From: David Parsons
The difference between topping off every day and when the tank is
empty is twofold.
1. Convenience. Do you really want to buy gas every single day, and
will you stick with it. I personally would find it really annoying to
always be buying gas, and it would really cause me to hyper focus on
the price of gas.
Convenience is why my existing style is to run the tank out and fill it
up again. But it's been so flaky recently I'm considering the
inconvenience to be the lessor evil.
2. You are dollar-cost-averaging your gas either way, but by topping
off every day, you are betting that there is more up/down volatility
day-to-day than week-to-week. Gas prices are trending upward for now,
but they will level out to a new stable point and it won't really
matter if you do it every day or when the tank is empty.
If it does reach a new stable point, I can always go back to running the
tank out & filling up.
As to the cheaper gas, you'll need to calculate how much out of your
way it costs you to go there. Map your route from the point where you
deviate from your normal route and where you rejoin your normal route.
That way, you know how the distance for the gas compares to not
getting the gas. For example, if your normal segment is 2 miles, the
cost is 20 cents, and if the gas route is 3 miles, your cost is 30
cents. If the cost of the route is more than you save on a tank of
gas, then it's not worth going out of the way for the cheaper gas.
Figure 12 gallons at $3.50 for the normal, and $3.30 for the cheaper.
$42 for the normal versus $39.60, the difference is $2.40. If the
differential remains the same as prices go up, you can go up to 24
miles out of your way (minus your normal route) and save money at 10
cents per mile. After that, you end up paying more. You also have to
consider the time that that adds to your commute.
That's where I'm trying to figure out if my math is valid. How many
miles can I afford to detour for the amount of gas I know I'm going to
buy for a given savings per gallon.
I've got a pretty solid idea how many gallons it takes to top off from
7/8, 3/4 ... E and I think a pretty good grasp of how much will be left
in the tank after I drive 'X' miles, so I know roughly how many gallons
I'm going to have to buy to top off when I get there.
The way it works out, the location is directly on the route between my
home and my apartment at school. No detour at all required to fill up
there if I'm going back and forth. I've already begun topping off every
time my route takes me past the station.
While staying in my apartment, it's about 8 miles round trip to the
station and back. It seems to still makes sense to go over there when
I've got to fill the tank up and can't wait until I'm headed back to
Raleigh to get gas.
I'm going to be keeping the apartment here in Asheboro while I do my
internship (it would cost more to let the lease terminate and get a new
apartment when I return for my final semester). I'll mostly be staying
at my home.
Going back and forth between home, hospital and internship it's
definitely too far away to detour there to fill up, even filling the tank.
But, I will occasionally need to come back here to my apartment during
my internship. If I come from where I'm going to be working, I have a
choice of two routes - one passes by this gas station, the other
doesn't. The route that passes by the station is about 10 miles longer
than the other route.
That's where I'm trying to figure out if it makes economic sense to make
the "detour". Commute times are about the same when you factor in the
speed limits on the different roads. The shorter route has more
stretches where the speed limit is 45 or 35.
Looks like if I need half a tank or more by the time I get there it
makes sense. Or I might take the short route down and the longer route
where I can fill the tank up on the way back.
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