How accurate is your Computer when averaging MPG ?
Thread Starter
Tech Contributor
Joined: Jan 2004
Posts: 20,030
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From: Wylie TX --> Less is More, except under the hood !
How accurate is your Computer when averaging MPG ?
Guys:
I normally just run my car around town so I have never had a chance to do a Highway MPG calculation for my car.
So I filled up friday night (reset my trip meter). Sat morning I drove about 10 miles of back roads to meet a buddy. I hit the highway at that point and "reset" my computers average MPG calculator. When I turned off the HW it read 33mpg (about 50 mile cruise). I was doing 65-70 the whole way there, 6th gear.
On the way home I continued to watch it...Did not reset it from first trip. This time 70-75 the whole way (about 60 mile cruise, 6th gear). When I pulled off the HW the average read 30mpg.
I go to the same gas station and fill up. For 125 miles driven It takes 4.9 gallons. The gas was filled to the filler neck both times. That equates to 25.5 miles per gallon.
The only thing I can think of is the 10 miles I drove from my house to meet my friend (did not reset the avg mpg until I hit the HW). It was a backroad, but not many lights, and cruising 40 or so mph.
I still think my manual calculation is more accurate than my car's computer. What do you guys think ?
Toque
I normally just run my car around town so I have never had a chance to do a Highway MPG calculation for my car.
So I filled up friday night (reset my trip meter). Sat morning I drove about 10 miles of back roads to meet a buddy. I hit the highway at that point and "reset" my computers average MPG calculator. When I turned off the HW it read 33mpg (about 50 mile cruise). I was doing 65-70 the whole way there, 6th gear.
On the way home I continued to watch it...Did not reset it from first trip. This time 70-75 the whole way (about 60 mile cruise, 6th gear). When I pulled off the HW the average read 30mpg.
I go to the same gas station and fill up. For 125 miles driven It takes 4.9 gallons. The gas was filled to the filler neck both times. That equates to 25.5 miles per gallon.
The only thing I can think of is the 10 miles I drove from my house to meet my friend (did not reset the avg mpg until I hit the HW). It was a backroad, but not many lights, and cruising 40 or so mph.
I still think my manual calculation is more accurate than my car's computer. What do you guys think ?
Toque
Tech Contributor
Joined: Dec 2003
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From: Horncastle Lincolnshire, England
2023 C5 of the Year Finalist - Unmodified
If I reset the trip and average over a full tank its OK.
Don't forget its dynamic so if you average over a short distance it should be spot on. If I let it run over a long period it takes a lot to shift it from about 20 on mine. Over a short run after a reset its moves up and down quite a lot.
The best way to check is to reset after a fill, run for a tank and cross check with the manual math.
Don't forget its dynamic so if you average over a short distance it should be spot on. If I let it run over a long period it takes a lot to shift it from about 20 on mine. Over a short run after a reset its moves up and down quite a lot.
The best way to check is to reset after a fill, run for a tank and cross check with the manual math.
Safety Car
Joined: Apr 2004
Posts: 3,996
Likes: 77
From: Louisville KY
I have always computed my mpg on every tank of gas in every car I have driven. Its just a mental math game I do.
My DIC and the manual computation are from .4 to .8 mpg off. The DIC being the higher of the two. My car has 38k on it and so I figure it is consistent with that type average.
My DIC and the manual computation are from .4 to .8 mpg off. The DIC being the higher of the two. My car has 38k on it and so I figure it is consistent with that type average.
Le Mans Master
Joined: Sep 2007
Posts: 7,604
Likes: 8
From: Belleville Il
I to check every tank doing the math to compare,,and the DIC is pretty consistently 1 to 1.5 happy.Just finished a 240 mi round trip Friday,and the DIC read 30.3,,actual was 28.73. This error factor is around that on every tank. Close,but no cigar!!
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Racer
Joined: Dec 2005
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From: Tucson Arizona
Pretty much spot on with what itzza427 said. 1-1.5 optimistic pretty much each time.
Tech Contributor
Joined: Oct 1999
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From: Charlotte, NC (formerly Endicott, NY)
Guys:
I normally just run my car around town so I have never had a chance to do a Highway MPG calculation for my car.
So I filled up friday night (reset my trip meter). Sat morning I drove about 10 miles of back roads to meet a buddy. I hit the highway at that point and "reset" my computers average MPG calculator. When I turned off the HW it read 33mpg (about 50 mile cruise). I was doing 65-70 the whole way there, 6th gear.
On the way home I continued to watch it...Did not reset it from first trip. This time 70-75 the whole way (about 60 mile cruise, 6th gear). When I pulled off the HW the average read 30mpg.
I go to the same gas station and fill up. For 125 miles driven It takes 4.9 gallons. The gas was filled to the filler neck both times. That equates to 25.5 miles per gallon.
The only thing I can think of is the 10 miles I drove from my house to meet my friend (did not reset the avg mpg until I hit the HW). It was a backroad, but not many lights, and cruising 40 or so mph.
I still think my manual calculation is more accurate than my car's computer. What do you guys think ?
Toque
I normally just run my car around town so I have never had a chance to do a Highway MPG calculation for my car.
So I filled up friday night (reset my trip meter). Sat morning I drove about 10 miles of back roads to meet a buddy. I hit the highway at that point and "reset" my computers average MPG calculator. When I turned off the HW it read 33mpg (about 50 mile cruise). I was doing 65-70 the whole way there, 6th gear.
On the way home I continued to watch it...Did not reset it from first trip. This time 70-75 the whole way (about 60 mile cruise, 6th gear). When I pulled off the HW the average read 30mpg.
I go to the same gas station and fill up. For 125 miles driven It takes 4.9 gallons. The gas was filled to the filler neck both times. That equates to 25.5 miles per gallon.
The only thing I can think of is the 10 miles I drove from my house to meet my friend (did not reset the avg mpg until I hit the HW). It was a backroad, but not many lights, and cruising 40 or so mph.
I still think my manual calculation is more accurate than my car's computer. What do you guys think ?
Toque
Bill
Le Mans Master
Joined: Sep 2007
Posts: 7,604
Likes: 8
From: Belleville Il
Probably the computer calculation is more accurate. With the expansion space above the fuel entry point in gas tanks it is very difficult to get an accurate refill at a gas pump. You can be off by more than a gallon even though you are filling all the way to the top of the filler neck. After getting to the point where a pump wouldn't pump any more gas I have taken a 5 gallon gas can full of gas and emptied it into the same gas tank without spilling anything. Everything depends on the speed of filling and how much pressure builds up inside the tank. Then you also have the problem with pump accuracy itself. If there is a problem with the pump's check valve you could be cheated out of several dollars worth of gas (http://consumerist.com/384414/faulty...s-gas-stations ).
Bill
Bill
Racer
Joined: Jun 2005
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Your calculation is correct. It only matters how much gas you put in and how far you drove, after all. My car does the same thing. I'd say it's about 5% high, usually 1.5 mpg off or so.
I think the problem is the C5 can only measure gas so accurately. Mine gas gauge fluctuates quite a bit when I drive.
The large difference you experience was probably from a little bit of city driving you did. I'll get 28 mpg on a 90 mile highway trip, but after 12 miles of 45 mph and three stoplights, I'm down to 25 mpg.
25.5 mpg is great mileage is for a performance car!
I think the problem is the C5 can only measure gas so accurately. Mine gas gauge fluctuates quite a bit when I drive.
The large difference you experience was probably from a little bit of city driving you did. I'll get 28 mpg on a 90 mile highway trip, but after 12 miles of 45 mph and three stoplights, I'm down to 25 mpg.
25.5 mpg is great mileage is for a performance car!
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I've found that mine is pretty accurate. Not enough difference to really worry about.
Team Owner
Joined: Sep 2001
Posts: 23,283
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From: Lake Elsinore, CA
My experience with EFI Live makes me believe the PCM comes pretty close. When you log data using tuning sw, you see that the PCM collates data in real time at fairly high accuracy levels. I seriously doubt fuel consumption is calculated using fuel sender level. More likely, the fuel delivery rates regulated via the injector controls are used to calculate mpg. The PCM always knows how long each injector is open for each intake event and what the fuel pressure is. A running tally can be generated to keep track of fuel used. JMHO
Thread Starter
Tech Contributor
Joined: Jan 2004
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From: Wylie TX --> Less is More, except under the hood !
Thanks guys for your advice ! I really need to do a long distance trip to REALLY determine how I will do on the Highway. I think I should do 25+mpg from this weekends test... Not too shabby
Thats very true ! Especially for a 450rwhp/430rwtq heads/cam car !
Toque
Thats very true ! Especially for a 450rwhp/430rwtq heads/cam car !
Toque
Pro
Joined: Feb 2006
Posts: 647
Likes: 2
From: Keystone, I.T.
The DIC is a ball park figure. Pick a gas station and fill your car up. Drive your car. Go back to the same gas station & use the same pump. Fill up with the trigger set in the same notch as before (I always use the first notch: slowest setting). Now devide the number if miles you drove by the number of gallons you just put in the car. This is your actual mpg. Now compare this to your DIC and then reset the DIC. Do this several times to get a good idea of how accurate your DIC is. Mine is off on the high side by 1 to 1.5 mpg. Oddly enough the one on my Cummins/Dodge 2500 is 1 to 1.5 on the low side.
Thread Starter
Tech Contributor
Joined: Jan 2004
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From: Wylie TX --> Less is More, except under the hood !
Seems to be a common statement on this thread.
Thanks guys !
Toque
