assume that you're going to get 40 mpg and you won't run out.


Fourth - my observations apply to the 2018 RX3. I don't know if all models use the same gas tank and same fuel gauge, they probably do, but please don't hold me responsible if your non-2018 model runs out of gas when using my observations!


Fifth - I've seen a thread on here where people were going in and modifying the float level for their gas gauge. If you've done that, obviously none of my observations here will apply.


So, with the preliminaries out of the way -- here's what those dots mean:



Where this becomes most important is with the bottom flashing dot (with the "E" in it). If that bottom dot is not flashing, if it's solid, then you've still got 1.2 gallons left (well, see the note below). If it's flashing, then you've got less than 1.2 gallons left. I've seen some folks say that the light starts flashing when there's 2 gallons left in the tank, and that just isn't so, at least not on my bike. When that light starts flashing, you're down to the last 1.2 gallons available.



Now, about those dots: they generally, on flat ground, will disappear according to the scheme I've shown above. But, if you're going up or down a hill, that will change when the dots disappear (or, even, reappear, sometimes). When going up a hill, the fuel all piles into the rear of the tank, making the system think it has more gas than it really does, so the dots will stay lit longer (or may even reappear, if one has disappeared already). When going down a hill, the gas piles into the front of the tank, and the float may be lower than it otherwise should be, making more dots disappear than are really reflective of what's in the tank. So the dots are at their most accurate on level ground.


Bottom line: when that last dot starts flashing, you had better fill up within 48 miles (at the most) or you could very well run out of gas. If there's no gas within 48 miles, then SLOW DOWN. At 45 mph, you should be able to eke out maybe as much as 60 to 70 miles before the tank runs dry, depending on headwinds or if you're going uphill etc.

[calvarez]

Very cool, I'm going to make a card from this, and put it in the window of my tank bag! When I do the 300cc kit, I was planning to also fully empty the tank, then measure it. We can compare.

Also, for sure I am going to call you if I run out of gas and expect you to bring me some!

[calvarez]

Thanks for your efforts, if you'd like a laminated card, I made a couple extra, PM me with an address.