Do you drive around for low gas prices? If a station offers a nickel-a-gallon discount for cash, do you pull out your Jacksons? Do you drive the first vehicle in the driveway?
With the price of gas passing $4.00 a gallon, you should rethink all these activities. It's a matter of percentages.
(For these calculations, we will assume the average car gets 25 MPG in a 15 gallon fillup and pays about $4.00 / gallon. As the price goes up, your mileage goes down, or you pump less gas at a stop, the logic gets stronger.)
If you get off your customary route to pay $3.99 rather than $4.05, you're saving 6c per gallon. With that fillup, you've saved $.90. That will buy you less than 1/4 gallon more or about 5.5 miles. That may be more than around the block, but it won't justify going to the next interstate exit and back.
The same numbers apply to savings for cash. Of course, if your credit card gives you a 1% or 3% rebate, not many stations will give that much discount. 3% of $4 is 12c. This assumes you pay off your credit card without interest. If you pay the bank 3 or 4 months' interest at 9% APR (a good rate; some people pay two to three times that), you've more than lost your 3% rebate.
Which car do you drive? If you have one car that gets 22 MPG and another at 27 MPG, should you make an effort to drive the more efficient one? If you have a 10 mile one-way commute, you'll save about $.67 per day. That may not sound like much; but come Friday, it's another beer at happy hour. Obviously a bigger spread means more money. Here we're talking about a 22% difference; 5 MPG is a bigger percentage if you're starting with less but only 13% between 42 and 37 MPG cars.
Download the spreadsheet for your own calculations.
(c) 2004-2008 Bill Barnes w/ public domain components. Free distribution with permanent acknowledgment.
More like this: http://3500a.blogspot.com/
Showing posts with label MPG. Show all posts
Showing posts with label MPG. Show all posts
Thursday, July 24, 2008
Wednesday, August 8, 2007
Miles per Gallon
OK, maybe I'm compulsive about keeping trivial records. But with $3+/gal gas, everyone's starting to be interested in how they're doing day-to-day.
This workbook+ contains one simple formula =(F8-F7)/E8 [ (mileage this fillup)-(mileage last fillup)/(gals this fillup) ] in col G to calculate MPG at your last fillup. Since you may not be consistent in how full you get your tank, I also included 2-tank and 3-tank averages to smooth it out.
My Audit column recalculates the cost as the price per gallon times the gallons to catch typos. If it's off by more than a few cents (to allow for the hundredths of a gallon), one of the numbers is wrong. Sometimes I know the audit is going to be invalid if I didn't actually fill up once and a "single tank" represents multiple purchases at different prices (this is common if I know I'm travelling to a low-price area, but need another 2 gallons to get there).
For MPG tracking to be accurate, you have to be faithful. If you regularly buy gas without filling up, you have to combine individual purchases to create a "tank" that you know all the miles were burned on.* And, if you forget to write down the gallons just once, not only is that tank invalid, but so is all your cumulative data. Like I said, I'm compulsive.
I haven't yet figured the simple formula (I can do it with programming) for row 1 to get an accurate average of all the non-0 entries in a column. If I set the average past the current bottom of the worksheet, it counts all the empty rows. As it is now, my "lifetime" is only to where I was last time I wrote the formula.
* If you don't fill up ...
You can only calculate MPG if you know exactly how many gallons you burned for a given number of miles. If you can only afford $5 at a time; one day that $5 may bring you up to 7/8 full and the next you leave the pump just 1/3 full. That means you burned more gas than you bought, but you don't know how much. If you occasionally miss a fill up, just add all the partials to the next time you top off and call it one "tank". If you never fill up ... well, your lifetime average will be close (because being off by 4 gal when you're calculating MPG for 400 gal isn't significant).
(c) 2007 Bill BarnesMore like this: http://3500a.blogspot.com/
+ To use the published workbook:Click the link to open a web picture - not a working spreadsheet - of my workbook. Type the content from the Formulas page into a spreadsheet. The heading "Date" goes in cell B3. You only really need the five formulas on row 7 that start =(F7-F6)/E7. Then you can use your spreadsheet's autofill function to replicate them on the rest of your rows. Be sure not to type the " ' " before the "=".
Even easier; if you have a Google account, you can open a live spreadsheet in Google Docs. From there you can export it for your own use with the formulas intact.
This workbook+ contains one simple formula =(F8-F7)/E8 [ (mileage this fillup)-(mileage last fillup)/(gals this fillup) ] in col G to calculate MPG at your last fillup. Since you may not be consistent in how full you get your tank, I also included 2-tank and 3-tank averages to smooth it out.
My Audit column recalculates the cost as the price per gallon times the gallons to catch typos. If it's off by more than a few cents (to allow for the hundredths of a gallon), one of the numbers is wrong. Sometimes I know the audit is going to be invalid if I didn't actually fill up once and a "single tank" represents multiple purchases at different prices (this is common if I know I'm travelling to a low-price area, but need another 2 gallons to get there).
For MPG tracking to be accurate, you have to be faithful. If you regularly buy gas without filling up, you have to combine individual purchases to create a "tank" that you know all the miles were burned on.* And, if you forget to write down the gallons just once, not only is that tank invalid, but so is all your cumulative data. Like I said, I'm compulsive.
I haven't yet figured the simple formula (I can do it with programming) for row 1 to get an accurate average of all the non-0 entries in a column. If I set the average past the current bottom of the worksheet, it counts all the empty rows. As it is now, my "lifetime" is only to where I was last time I wrote the formula.
* If you don't fill up ...
You can only calculate MPG if you know exactly how many gallons you burned for a given number of miles. If you can only afford $5 at a time; one day that $5 may bring you up to 7/8 full and the next you leave the pump just 1/3 full. That means you burned more gas than you bought, but you don't know how much. If you occasionally miss a fill up, just add all the partials to the next time you top off and call it one "tank". If you never fill up ... well, your lifetime average will be close (because being off by 4 gal when you're calculating MPG for 400 gal isn't significant).
(c) 2007 Bill BarnesMore like this: http://3500a.blogspot.com/
+ To use the published workbook:Click the link to open a web picture - not a working spreadsheet - of my workbook. Type the content from the Formulas page into a spreadsheet. The heading "Date" goes in cell B3. You only really need the five formulas on row 7 that start =(F7-F6)/E7. Then you can use your spreadsheet's autofill function to replicate them on the rest of your rows. Be sure not to type the " ' " before the "=".
Even easier; if you have a Google account, you can open a live spreadsheet in Google Docs. From there you can export it for your own use with the formulas intact.
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