How truck calculates mileage

[wsurf]

Was wondering how the truck calculates fuel mileage? Does it meter the fuel and distance then do the math? I searched on this and of course came up with more hits than I could read. I noticed that the mileage will change often (in tenths) if I reset after each tank, but will seldom change if I leave it for many tanks. It must also average mpg between resets. Anyone know? Thanks for the help.

 

[klenger]

You are correct that the display is the average since the last reset. I always reset my '01 at every fillup to compare it with the actual milage. I am pretty sure that the computer is tracking fuel used and milage and doing a mathamatical integration of those numbers over time. Question is, how does it measure fuel used? I think (big guess here) that the overhead computer is getting informaiton from the ECM on fuel delivered to the injectors each time a cylinder fires. Even though each firing is a very small amount of fuel, I think that it is very precisely controlled by the ECM, and therefore very measurable.



On a turbine aircraft, there are transducers installed in-line with the fuel line that have a small turbine that the fuel passes through. The more fuel that is being used, the faster the turbine spins. This electrical output of the transducer is fed to the fuel flow gauges which display fuel used in lbs / hour. The company I work for builds flight management system (FMS) which uses GPS as it's primary position sensor. The FMS also watches fuel used over time and can project fuel burn along the flight plan, and even tell the pilot fuel remaining at landing. Probably more than you wanted to know.

 

[rbattelle]

I'm not sure it's quite that sophisticated, since if it were I would expect the overhead to be a lot more accurate than it is. I always figured it was just dividing the total mileage since reset by the fuel consumed from the fuel level gauge signal.



Although, as klenger says, it would be easy for the ECM to calculate almost exactly the fuel flow rate, since it knows the rail pressure and the opening time for each injector. I just don't have faith that the ECM programmers took the time to be that precise... remember, extra lines of code cost more money!

 

[JHardwick]

I read awhile back that the calculations come from the TPS, hence the reason a fuel box makes the overhead readings high.



You need less throttle, therefore the computer thinks you are using less fuel per mile.

 

[Prairie Dog]

No transducers on our trucks, it is indeed just a measure of throttle position, speed and an on board data base of avg economy under stock eng conditions. Expensive auto's have optical transducers that actually measure fuel flow - If you have one of these cars, sometimes you can open the hood in the dark and actually see it flickering.



Scotty

 

[klenger]

My '01 was usually within a couple of tenths of actual, unless I had a mix of towing and empty. They it got confused.

 

[DIESELMAN]

I would think Klenger is correct, TPS only commands the ECM to a requested torque or power level, and it is not always delivering the same amount of fuel for a given TPS position, therefore, TPS cannot provide an degree of accuracy.



The ECM doesn't care about individual cylinders, but it has all the information for fuel consumption in every revolution of the engine.



Flow is known at a given pressure and specific orofice size, and the ECM that is already controlling time and pressure will simply use a fixed value for the orofice size, so calculating fuel consumed is only about 8 lines of code and the value stored in a non-volatile accumulator register. The display would simply divide fuel and miles.



This is childs play.



So why does a box screw up the MPG? Because the ECM is being lied to as to the actual pressure, and therefore is calling for a pressure that is less than actual, even though it takes a certain amount of BTU's to move our sled down the highway, and the ECM thinks its using less, therefore reported MPG is higher.



The reality is, regardless of what you believe, you will use the same amount of fuel with an EZ on 0 as on 4 or 6, IF, you drive exactly the same, never, ever, floor the pedal, have the same load and essentially accomplish the same workload.



And if you enjoy that extra power, you will use more fuel. While there are some variations that occur as to how efficiently that fuel may be burned, it would be small.



Science, the only true religion.

 

[cpizz]

This is how accurate the computer is. I have an '04 with a 1K miles on it. Yesterday I filled up and calculated the mileage. I got 13. 9mpg. The overhead computer said 15. 3mpg. This truck is stock. No boxes on it. I think these are the HIGH readings some people are claiming they are getting on mileage. I have heard some real tall tales on here!

 

[DIESELMAN]

With my 133 gallons on board, I refuel at pretty long intervals, and this really makes up for innacuracy during the refill.



I filled 121. 4 ($185!) Gallons into my truck last week, and logged 2286. 9 miles. That is 18. 83 MPG, the overhead showed 17. 9, not bad for long term accuracy. And if I was over or under by say 10 gallons on the refill, then it would be a range of 17. 4 to 20. 5 MPG.



I do have an EZ on 3, and I have 315's, I didn't recal the speedo so these actually offset each other, and my warranty is extended to boot!

 

[sag2]

I was fooling around Friday on my way home from work. Just wanted to see what the mileage change is at different speeds, as the new 04 (HO vs SO 03) is getting worse mileage (15 vs 18). I used the overhead (which was very accurate on the 03) and ran each speed for one mile. The road was straight and flat, and there was no wind, ambient 57 degrees.

80 MPH-14. 8

70 MPH- 18. 4

65 MPH-19. 2

60 MPH-19. 6

55 MPH-20. 4

50 MPH 25. 3

46 MPH (slowest speed to stay locked in high) 27. 2



So the actual numbers are not that important, but what is important is you can't compare your mileage to anyone else. You can see that the guy who drives 50 no mater what, is going to get significantly better mileage than I do. I drive this truck faster and harder than the 03 because it has more power.

 

[fox]

I really trust my over head computer.

THe other day I reset it and realized I was getting 99 miles to the gallon!!!!!!!!!!!!



Just haven fun of course!

Mine does read high with the box on it.

So I use a PENCIL.

 

[Moterhead]

I think there is a specific number of miles the system uses for calculations if the reset function is not used. I base this on my experience on a trip to Michigan from my home in the Florida Keys. Left Big Pine with 222 miles on the truck. Arrived at my daughter's condo in Roseville, Michigan the next day. I covered 1610 miles and used almost exactly 67 gallons of fuel. Filled when I got there. The computer said 24. 6. Manual calculation comes out to 24. I did not reset for the drive up. I did not reset at my fillup upon arrival. As I drove around the Detroit area, the displayed fuel economy dropped fairly rapidly. When I left for home a week later, I had put one tank of fuel thru the truck, about 25 gallons. The displayed mileage had dropped to a little over 20. Miles covered on that tank was almost exactly 500. That's an even 20 mpg. This set of circumstances indicates the system uses perhaps the last 500 or 600 miles for calculation purposes barring reset. I believe I remember reading somewhere that on my '98, the last 600 was the number. BTW, the trip back to the Keys was an average of 21 for the trip. Drove much faster. Snow in the forecast & I wanted to get back to the sunshine!

 

[wsurf]

I always learn when I read TDR. Great relpys. As toward the pencil method, how do you know the amount of fuel in your tank? I have never used this method (use to on the Fo_d) and figure it's all relative, soemtimes I get better mileage (light foot) and sometimes I get bad mileage (heavy foot).

 

[wsurf]

Sag2, forgot to ask, how is the 04 HO compared to the SO? I bought in CA so have the SO, that big of difference? I probably shouldn't ask, will probably end up costing me big bucks.

 

[Moterhead]

Originally posted by wsurf

I always learn when I read TDR. Great relpys. As toward the pencil method, how do you know the amount of fuel in your tank? I have never used this method (use to on the Fo_d) and figure it's all relative, soemtimes I get better mileage (light foot) and sometimes I get bad mileage (heavy foot).

In using the pencil method, it's not necessary to know how much fuel is in your tank. You use how much you put in at the fillup and divide that number into the miles you drove on that tank. Naturally, how well you fill your tank greatly influences your caluculations. The gen 3 Rams are so hard to completley fill that I mostly just use the overhead. After I read the many posts about how much additional fuel you can squeeze in after the pump shutoff, I tried it out. I added an additional four gallons before I got tired of fooling with it and the glares of others waiting for the diesel pump.

 

[rbattelle]

I usually just pump an extra gallon or 2 after initial click off of the pump. I'm really interested in trending: how's this tank compared to the average. Over a long time, the average will stabilize and won't be affected too much by swings in how full you fill the tank. And I figure any problems in the engine will show up as a large change in fuel mileage over a few tanks.



BTW, I use a handheld database on my PDA to calculate all sorts of statistics for my fueling. Things like mean MPG, mean gal/hr, mean miles/day, mean hrs/day, etc... If anyone has a handheld and is interested in this, I'll give it to you free. The catch is, it's a HanDbase database, so you'd have to buy a copy of HanDbase, available here.

 

[JHardwick]

I don't agree with you DIESELMAN.



I can't site specifics, but I don't buy it.



What you are telling me is that this Cummins will always get the same MPG regardless of who is driving it ... ... ... ... leadfoot/*****foot.



Put it to the floor and the ECM will only allow a certain fuel flow regardless, only dependant on what the programming will allow.



I agree that a TPS to an ECM is nothing like a cable to an 850 double pumper ... ... ... ... ... ... . but come on.

 

[Rosco]

I just calculated the mileage for a trip I made this weekend from my home in Rhode Island up to the White Mountains in New Hampshire and back. Before I left I filled the truck up until I had fuel in the filler tube and reset the odometer and overhead. When I got home today I filled the truck up again until I had fuel in the filler tube. Here's my results.



Total Miles: 466. 7

Total gallons of fuel: 32. 029 (cut it kind of close)

Overhead reading: 16. 0 mpg

Actual mileage: 14. 56 mpg



Rosco

 

[cpizz]

See what I mean! Rosco gets it. I have had 4 CTD's and all of the computers were inaccurate. I don't think it is a very sophisticated device. A calculator and pencil is still the best way!

 

[sag2]

wsurf, the HO has a noticeable difference in power. I had both trucks for about a week, and should have raced them. But after 40,000 miles the 03 did run great.