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Old 08-24-2012, 06:53 PM   #1
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Crazy MPG?

So I hand out paychecks and hit the road this afternoon. I fill up with Shell diesel and head on out up the turnpike. Usually I'm a cruise control guy, set it at 65 or 70 and relax. Today I'm on a conference call (handsfree) with Ezra (lawyer) and 2 from my staff. It gets heated, and of course, I'm paying no attention and certainly no cruise. I've been running along around 80 to 85 most of the way. No traffic really. Miracle didn't get a ticket.

So when I get where I'm going, the computer says 23.2. Usually it's 19-20 when I'm behaving myself. This just seems impossible. Slower should not be less mileage. Does diesel vary that much? Weather is about the same as usual.

I've read about studies in the 70's where they said engines have peak efficiency at 75% load. THat's how they numermagically came up with 55 as a fuel saving speed limit back then.

I'm sure this isn't particular to Chevy's, but the Fords and Fiats too.

Anyone experience this? I have guys at work that say their trucks get the same mileage no matter what they do.
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Old 08-24-2012, 07:15 PM   #2
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Since there is no actual fuel flow meter on any factory truck, the computer readings are useless, and usually off 2 mpg or more. The only way to measure fuel mileage is fill up, right down mileage. Drive until you need fuel again, going to the same station and pump, parked in the same place, same temp, and fill up. Note mileage, and figure miles traveled divided by gallons burned. The one and only true reading, and even it has a some error in it, usually only a 1/10 or so.
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Old 08-24-2012, 07:20 PM   #3
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I agree a true measure is to do it manually. I have found the computer pretty close in the past. Would assume it's either both high or both low.
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Old 08-24-2012, 07:31 PM   #4
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windrider, do you know how the fuel mileage "computer" actually works?
Mine is pretty darn accurate, and seems consistant as well. Is that just dumb luck?
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Old 08-24-2012, 08:42 PM   #5
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the factory computers i have had will give you and instant (sometimes) and an average mpg after you reset them. if you keep resetting you get instant mileage for awhile, or leave it for the whole tank if you want the average.
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Old 08-24-2012, 08:51 PM   #6
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the GM instruments are pretty cool right now. I have an oil life monitor, a fuel filter monitor, instant and ave mpg, and a bunch of other readouts. We shouldn't be so amazed - the computers are relatively simple - it's all the instruments/sensors that can make things complicated.

One of the things I thought would be annoying is the DEF and the limits it puts on you. If you run it out, the computer can limit the truck to 3 mph until you refill it! I called Onstar one time in a panic, but luckily found a store to buy some more.
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Old 08-24-2012, 10:24 PM   #7
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I had a 1998 Chev Malibu with a V6 and I got 38 mpg when traveling about 72/73 mph.
Maybe you should try a run at 65 - 70 in a lower gear to see how the rpm's affect mpg.

Edit
...hand calculated...
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Old 08-24-2012, 10:35 PM   #8
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Factory computers figure gas mileage mathematically. They really have no idea how much fuel you are burning. There are closely guarded math formulas to do with fuel burn at certain loads and rpm's, and gear, etc. As mentioned, right after a reset you get more of an instant rather than average. Once it has several readings, it starts to average all of these together. Most are a lot closer on the highway, but suffer around town due to too much variation. Been a few years since I really messed with one, may have improved a lot since then. Hope this explains a little without getting real technical.
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Old 08-24-2012, 11:16 PM   #9
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windrider, so the computer is just using the throttle position, engine rpm, and the vehicle speed to "guess" at the mpg? I would suppose that at any given throttle position and engine rpm, they have a pretty good idea how much fuel is being used.
At any rate, it is interesting to watch how different speeds and conditions affects the dash mpg, even if it isn't perfect.
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Old 08-24-2012, 11:37 PM   #10
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In the GM, also measures fuel consumed. I get a readout that shows the gallons down to the tenth. If I were to guess, I'd say these items are measured from the mass-air flow sensor. I suppose with computer controlled fuel injection and air regulation, that would be fairly simple to actually measure, and not some kind of back-calculation.
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