Monday, July 14, 2014

The Fuel Light

My Subaru Forester has real-time fuel economy data, and with that, comes an estimate of how many miles I have left on my current tank of gas (rounded to the nearest 10). So, I can take it all the way down to nearly zero before I refuel, right?

In theory, yes. The thing is, though, after the number drops from 40 to 30 (since it's rounded to the nearest 10), the next step below 30 is ----:

By the way, this is well after the traditional fuel gauge went to E. I think that happened with around 40 miles to go. And, the fuel light went on even earlier than that, with still 70 miles to go:

I refueled less than 10 miles after the fuel range indicator went from 30 to ----, and my 15.9-gallon tank took 15.6 gallons. Close! But not that close. I only took it that close because I read that the indication of 30 miles is reasonably accurate, such that you're good for at least another 20 miles after the display goes from 30 to ---- under normal driving conditions. And, I knew I'd be driving by multiple gas stations in another 5 to 10 miles. (Obviously, if you try to go the full 30 after the display goes to ----, that's pretty risky.)

I almost never took my Honda Civic this low on fuel - maybe only saw the fuel light two or three times a year, and only once in the life of the car did I run the fuel gauge all the way to E. I guess I'm more willing to do that with the Subaru since it actually tells me how many miles I have left on my tank (up to a point).

The fuel light in the Subaru comes on with 2.6 gallons remaining, according to the manual. That actually seems kind of early to me, so I'm pretty comfortable taking it down to the fuel light most every time, at least when I'm driving locally. On longer road trips, I'll usually refuel earlier than that.

By the way, continuing the topic of how accurate my car's real time fuel economy data is: my car reported 30.0 mpg for that tank, but my calculator reported 29.4 mpg (459 miles / 15.6 gallons). I'll wait another two or three more tanks before declaring that the Subaru's fuel economy numbers are inflated.


Spartangoogle said...

Since I rarely let my car drop below 1/4 full, you must have inherited this risky behavior from someone else.

Leah said...

Speaking of your mileage calculations, I would like to see an analysis of the effect of cruise control. Please and thank you :)

Chris Allen said...

So, this is purely qualitative, but here's what I've learned about cruise control so far:
- On flat roads (i.e. most of Florida), the car's cruise control is more fuel efficient than I could be.
- On hilly roads, cruise control on automatic transmissions has a tendency to push the RPMs really high going up hills to maintain speed, and that's not particularly friendly to my MPG. I've gotten slightly better MPG numbers controlling the throttle myself up and down hills, letting the car speed up a little going downhill (coasting in neutral where practical) and slowing down slightly going up hill.