# How is milage determined **URL:** \ **Category:** Factual Questions\ **Created:** [September 15, 2005, 7:45am UTC](https://boards.straightdope.com/t/how-is-milage-determined/321748 "2005-09-15T07:45:41Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Declan](https://avatars.discourse-cdn.com/v4/letter/d/ecae2f/32.png) [@Declan](https://boards.straightdope.com/u/Declan)\ **Post date:** [September 15, 2005, 7:45am UTC](https://boards.straightdope.com/t/how-is-milage-determined/321748/1 "2005-09-15T07:45:41Z")
All car manufactures have a rated milage value attached to vehicles to determine how many miles per gallon one should expect with a properly tuned vehicle in optimum conditions. Is that determined by the max capacity of the vehicle, so that a sedan that seats five adults, weighs 2500 pounds, will or should get X mpg city and Y mpg highway? Now I know that certain amount of slippage is taken for granted , so the true mpg will be either less ,due to A/C , stop and go traffic , etc, but should it also rise if only one occupant in that sedan driving highway. Here is my thinking in a nutshell One sedan weighs 2500 pounds dry 5 adults, average 200 pounds=1000 pounds Full tank of gas 14 gallons or 56 pounds If this vehicle is rated at 18 mpg city or 22 hwy, would the above be the true factors in determining the mpg or is it just taken from an average sample , prior to coming to market. Declan ---
**Author:** ![WonJohnSoup](https://avatars.discourse-cdn.com/v4/letter/w/f17d59/32.png) [@WonJohnSoup](https://boards.straightdope.com/u/WonJohnSoup)\ **Post date:** [September 15, 2005, 7:53am UTC](https://boards.straightdope.com/t/how-is-milage-determined/321748/2 "2005-09-15T07:53:47Z")
I was flipping through a Consumer Reports issue at the magazine rack today and they had some article about this. [http://www.consumerreports.org/main/content/display\_report.jsp?FOLDER\<\>folder\_id=772749&bmUID=1126771275964](http://www.consumerreports.org/main/content/display_report.jsp?FOLDER%3C%3Efolder_id=772749&bmUID=1126771275964) I only skimmed through the article, but I know that the above link doesn’t show the whole thing (you’d have to subscribe). But here’s one thing they mention: > [@](#): > > • The EPA ratings are the result of 1970s-era test assumptions that don’t account for how people drive today. Automakers also test prototype vehicles that can yield better mileage than a consumer could get. ---
**Author:** ![Padeye](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@Padeye](https://boards.straightdope.com/u/Padeye)\ **Post date:** [September 15, 2005, 11:56am UTC](https://boards.straightdope.com/t/how-is-milage-determined/321748/3 "2005-09-15T11:56:14Z")
Because of all the variability the EPA ratings can’t be used to determine precisely how much fuel economy you will get but as a guide to comparing one car to another and even then it isn’t a perfect system. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [September 15, 2005, 12:41pm UTC](https://boards.straightdope.com/t/how-is-milage-determined/321748/4 "2005-09-15T12:41:51Z")
Here is how the testing is done: [http://www.fueleconomy.gov/feg/how\_tested.shtml](http://www.fueleconomy.gov/feg/how_tested.shtml) They adjust for vehicle weight (I don’t think they account for passengers or cargo) and run through a “schedule” which simulates driving. That schedule is probably not the best match to real world driving conditions. ---
**Author:** ![FatBaldGuy](https://avatars.discourse-cdn.com/v4/letter/f/ecd19e/32.png) [@FatBaldGuy](https://boards.straightdope.com/u/FatBaldGuy)\ **Post date:** [September 15, 2005, 1:05pm UTC](https://boards.straightdope.com/t/how-is-milage-determined/321748/5 "2005-09-15T13:05:17Z")
I had also heard, although I have no clue as to accuracy, that some/many/all(?) auto manufacturers tweak their engines to perform well on the EPA criteria, even though that may not give the best “real world” mileage.