: It actually works reasonably well.
Previous research by the author (emzed, 2007, 2008) showed that for purposes of estimating miles to exhaustion, both the Reserve indication and the BC’s Miles Remaining feature are about as accurate as darts thrown by a drunken blind hippopotamus. The BC’s MPG feature in theory could be used to estimate miles remaining but only if the values it presents are reasonably consistent if not entirely accurate. It seems incredibly unlikely to be the case, but in the interests of science, this paper compares the BC’s MPG against actual mpg calculated for each fill-up.
2007 BMW K1200GT, blue.
From 6/13/2007 through 6/11/2008 the following data were recorded with each refueling, except when I forgot:
- F: Fuel to fill tank.
- T: Tripo miles at refueling.
- M: BC reported mpg.
This resulted in data for 82 fuelings. From these data, actual mpg was calculated as T/F.
Figure 1 plots actual versus BC MPG as one blue diamond per refueling. The dashed line represents perfect accuracy. As can be seen, the BC tends to over-estimate the mpg, but fairly consistently so. A least-squares linear regression found that you can predict actual mpg from the BC MPG by using the following equation:
Actual mpg = 0.897 * M + 3.266
The line corresponding to this equation is shown as a thin blue line in Figure 1. The predictions from this equation are accurate to plus-or-minus 1.3 mpg (at 95% confidence). Note that about 0.9 of that 1.3 can be attributed to the granularity of the BC display (e.g., it shows either 46.1 or 47.0 mpg, but never anything in between), suggesting the underlying computation of the BC may be consistent to within less the 0.5 mpg. Not that it helps you any.
Astonishingly, it appears that the BC might actually be useful for something. The display of the Reserve indicator can mean you have anywhere from 47 to 73 miles remaining to exhaustion, if you get the mileage I typically get (emzed, 2008). The BC Miles Remaining feature tells you…, well, I don’t know what the hell it’s telling you. However, if you reset your BC MPG with each refueling, the BC MPG feature along with the formula above can be used to tell you the total miles you can expect from your tank to an accuracy of plus-or-minus 8 miles (6.3 gallon fuel tank at +/-1.3 mpg; ). The estimate ranges from this formula for each BC MPG reading are provided in Table 1. Using the BC MPG in this way is actually more accurate than using the Reserve indicator.
Alternatively, if playing chicken with your gas tank is not your idea of thrills, the above formula can be used to provide the minimal number of miles remaining when the Reserve indicator appears (i.e., under the assumption that only 1.1 gallons remain). This is provided in Table 2.
When using these tables, remember that they assume your MPG remains steady. If you’re getting 50 BC MPG at 100 mi, and then hit a mother of a headwind for the next 150 miles, dropping BC MPG to 39.1, well, you’re not going to make it that far. And the usual disclaimers, these results are based on the performance of a single rider on a single bike, YMMV, blah, blah, don’t sue me if buzzards have you for breakfast on a lonely desert road.
emzed (2008). Irregularities in the Point of Reserve. http://www.k-bikes.com/forums/showthread.php?t=13313
emzed (2007). Demonstrated Usable Fuel Capacity. http://www.k-bikes.com/forums/showthread.php?t=9886
1. A quadratic regression resulted in the squared term of the equation being marginally statistically significant, but it only reduced the standard error of regression by 0.02 mpg, to which I say, “Who cares?” I mean, it’s not like I’m really anal or anything.