# How is mpg measured?



## xphobe (Aug 17, 2003)

This question actually has two parts.

First part: I assumed my Alty would have a flow meter in the fuel line, in which case it would be a simple direct calculation. But my friend says in most cars it's calculated indirectly by some kind of air flow and is notoriously inaccurate.

Second part: over what period of time or distance is the mpg calculated? I know I can reset it via the button, and it does change a lot at first after a reset, indicating maybe it's over the entire time since reset. On the other hand, I sometimes notice it changing quickly under certain driving conditions, even after having driven hundreds of miles.

Thanks!


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## Alti9 (Apr 13, 2005)

xphobe said:


> This question actually has two parts.
> 
> First part: I assumed my Alty would have a flow meter in the fuel line, in which case it would be a simple direct calculation. But my friend says in most cars it's calculated indirectly by some kind of air flow and is notoriously inaccurate.
> 
> ...


It is over the entire time since re-set. Of course it would change quickly under certain conditions. If you drive like an old lady for a couple hundred miles, all it takes it one punch to the floor to make the average drop like a rock.


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## 1AltimaR (Aug 12, 2005)

xphobe said:


> This question actually has two parts.
> 
> Second part: over what period of time or distance is the mpg calculated? I know I can reset it via the button, and it does change a lot at first after a reset, indicating maybe it's over the entire time since reset. On the other hand, I sometimes notice it changing quickly under certain driving conditions, even after having driven hundreds of miles.
> 
> Thanks!


In the Owner's manual it say's that the trip computer for mpg resets every 30secs. regardless of how you are driving. So, reset it before you leave your house for work such as while warming up, then drive like you normally would there. If you have city/highway travel like me you can reset it several times. Reset before you leave/before you get on the highway/ after you get off the highway/ repeat on the way home. Every 30 secs it updates so heck from light to light if you want  . The equipment used to measure this could be a number of differnet sensors I don't know for sure. Like the throttle position sensor(drive-by-wire) , air/ fuel pressure levels, of course distance, braking, hills etc.....but there is a formula for m.p.g.
:givebeer: :cheers:


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## Alti9 (Apr 13, 2005)

lets not make this any more complicated than it has to be. Your trip computer knows how far you've travelled(odometer), and it knows how much gas you used(gallons to empty). Thats the two things required for a MPG estimate.


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## 1AltimaR (Aug 12, 2005)

Properly, would actually be yourself tracking how many gallons you put into the tank. Then reset the trip odometer. Drive until you refill the gas. Take the miles driven divided by gallons used. So 12 gallons used over 240 miles is 20 mpg. You can use this to see if your computer ic correct. Or go here http://www.fueleconomy.gov/mpg/MPG.do?action=calcMPG , now
:givebeer: :cheers:


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## blitzboi (Apr 13, 2004)

I always wondered if the ECU uses the info from the fuel injectors to measure fuel consumption, and if not why. The injectors are metering out specific amounts of fuel as instructed by the ECU, thus over time simply adding these amounts together would determine how much fuel is being used. Then, dividing by distance travelled, which the car already knows thanks to the odometer, would provide averaged mpg numbers.

I would think it would be more accurate than guestimating based on throttle position, engine load, or other variables. Of course, the most accurate of all, as 1AltimaR pointed out, is to do the calculations yourself... but that actually takes effort. 

Cheers!


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## Alti9 (Apr 13, 2005)

Whats wrong with using the trip computer if you have it? Are they that in-accurate?


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