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Discussion Starter · #1 ·
My car gets empty at 320 miles indicated on the dash. It takes 16 gallons to fill up my tank. 320 miles / 16 gallons = 20 mpg. My computer tells me my milleage is 23.4 mpg. I get close to 100 miles before the gas indicates 1/4 gone:). once it reaches the last quarter it only runs for about 50 miles:cry:.

So how am I supposed to figure out the true mpg on my car:mad:
 

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"Clunker" *****
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My car gets empty at 320 miles indicated on the dash. It takes 16 gallons to fill up my tank. 320 miles / 16 gallons = 20 mpg. My computer tells me my milleage is 23.4 mpg. I get close to 100 miles before the gas indicates 1/4 gone:). once it reaches the last quarter it only runs for about 50 miles:cry:.

So how am I supposed to figure out the true mpg on my car:mad:
It's my understanding that the computer measures mpg while the car is moving. When you do the calculation of gas used per miles traveled you are including the periods while the car is idling. I have always assumed that this accounted for at least some of the difference.
 

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you could always use mapquest to see the distance you are traveling, and travel that same distance back and forth until you run out of gas. Then do the calculations.
 

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The way I get MPG is, I fill up my tank, zero out the mileage; then when I go fill up, I take the mileage (shown on the dash) I have driven and divide by the total gallons it takes to fill up (shown on the gas station) to get the MPG. MPG is an average of all your driving (highway & city). This is definitely more accurate then what the computer in your car says because you're using actual numbers that you can see, whereas how did the computer computed your mpg and what did it account for?
 

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The way I get MPG is, I fill up my tank, zero out the mileage; then when I go fill up, I take the mileage (shown on the dash) I have driven and divide by the total gallons it takes to fill up (shown on the gas station) to get the MPG. MPG is an average of all your driving (highway & city). This is definitely more accurate then what the computer in your car says because you're using actual numbers that you can see, whereas how did the computer computed your mpg and what did it account for?
That's how I do it, my obc is usually 1-3 mpg off.
 

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Discussion Starter · #7 ·
Hi guys. Thanks for your answers. One more question then. How acurate is the distance measured on the milleage reading? Is it measuring the number of rotation the tire makes? Would the tire size make a difference or whehter its pumped up at high psi make any difference?
 

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I'm here all week
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Wheel size can make a difference. People who go from 13" to 18" on a little hatch-back don't realize that their speedo is completely out!

1 Rotation of a big wheel covers more distance than a smaller one, so if the computer goes by rotations, you need to keep the wheel diameter the same as stock.

Possibly (sounds good tho!)
 

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The traditional way is simply divide the gallons into the miles travelled. Just yesterday, I travelled from Hartford CT to New Hampshire and traveled a little more than 171 miles. The computer read 29.2 mpg. I filled up and put in slightly more than 6 gallons. Doing the math, that's about 28.5 mpg. On the way back the computer read 30.2. Haven't filled up again. The route I took was mostly highway and I was going about 70 mph. Overall, I think the computer is accurate enough.
 

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My speedometer reading has been giving me variations in speed between 20 and 100mph 5-15mph slower than my sat-nav.
e.g my speedo reads 70mph and my tom tom reads 82mph

Anyone know which would be the true speed to use for more accurate calculations?
 

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Monacoblau mit beige
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My speedometer reading has been giving me variations in speed between 20 and 100mph 5-15mph slower than my sat-nav.
e.g my speedo reads 70mph and my tom tom reads 82mph

Anyone know which would be the true speed to use for more accurate calculations?
Yeah the sat nav is the most accurate, speedos are always 10% under the actual speed. Who knows whether speed cameras accurately measure true speed!?
 

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Yeah the sat nav is the most accurate, speedos are always 10% under the actual speed. Who knows whether speed cameras accurately measure true speed!?
Nice. I'd like a navigation, but I wouldn't really use it until I graduate college but maybe a couple times, so it would be a waste. When I graduate I want an E46 M3. Red, or the same color as mine (like a maroon).
 

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the only way i think to get a great measurement is to divide the miles drove by the gallons to fill back up. You drive 60 miles and it only takes two gallons to fill back up then you got 30 miles to tank. 60/2=30
 

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Does anyone know how the dashboard light for low fuel works? I take it its when the float hits a certain level in the tank. (is it reliable?)

I've tried the full tank thing but was unsure how true a reading I got as the full tank fuel fill cut off can be different depending on the speed of fuel fill.(if you understand what I'm saying)
 

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the only way i think to get a great measurement is to divide the miles drove by the gallons to fill back up. You drive 60 miles and it only takes two gallons to fill back up then you got 30 miles to tank. 60/2=30
that's good, but one of the problems we are encountering is that either the dash is wrong, the OBC is wrong, or both, for various reasons including wheel size and being incorrect from the factory(a known problem).
 

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Monacoblau mit beige
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Does anyone know how the dashboard light for low fuel works? I take it its when the float hits a certain level in the tank. (is it reliable?)

I've tried the full tank thing but was unsure how true a reading I got as the full tank fuel fill cut off can be different depending on the speed of fuel fill.(if you understand what I'm saying)
The fuel light comes on when 8 litres remains in the tank. It says that in the Owner's Manual. It might be reliable.

Just fill up the same each time and you won't have a problem. It's going to fill up to pretty much the same level each time.
 

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The fuel light comes on when 8 litres remains in the tank. It says that in the Owner's Manual. It might be reliable.

Just fill up the same each time and you won't have a problem. It's going to fill up to pretty much the same level each time.
For those Americans here, 8 liters is 2.11 gallons :)
 

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Yeah the sat nav is the most accurate, speedos are always 10% under the actual speed. Who knows whether speed cameras accurately measure true speed!?
The generally accepted speedo discrepancy is 10% + or - 2.5 mph OVER the actual speed. :)
Do a search, it's been covered many times.

dj
 

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The generally accepted speedo discrepancy is 10% + or - 2.5 mph OVER the actual speed. :)
Do a search, it's been covered many times.

dj
Yeah I meant under and that was what I was thinking, for some reason I wrote it down the wrong way around! 10% was just a rough guess.

Bimmer M.D. said:
For those Americans here, 8 liters is 2.11 gallons
The light could trip at a totally different level for US cars, check your owner's manual where it gives info on the fuel gauge and see what it says. It might say 2 gallons!
 
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