Crazy MPG?

[crasher]

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.

[LadyWindrider]

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.

[crasher]

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.

[bakken]

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?

[dbuzz77]

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.

[crasher]

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.

[Twisty]

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...

[LadyWindrider]

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.

[bakken]

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.

[crasher]

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.

[Dave_Monica]

The computer generated mpg always seems optimistic compared to hand calculated. Has anyone ever had the computer indicate worse mpg than the hand calculated number...I've never see it!

Dave

[ironj]

I have had 2 occasions where the puter was 1/10th lower than hand calculated.....im not sure how tge new fords data mine, but i get average, instant, and last 5 min graph type readout......i have never had it off more than 3/10ths vs hand calculated......im happy with that !....of course if i reset it while slowing down i can get 99mpg......lol

[Filthy Beast]

Originally Posted by windrider

Since there is no actual fuel flow meter on any factory truck, the computer readings are useless, and usually off 2 mpg or more.

Sorry, but my DIC is always within 1/2 mpg of my hand calc.

[LadyWindrider]

I don't think anyone in the general public knows what all the formulas the computer is using to calculate mileage, closely guarded trade secret. Yes the fuel gauge comes to play, Mass Air Flow, throttle position, O2 sensors, (rich or lean), intake vacuum, transmission gear, torque converter lock up, outside air temp, coolant temp, and others sensor readings come to play. They probably do dyno runs, ant various rpm and load, and measure fuel mileage at several points. Then they can make a graph, plot the curve, then go work on all the formulas to get somewhat close to real world. I haven't owned a vehicle in a few years with a computer readout, but Dad's 2009 Explorer is reasonably close, a little higher than actual. May be due to wind, as I'm sure the wind tunnel test give drag coefficient and that is in the formula also. Of course extra wind would drop mileage. As you can see, the formula starts to get rather lengthy, with a lot of changing data. Glad to hear they are making it closer to real life.

[ironj]

I dunno how it works, but it does......+/- 3/10ths in my case?....towing, hwy, idling, all pretty accurate...with all the sensors and computing power in these new things, its not terribly surprising....

[wayne anthony]

My 06 Chev truck was .3 optimistic when it was new, last check in 09 it was .2 optimistic. My 08 Taurus was .2 optimistic when new, last fall was off by 1.5 mpg (optimistic). This summer its reading about .2 low. The truck always takes more fuel than it says it burned.

[crasher]

Going downhill, it cuts back to 4 cylinders and I get 99 mpg. You'd think running down to the beach I'd get at least 50 mpg!

[blw2]

Isn't the fuel injection system fed by a constant displacement pump? That means that fuel flow rate is known as long as pump speed is known. Easy.

[Dave_Monica]

Originally Posted by blw2

Isn't the fuel injection system fed by a constant displacement pump? That means that fuel flow rate is known as long as pump speed is known. Easy.

The fuel pump maintains the required fuel pressure needed for the injectors. The injectors determine the flow, not the pump.

Dave

[MtnGuy]

In my case, I use a ScanGauge II. The mileage differences can be adjusted with the ScanGauge. I keep a spread sheet of actual vs. computer mileage so I can get a good average over time, and have already made several adjustments to the ScanGauge since I got it. It might due for another adjustment, although it seems to be pretty accurate with my non-towing mileages. Here are my results from the trip pulling my Surveyor to the FROG Rally and back:

A low of 9.1 mpg (actual) vs. 9.6 mpg (ScanGauge) in the mountains of WVa. and eastern KY…..there were some long, hard pulls on that section of I-64. High was 11.7 mpg (actual) vs. 12.1 mpg (ScanGauge) for a section of mainly 2 lane road through rolling hills, with some stop and go in small towns and getting lost in downtown Winchester, VA. Overall, I had a 10.2 mpg (actual) average, vs. a 10.5 mpg (ScanGauge) average. Yep….the Scan Gauge is due for a little adjustment.