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Old 10-27-2014, 09:39 AM
 
33,387 posts, read 34,622,442 times
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Quote:
Originally Posted by HappyRider View Post
The other mystery is how it calculates DTE. It must know how much gas is in the tank. I am not aware of any sensor that sends that measurement to the computer. If it is getting it from where the fuel gauge is getting it then then we are in trouble.
it in fact does take the fuel level from the fuel gauge sending unit. in the any calculation by the computer as to fuel economy is a rough estimate based on inaccurate parameters.
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Old 10-27-2014, 11:01 AM
 
Location: Myrtle Creek, Oregon
15,293 posts, read 17,563,577 times
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Quote:
Originally Posted by mikey2 View Post
The one on my car always reads a bit high....

for example it typically says I'm getting 42 mpg average for that tank of gas.

When I stop to fill up and calculate it manually, I usually get 37.5 mpg for that tank of gas. Highest ever was 39.

Is that true for others or is it just my car?

Mikey
When you fill up it's an average. When you are sitting at a stop light, your mpg is 0. When you are accelerating it drops quite a bit. It sounds like your typical mileage is pretty accurate.
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Old 10-27-2014, 11:04 AM
 
Location: Myrtle Creek, Oregon
15,293 posts, read 17,563,577 times
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Quote:
Originally Posted by rbohm View Post
it in fact does take the fuel level from the fuel gauge sending unit. in the any calculation by the computer as to fuel economy is a rough estimate based on inaccurate parameters.
Why would it measure the gas in the tank when the computer can get data directly from the injectors? Miles remaining until the next fill-up does come from the tank sensor, but it has a built-in safety margin just like the gas gauge.
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Old 10-27-2014, 11:13 AM
 
4,833 posts, read 5,673,562 times
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When I calculate my MPG it is always 2mpg lower than computer
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Old 10-27-2014, 11:28 AM
 
Location: U.S.A.
3,306 posts, read 12,150,127 times
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Quote:
Originally Posted by IShootNikon View Post
When I calculate my MPG it is always 2mpg lower than computer
Due to measurement error in determining amount of fuel remaining in the tank it is expected that you see these discrepancies. The most accurate way is likely the way you have been doing it: Track mileage between fill ups, divide by gallons filled. Fuel pumps are constantly calibrated and then checked by a state approved department of weights and measures or third party.

Live calculations of MPG are very accurate, as mentioned they include injector duty cycle data as well as engine and vehicle speed.
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Old 10-27-2014, 11:50 AM
 
Location: Richmond
419 posts, read 896,446 times
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My infiniti gas sending unit is not working. I use the DTE to make sure I dont run out of gas. It seems to be very accurate. I would guess it is using the fuel flow to the engine or the injector info as there is no other place to know what the rate of use is. Then it probabaly looks at the fuel capacioty as a fiuxed value. My tank is 20 gallons and has not changed so that plus the miles travelled should give the CPU all the info it needs to spit out how long til I am stuck on the side of the road.
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Old 10-27-2014, 12:07 PM
 
Location: Denver
3,373 posts, read 9,151,958 times
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Quote:
Originally Posted by mikey2 View Post
The one on my car always reads a bit high....

for example it typically says I'm getting 42 mpg average for that tank of gas.

When I stop to fill up and calculate it manually, I usually get 37.5 mpg for that tank of gas. Highest ever was 39.

Is that true for others or is it just my car?

Mikey
Driving a 2014 Jetta rental right now. Hand calculated 24.5 and the display said it averaged 34.
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Old 10-27-2014, 12:41 PM
 
227 posts, read 389,374 times
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Quote:
Originally Posted by IShootNikon View Post
When I calculate my MPG it is always 2mpg lower than computer
I have a 2014 Subaru Forester and it's typically about 1mpg lower than the computer. On the most recent fill-up it was a full 2mpg lower, but sometimes it's within 0.5mpg. I've never calculated it over the computer's number though.
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Old 10-27-2014, 01:19 PM
 
Location: U.S.A.
3,306 posts, read 12,150,127 times
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Quote:
Originally Posted by gregir View Post
My infiniti gas sending unit is not working. I use the DTE to make sure I dont run out of gas. It seems to be very accurate. I would guess it is using the fuel flow to the engine or the injector info as there is no other place to know what the rate of use is. Then it probabaly looks at the fuel capacioty as a fiuxed value. My tank is 20 gallons and has not changed so that plus the miles travelled should give the CPU all the info it needs to spit out how long til I am stuck on the side of the road.
No need for flow meters, using injector displacement is a very precise method. The overall precision of the DTE measurement goes out the window however when using the crude instruments for fuel volume in the tank. In your case I would just get a good grasp of average MPG through hand calculations and then use a buffered mileage amount to refill at.
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Old 10-27-2014, 01:49 PM
 
33,387 posts, read 34,622,442 times
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Quote:
Originally Posted by Larry Caldwell View Post
Why would it measure the gas in the tank when the computer can get data directly from the injectors? Miles remaining until the next fill-up does come from the tank sensor, but it has a built-in safety margin just like the gas gauge.
it is just another parameter that the computer uses to calculate fuel economy. even though it uses the flow through the injectors to make its primary calculations, it still needs the amount of fuel in the tank to start with, and the amount of fuel left in the tank at regular intervals.
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