How to Really Calculate Miles per Gallon and Cost per Mile Using Formula Template

For folks who want accurate miles-per-gallon and cost-per-mile answers.

For quick, approximate answers; simply use the templates and you're done.

Not pretty.

This page will serve you well if your gas gauge is broken, inaccurate, or is otherwise giving you problems. Or if you just want to know how well your car is doing. Can also be used for possibly figuring out ways to improve your mileage.

Needless to say, one needs to know the miles driven, how much gas was used, and the price of the gas before the templates will be of any use to you. If you do not already have these numbers, Section I below has everything you need to know on how to get started.

" Ω " Handy Google calculator. Opens in a separate tab or window.
  • Both " / " and " ÷ " means divide.
  • After arriving at the calculator and before entering numbers, you will need to click its numbers box first to get its attention.

Distance Traveled Precalculation

______________________  -   ______________________  =  ______________________ 
New Odometer Reading            Previous Odometer Reading                Miles Driven
(When you refill the tank)        (From previous fill-up of tank)

As previously mentioned, if you are just looking for approximate answers, then you can simply use the templates and call it a day. If you are looking for the most accurate results possible, see Section I.

Miles per Gallon Formula and Template

___________________  /  ___________________  =  ___________________ 
Miles Driven                        Gallons of Gas Used                Miles per Gallon


Example

  1. You drove 100 miles and used 5 gallons of gas.
  2. Your intuitive answer would be 20 miles-per-gallon. Your intuitive answer would be correct.
  3. 100 miles traveled, divided by 5 gallons of gas used, gives you 20 miles per gallon.
  4. 100/5 = 20 mpg.

Gas Cost per Mile Formula and Template

___________________  /  ___________________  =  ___________________ 
Price per Gallon                  Miles per Gallon                          Cost per Mile


Example

  1. You paid $5 for a gallon of gas, and you get 10 miles-per-gallon (mpg).
  2. Your intuitive answer would be $.50 a mile. Your intuitive answer would be correct.
  3. $5 paid, divided by 10 miles traveled per gallon (mpg), gives you $.50 cost per mile.
  4. 5/10 = $.50 cost-per-mile

About Your Miles per Gallon Results...

So what does the miles-per-gallon answer actually tell you? It tells you that...

  • This particular vehicle,
  • being mechanically maintained at a given level of efficiency,
  • using a specific brand and grade of gasoline,
  • being filled at a particular time of day,
  • from a particular gas station and a particular pump at a particular  fill speed,
  • and being driven a certain commute route,
  • by a specific driver...
...gets so many miles per gallon.

If the numbers used to make the calculation were accurate, this can lead to some interesting experimentation. What if...

  • A different gas station or pump was used? Not all stations and pumps are the same.
  • A different pump fill speed was used? 
  • The tank was filled at a different time of day? Temperature affects fluid density, first thing in the morning is best; that is when the gas is coldest and most dense.
  • A different brand and/or grade of gasoline was used? See Section II.
  • A different commute route was tried?
  • Deficiencies were found as to the vehicle's maintenance?
  • The vehicle's ignition timing was experimented with (but staying within smog emission specifications)? See Section II.
  • The driver notices and alters a particular driving habit?
Probably other ideas might also come to mind over time. 

Section I - Using a Reasonably Scientific Method and Mistakes to Avoid

If you are looking to get the most accurate results possible, this procedure will help you do that. If you are just looking for an approximation, then you can skip it all and fill in the templates with your existing numbers.
  1. Pick a week, or other time period, when you will be doing your most typical driving pattern.
  2. Have two pens and paper in the car.
  3. Use the gas station you normally use. Fill the gas tank at your usual time. Note the pump number you are using. Note the pump speed you normally use. Do not top off. While waiting, write down your odometer reading, include tenths. If you have a trip-odometer, reset it to zero. Remember to not be distracted by all this to the point you forget to put back the gas cap.
  4. Commence with your week; the usual work commute, errands, etc. Combining your work commute with errands will increase your gas mileage, but only do so if it is what you intend to usually do. Continue your routine until you have less than a quarter-tank. Don't strive for a gas-gauge reading of empty unless it is what you normally do.
  5. Make sure you still have the pens and paper in your car.
  6. At the same time of day as before, return to your previous gas station.
  7. Attempt to use the same pump number you used before. Set to the same pump speed as before. While waiting: write down your odometer reading; write down your trip-odometer reading; include tenths from both. When the pump-handle clicks: write down how many gallons; and very definitely include tenths. Write down the price you paid per gallon. Save the receipt; if the gas station is at least half-coordinated, some or all of this information will be printed there for you. Does it match what's showing on the pump? Do not top off. If so inclined, reset trip-odometer to zero. And the gas cap thing again...
  8. Proceed with your normal routine. You'll do the calculations with the templates at your leisure.

Section II - List of Notes About Fuel Economy, Improving Gas Mileage and Saving Money

Tune-ups and tire pressure: These are the Big Two as to getting the best mileage. A couple notes...  Over-inflating tires increases gas mileage, but causes an immediate and significant increase in tire wear; so don't do that. Under-inflated tires reduce your mileage; and it doesn't do your sidewalls any good either. As for tune ups, spark plugs are especially important. A fouled or carbon-built-up plug reduces mileage drastically, not to mention it will probably cause you to flunk a smog check. A personal note: Two different mechanics quoted me a price of over $100 to change a set of 6 spark plugs, plus the inflated cost of the plugs. In both cases, I departed the premises immediately. I ended up changing the plugs myself, it's not that hard to learn to do. Buy yourself a Chilton or Haynes manual for your particular make and model of car, they have all sorts of useful information. Some auto parts stores even have tool-loaner programs if you don't want to buy your own.

Looking ahead and coasting up to stop lights: Is a close third.

Speed: Once you are above 40 mph or so; the faster you go, the lower your mileage.

Weight: If you are carrying excess, unnecessary weight in the trunk, it will:
  • Reduce mileage
  • Increase engine wear and tear
  • Wear out your brakes faster
Ethanol: Do you have ethanol-times-of-year versus non-ethanol-times-of-year? It can be interesting to make mileage comparisons between the two. You probably won't be happy with the ethanol results.

Gasoline Grade: Putting premium in a car that takes regular will do absolutely nothing for your mileage. However, if your car is in the midgrade octane category and what with there being some octane rating overlap, it might be worth experimenting with trying both the lower and higher octanes; especially if you are also experimenting with the ignition timing.

Temperature and humidity: Mileage is better during cooler times of the year than during heatwaves. And the higher the humidity, the better the mileage. Yep, one does get better mileage on rainy days.

Air filter: When is the last time you replaced the air filter? A clogged air filter does reduce mileage.

Fuel density and time of day: As mentioned earlier, always fill your tank first thing in the morning. Fluid density is affected by temperature. The colder it is, the more gas you get per gallon.

Logbook: If so inclined, this is as good a time as any to start one, especially if you want to try any of the aforementioned experiments..

A Couple of Relevant Federal Websites

I thought I'd include a couple of useful federal websites for your future reference. Both are worth browsing the next time you have some time to kill.

If only urban freeway traffic looked like this...
From www.epa.gov/air-pollution-transportation. Has all sorts of links regarding vehicles and fuel efficiency and saving gas in general.

For when planning your next road trip...
From www.fueleconomy.gov/trip. This goes directly to their trip calculator page. What makes this calculator unique is you can specify the make and model of car you are using or are curious about. In addition to the total fuel cost calculation, they throw in a map and text directions as well. The rest of the site is also worth browsing.

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