Crazy MPG?

[Len & Cheri]

Originally Posted by mtnguy

Yep….the Scan Gauge is due for a little adjustment.

I've only had mine long enough to make 1 adjustment. How many times hve you adjusted yours? I was hoping after 2 or 3 it wouldn't need anymore.

[crasher]

This has gone way down the rabbit hole. Can anyone comment on the original question?

[MtnGuy]

Originally Posted by Len & Cheri

I've only had mine long enough to make 1 adjustment. How many times hve you adjusted yours? I was hoping after 2 or 3 it wouldn't need anymore.

I adjusted 1 time after the 1st couple of fill-ups, and 1 time a year or 2 ago after I started keeping a spreadsheet.

[MtnGuy]

Originally Posted by wincrasher

This has gone way down the rabbit hole. Can anyone comment on the original question?

Threads have a way of doing that, I am myself am guilty of contributing to that problem...sorry about that.

I think once you mentioned "the computer says 23.2", that started the discussions on the accuracy of onboard computers. But it has been an interesting discussion.

Hopefully the thread will get back to your original questions.

[acadianbob]

The ScanGauge worke great on my '05 5.4. It was always within a couple of tenths on the number of gallons it computed vs what went in the tank. Unfortunately, I can't seem to get it adjusted for the Ecoboost. I can get it zeroed in for non-towing but then it is way off when towing. Not sure why.

As for the truck's computation of fuel consumed, it is always within a few tenths of a gallon vs the actual; whether I'm towing or not. On a 36 gallon tank, the error is negligible.

[blw2]

Originally Posted by Dave_Monica

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

Dave

Well that makes sense I suppose... Last time I worked on engines was the days of carborators. So, constant pressure pumps feed a metering device..... Still the same. Fuel flow is known.....

[LadyWindrider]

Nope, fuel flow is not known, but is calculated. In a gas engine, the fuel pump is capable of over 80 psi. This pressure goes to the fuel rail that feeds the injectors, but is controlled to around 40 psi at idle, and the rest of the fuel takes the return trip back to the tank. The pressure regulator is controlled by vacuum, and will allow an increase in pressure, under load, low vacuum, to allow more fuel to be fed by the injectors. The computer controls the timing of the injectors, thus knowing how long each one is open, and knows the fuel flow at given pressure. The problem is, the regulator has a tolerance of +/- maybe 5 psi for given vacuum. The injectors also have a tolerance. 20 lbs per hour may be 20.5 pound per hour. Depending on the vehicle, the accuracy of the injectors, and how well they match, (the computer assumes all match set), and the accuracy of the pressure regulator, determines how close the calculations are. Basically, the computer is dead on with the calculations, it is the tolerances in the equipment that cause the error. Still, without a true fuel flow meter, it is all in the math. Tank gauges are not accurate enough. Hope this helps, and gets back to the original post.

[Coops1]

On the newer trucks the Driver Information Center measurement of gallons used is VERY accurate. It actually measures the fuel going to the injectors. Therefore, as long as you don't reset the gallons used, from new, you can divide the miles on the odometer by the gallons used to get an ACCURATE mpg. On mine I have seen the same mileage as the OP but over the entire tank the best I've seen is 17.5 (combined - iPhone app)

[blw2]

Yeah, like I said..... Measured/known... And of course it's considering tolerances of the system. I admittedly don't know what the tolerances are, but my Chevy reads good.

And I think this is along the topic of the thread.... Why did the op's reading get whacky?
Perhaps something squirrelly with something in the system at a particular speed... Such as a weird fluctuation in fuel pressure at a given speed .....
I would wonder, tail wind? Could be more likely.

When I bought my silverado a few years back, I plotted several data points at various speeds.... Indicated avg MPG at something like 35mph, 45,50, 55. 60, 65, 70, 80
All points on cruise control sets, and several points at each speed to average out terrain, headwinds, etc... Interesting bell curve.

[crasher]

I suspect that at 80-85 mph, in an unloaded condition, the engine is at it's optimal efficiency, but that is only a guess. I know guys that get the same mileage no matter what they do. In my Suburban, the faster I go, the worse it gets. So go figure.

I've also found, that some times, I do get better mileage not using the cruise control, so it could be that. My "natural" driving style is pretty efficient I guess.

[blw2]

My silverado peaks at something around 55mph. I would have guessed better mpg at slower speeds, but it actually is better at around 50-55. Plot is a nice bell curve....

I can believe better mpg without cruise control in hilly terrain. Mine kicks in overdrive to get back up to speed or maintain on a hill.... Even when its not necessary to maintain the speed.
I have been paying a lot of attention to this lately, and have noticed that mpg really drops in stop and go city traffic. Also, slowing down to make turns really makes a hit. Steady state speeds and very slow acceleration help.

[OldCoot]

Originally Posted by blw2

My silverado peaks at something around 55mph. I would have guessed better mpg at slower speeds, but it actually is better at around 50-55. Plot is a nice bell curve....

I can believe better mpg without cruise control in hilly terrain. Mine kicks in overdrive to get back up to speed or maintain on a hill.... Even when its not necessary to maintain the speed.
I have been paying a lot of attention to this lately, and have noticed that mpg really drops in stop and go city traffic. Also, slowing down to make turns really makes a hit. Steady state speeds and very slow acceleration help.

Do you mean it kicks OUT of overdrive? At least mine will, even tho' it's only lost about 1-2 mph.

[awjoker]

Originally Posted by wincrasher

I've also found, that some times, I do get better mileage not using the cruise control, so it could be that. My "natural" driving style is pretty efficient I guess.

That's a very good point, especially when pulling a load. Cruise control on my Dodge is more accurately named speed control. It wants to maintain the constant speed. If there's a slow down, it may often run hard to get back to that speed. My truck also has the tow/haul mode and I know how to get it to shift to OD but cruise control does not. It will run in a lower gear at higher RPMs for a longer period than going slightly faster and shifting sooner. When pulling a load with this truck, I can ALWAYS get better fuel mileage by NOT using cruise control.

[Herk7769]

My "read"?

Downhill with a tailwind will get better mileage than UPhill with a headwind.

When your computer is giving you numbers that do not sound right; most likely the conditions you are seeing are specific to the stretch of road you are on and the weather (wind) conditions at the time.

A tail wind or downhill will reduce drag, alter weight vector, and reduce rolling friction. This will reduce the amount of throttle required for a given speed; resulting in better gas mileage.

[crasher]

My trip back this morning - everything about the same, including yakking at Ezra the entire way as well - but this time set cruise at 70. Same old 21 mpg as I always get.

[MikeDett]

I think we finally figured it out. The 2008 F150 Limite 5.4 w/ 4 sp 3.73 is doing better. We are now getting 11.2 MPG where before the mods we got 8 mpg.

Our TT is a Rockwood Sig Lt. 8315 curb wt. of 7200 lb ready to go 8050 lb. The changes I made are as follows.

Banks Ram Air system at $450.00 and ECM reprograming tool at $300.00. Reprogram to Heavy Tow... Before the mods the TV would struggle up the hill here in S. Calif. At home base we are 800 ft. elevation. On a trip before the mod to Idyllwild CA at 6500 ft. we had some hills we pulled at 30 mph and were lucky to get 6-8 mpg. After to mod we made our last trip to Yosemite National Park. Elvation to 5500 to 6000 ft. One grade called Priest Grade was 5 miles of 5-6% switch backs going into Groveland the TV did great pulling the hill at 45-55 mph. The entire round trip of 1100 miles the average was 11.2 MPG.

We had no over heat even with temps of 95 deg. The TV is a 2008 F150 Limited with tow pkg. trans cooler 22" tires and 3.73 axle. We use a Propride hitch and P3 brake controler.

I'M HAPPY

[MillerTime]

To the original question. I have not heard of thus happening but I will talk to my heavy line big truck guy (work in a GM dealership) and see if he knows anything. Possibly just got a good clean burn on the last regin. cycle.

I thing that maybe we need to go to another forum and ask tech questions to actual in the field TODAY techs. Seems there is a lot of opinion based bs and less actual real world knowledge and facts.--- But, that is just my opinion!

[Mark and Linda]

Last week on a trip I noticed mine shifting more than it normally does in the Tow/Haul Mode. My DW even asked me about it. I have found that mine does better when towing at 60 mph. Around 55 it was shifting up...at 60 RPM's were lower. Part of my problem was the road I was on...because of the bouncing up and down I was trying to stay below 60 mph.