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. 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. 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. On Wed, Mar 9, 2011 at 6:40 PM, John Sessoms <[email protected]> wrote: > Money's got tight, every penny I can save is important. > > With gas prices rising the way they are, I have a couple of ideas on how to > save as much money as I can on gas, but I'm not sure how to prove or > disprove it mathematically. > > I know there's some people around here who are better at math than I am. I'd > appreciate it if the engineering-scientific types would look at what I'm > thinking to see if it makes sense. > > Here's the deal. I average around 31 mpg in my car. That gives me about > $0.10/mile cost for gas at current local gas prices. It was $0.08/mile last > year. > > Currently, gas prices in the area are around $3.55/gal for 87 regular ... > except for one station that's currently at $3.35/gal. I'll get to that a bit > later. > > My style of driving has been to empty the tank before filling up. That gives > me an average range of around 400 miles and 12 gallons per fill up. > > Part I - I'm considering changing my style in order to minimize the effects > of rising gas costs. > > I figure that when gas prices are rising, it makes more sense to top off the > tank nearly every day, so that the tank is always nearly full at the lower > price (i.e. before the price gets raised again). > > The incremental change in the cost of the gas will be less than the change > from waiting till the tank is empty. The aggregate cost for a given volume > of gas should be less if you buy gas more frequently when prices are rising, > and if you run the tank all the way out before filling up when prices are > falling. shouldn't it? > > Or am I missing something obvious? > > Part II - I will be making approximately a daily 60 mile round trip commute, > 7 days a week for the next 8 weeks, with a 120 mile round trip M-F for the > following 5 weeks - my internship & radiation therapy overlap. > > The location where I will be doing my internship is only 4 miles from the > hospital, so I can't make any significant savings by dropping out of school > until the radiation therapy is complete. I'd only save the cost of 2 gallons > of gas per week. > > Plus I'd still only be deferring the cost of commuting to the internship > until a later date when gas prices will probably be higher still. > > With my gas mileage at around 31 mpg, I burn not quite 2 gallons in 60 > miles; not quite 4 gallons in 120 miles. The cost per mile for gas is > averaging about $0.10 per mile at today's prices. > > There is a single station in my area that has gas for $0.20/gallon less than > any other station. Part of a chain, but its prices have been consistently > that much lower for over a year. I pass it on the way between my home in > Raleigh, and my apartment down here at school. I've been filling up there, > and that $0.10/mile cost if figured on that cost. > > But it's a bit out of the way for the commute to my internship. > > Ignoring the time involved ... > > How many miles out of my way does it make sense for me to drive - miles > added to the commute - to get my gas from that station selling gas at > $0.20/gallon less?. > > I have a good idea how many gallons I'm going to need every day of the > commute. If I divide the savings per sale ($0.20 for 1 gal, $0.40 for 2 gal, > ...) by my cost per mile ($0.11), does that give me the correct number of > miles I can afford to detour before the cost of the detour exceeds the > savings? > > Does ... > > "((Total Savings on the sale/cost per mile = number of miles)*0.9)" > > ... work to give me the number of miles I can drive without burning up the > savings? > > I'm figuring I can afford to detour about 3.25 miles for 2 gallons, 6.55 > miles for 4 gallons ... up to 19.64 miles for 12 gallons. > > Does that look right? > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 10.0.1204 / Virus Database: 1497/3492 - Release Date: 03/08/11 > > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and > follow the directions. > -- David Parsons Photography http://www.davidparsonsphoto.com Aloha Photographer Photoblog http://alohaphotog.blogspot.com/ -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
