My trip computer is darned accurate. Within 0.5% since new.

I reset my 'B' trip odometer when I got the truck. Haven't touched it since. I've put 26,410 miles on it since new I had to stop for fuel today. The 'B' trip odometer shows (2)6,410 miles, 1,610 gallons used, 16.4 MPG. My tally of fuel used since day one is 1,617 gallons. That is 16.33 MPG.

Pretty close IMO.

Like almost everyone, I can get 20-21 MPG on the highway to no end, but as soon as I pull off to work, eat, refuel, etc., the mileage begins to nosedive rapidly. Before you know it, I'm down to 18 MPG. A few cold starts later with no highway driving and I'm down to 17. No highway for a week and I'm down to 15.8.

So, as far as I'm concerned, 16.4 is my real world MPG. I would never tell anyone that I get 21 MPG with this truck. If they were to buy one looking to get 21 MPG, they would be very disappointed in the truck and me.

 

Great data, thanks for sharing! I wish I tracked all of my gallons pumped since I bought the truck.

But you did tell us you get 21 mpg. It's perfectly fine that you told us that however, because you included qualifying information. If you told us your tank average or whatever was 21 mpg it would be a lie and you'd feel like a dirtbag. If you tell someone you get 21 mpg on the highway, that's a true statement. We should be able to state what mpg we get in any given set of conditions without some simpleton interpreting that incorrectly as a tank average and then blaming his hurt feelings on us. The world would be a better place.

 

Second fill up is done. As expected it came out much closer to what the truck reported. The first fill up was 0.4 gallons over, which is huge, this one was 0.1 gallons over. I would bet a third one would be even closer, then no further change after that and the results would be trustworthy. That will have to wait until next week, if I can even keep up the motivation to keep stopping every day to pump 2.5 measly gallons.

The results are obviously worthless with the demonstrated variability in pump shutoff, but I'll post it anyway for curiosity's sake. To recap the test is daily round trips to work of about 45 miles each day, roughly half back roads and half highway. Weather is in the 75 degree range in the morning on my way home, and 85 degree range at night going in. This includes all cold starts and warm ups etc, nothing was left out. All three fill ups were at the same pump facing the same direction parked in exactly the same spot.

Day 1: 45.3 miles. 2.2 gallons reported consumption, 2.6 gallons pumped (foaming issue due to pumping 20+ gallons on the previous fill up). 20.6 mpg reported, 17.42 mpg calculated (reduced by inflated gallons pumped). Traffic was really light and I had pretty good luck at the lights.

Day 2: 45.4 miles (0.1 difference from day 1 likely due to rounding, due to single decimal place precision of the odometer). 2.4 gallons reported consumption, 2.5 gallons pumped (reduced discrepancy due to pumping only 2.6 gallons previously). 18.9 mpg reported, 18.2 mpg calculated (much closer). Traffic was unusually heavy, less than stellar luck at the lights.

Total for 2 day test: 90.7 miles. 4.6 gallons consumption reported. 5.1 gallons pumped. 19.7 mpg reported. 17.8 mpg calculated. Note that when you average the mpg for the two days, you get the same average as the total average. This simple math works because the mileage on both days was identical, naturally. Even if you don't like math, this illustrates the point I made earlier that if you average 20 mpg every day, your tank average will still be 20 mpg. If you do like math, 20 mpg times 10 days divided by 10 is still the 20mpg you started with.

I think everyone can see the trend here. A third day is really needed to be sure the results are being interpreted correctly and that the foaming model is what's really happening.

Note that even though the second day showed a lower reported MPG and was done in appreciably worse conditions, the calculated mpg was higher than day 1. This leaves no doubt in my mind that the truck's EFI system is much more accurate at tracking fuel economy than hand calculations, thanks to variability in where the pump shuts you off. It should be clear that this is a worst case scenario for the foaming model though, going from 20+ gallons on the first fill up to 2.6 gallons on the second fill up. It's often helpful to use extreme examples to illustrate a point though, and I'm glad I did the test this way since it highlights the difference so well.

If you're still reading this far, you're a true champion. There's one more thing I'd like to mention though and include in next week's test if I do it. My mpg numbers are lower than what I usually get round trip to work, because I usually use my house as the start/end point, and I don't have a gas station at my house. It's in the middle of my commute, right where the back roads meet the highway, near the top of a very important hill. That downhill section is a big part of my strategy to improve fuel economy, and I have to stop right in the middle of it and completely throw out that advantage. Some people might think "hey, that's cheating, coasting down hill!" But of course it's not. It's included in my round trip commute, and I have to spend fuel climbing it, so it's only fair that the reciprocal is included on the return trip. Ideally the gas station would be at my house where I'm already stopped anyway (I just realized that this could be accomplished with 5 gallon jugs since I'm around 2.5 gallons per fill up. Oh boy...). By switching my daily fill up to the trip TO work, stopping where I'm already taking it square in the bollocks climbing the hill, the effect of this will be minimized (mpg can't drop below zero, and zero is not far from the 5-10 I'm getting at that point, and very far from the infinity I'd be getting on the dowhill). Now obviously this won't have any effect on the test of computer mpg vs hand calc mpg, but it will have an effect on my pride. Posting mpg numbers below 20 is somewhat embarrassing. The last 2 days wasn't my best work ever, but I made a reasonable effort to get good mileage and it can easily be duplicated. I'm hoping it's a 1-2 mpg boost based on what I have gotten in the past, but we'll see what effect this change has next week. To truly complete the test I would have to make a couple trips without stopping for fuel at all to see what difference there is between that and both ways of stopping (the way I normally check my round trip commute mpg that produces the numbers I'm used to seeing, which are arguably more relevant since I wouldn't normally be stopping there every day).

Ok I'm done for now...

 

I have been keeping up with this thread but have some questions...

It has been said that the truck computer should be accurate because the computer is feeding the injectors and metering fuel flow - Or at least that was my take on a couple of the post in this thread.

My Lariat is brand new - only 3200 miles - maybe 6 weeks old. I have left the Trip "B" screen alone since new - resetting the trip "A" on each tank fill.

Trip B shows my 3206 miles accurately and shows 228.9 gallons used.

My spreadsheet shows 276.76 gallons bought - but I have 37.5 gallons at 1 mile because the dealer filled it (it did show full also - so I am trusting them).

So if you take the total Fuel 276.76 gal. - the 37.5 in the tank - although there is about 5 gallons less today on this tank - that makes like 239.26 gallons USED . Using these numbers the computer says that I have used 10.3 gallons less than my spreadsheet shows..

Can anyone explain why these numbers would differ?

 

My computer isn't accurate, it's always off 1 full mpg. I will admit it is more accurate than my Dodge was though. It was always off 5-7mpg.

 

I would do another test over the next few tanks from a known good starting point. Try to fill up at the same pump each time parked in the same spot. If you do this, please post the result. Alternatively, does your spreadsheet let you look at the error at the 1000 and 2000 mile mark?

 

Hand calcs are always screwed due to the difference between pumps, angle (both pitch and roll) of the vehicle at a stop, the temperature outside, and who knows what else. Going to a "click" isn't particularly helpful.

 

I really want to try filling up to the neck using a jug. I'd have to buy another one though. My only clear jug, which could easily be graduated to half tenths or so, is for ethanol for the race car only, I don't want to use it for diesel. Filling up to the filler neck is the only way to eliminate the foaming variable. Vehicle angle is still important but perhaps a little less. Either way, if I can do it at home with a jug, I can park it in the same spot every time without getting mean mugged by some lady in a minivan that insists on taking up a diesel pump for some unknown reason. I may get around to this some day.

 

I did not note the truck computer info on the spreadsheet - Only miles , date, quantity (gal purchased)..

 

Do you guys with the foaming & can't fill to the neck all have short beds? My LWB fills to the neck everytime and I don't ever get foaming. The only time I've ever had any foam at all is when I put in some PM22. Maybe the diesel doesn't foam here in Texas because of the biodiesel mandate?

 

That's too bad, but understandable. The trend in the error would have shown if it was mostly from the first tank/whatever, or if it was a continuous error (gap widens as time goes on), or what mixture of the two it was. The same can be done from the additional measurements I mentioned though.

Regarding filling up from a jug, it occurred to me on the way home that I can weigh the jug instead of marking it. Diesel fuel as a Specific Gravity of roughly 0.85 (a range is given in all online references), giving it a specific weight of about 7.1 lbs/gal. With a typical scale with precision to 0.1 lbs, you'd get a resolution of about 0.014 gallons. Way more than we would ever need for something like this. No marking the jugs, and any jug can be used.

The above numbers are good for a rough estimate, but I can determine the specific gravity and weight of the fuel I get at my pump at the temperature at the time (SG/SW vary with temp). As long as I can get repeatable fillups this way, and I can't see why not, I'd say all due diligence has been done and this is the highest degree of accuracy a regular jabrone like me is likely to get in such an experiment. I'll post whatever data I collect as I find it, so we can hopefully all come to the same conclusion. If it turns out that my faith in the compyooter has been misplaced, so be it. My feelings will be a little hurt, but I'd rather know the truth.

 

That's a good question! And another set of variables to consider. I have a long bed CC DRW and live in New England where the fuel blend changes with the seasons. As I mentioned previously my 2004 would take another 4-5 gallons to fill it all the way up after the first shutoff, but I haven't tried that with this truck. I may do so tonight, which will also kick off my other experiment.

 

Same with me - I get very little foaming - I can get to auto shutoff - the gently feed maybe $.50 -$1.00 & it is almost spilling out.... I don't use the PM22.

That being said - my tractor tank is a 105 gal. mounted on my GN trailer. You can see the foam building quite a "head" when filling the tractor. But the tank is more of a vertical tank when low/empty the fuel will "fall" about 2 ft. to the bottom of the tank - so that would create more splashing/foam. The GN tank gets treated also with JD equivilent of PM22..

 

For a while I had a bad oil tank in the new house, oil companies wouldn't deliver to it, and had to fill it myself for a while with 5 gallon jugs. When filling a 5 gallon jug I'd be lucky to get to 3 gallons before I had to stop and let the foam die down. I didn't notice a difference between summer and winter diesel, but I wasn't looking for one either. It would be really interesting if that turned out to be a regional thing.

 

I used to worry about the differences, then I realized it doesn't matter that much. The computer, whether it is "correct" or not will show you the relative MPG. In other words, if you normally show 16 MPG and now are showing 13, then you know you are not being as efficient, whereas if it's showing 20, you are doing a nice job driving downhill with a tailwind.