Specifically, i have a 2011 Hyundai Elantra, but would be interested in other cars as well.
One of the functions on the dash after “MPG” is “Range”. Now, the MPG is a cumulative average since the last refill. But does anybody know how the “Range” is calculated?
I just filled up and, pulling away from the station, getting 8 mpg, the range said 380. Once up to speed, and now with 30 or so mpg cumulative on this tank, the range said 379. So clearly it’s not really based on the current cumulative mpg, or it would have shown something like 100 at first, then 380 later.
I can think of basically three ways to do this calculation, to varying degrees of accuracy and usefulness:
When the tank is filled, the range just resets to 380 as a typical range, and counts down based solely on the odometer. Not very accurate, but easy.
Calculate based on the average mpg for that tank, and the amount of gas left in the tank.
Calculate based on the average mpg since the engine was started, or that trip, day, etc. and the amount of gas left.
Obviously, since its already calculating mpg, it has some method to measure or at least approximate fuel flow, and so calculating how much fuel remains is just a matter of fuel flow and time.
2 and 3 are the better solutions, but neither of these seem to be what my car is doing.
Does anybody know how it is typically calculated, for most cars, or for a 2011 Elantra specifically?
From experimentation, it’s #3 for my Dodge truck anyways. It has some sort of average MPG figure kept in the computer separate from the average MPG reading that is displayed (and that you can reset). When I changed my battery and the computer reset, the DTE readings jumped all over the place for the first couple of tanks.
I don’t know precisely, but my car at least uses a moving average of the last 10-20 miles. Or, it may be using an exponentially decaying average which gives almost no weight to conditions 10-20 miles ago. This latter approach is especially easy since it requires almost no memory and no detection for filling the tank or the like.
You are correct that every modern car can compute an instantaneous MPG since it knows both the fuel flow rate (via electronic fuel injection) and the speed.
The first car I ever had that had one of those based it on the MPG for the last 20 miles, according to the manual. It seems to me that the others I’ve had do something similar. I customarily leave it on “range”, and consult that rather than the fuel gauge.
“This mode indicates the estimated distance to empty based on the current fuel in the fuel tank and the amount of fuel delivered to the engine.”
Somewhat less than helpful, and quite possibly written by someone completely unaware of the actual algorithm involved.
On my other car I have a “Scanguage”, a wonderful piece of technology that tells me all kinds of stuff. Among others, it shows instantaneous MPG, current trip MPG, and current tank MPG, as well as distance-to-empty. The DTE is calculated based on the current tank MPG, which may or may not be the best method, but at least it’s obvious what it’s based on. Fill the tank, pull away, and as you accelerate away from the gas station, with correspondingly dismal tank MPG, the DTE is like 100 miles. Get up to speed, and it increases. Sit at a long red light and you can see it gradually go down.