Instant mpg

[Daytripper]

How accurate is the instant mpg readout on the prostar dash? Or should I rely on the performance monitor in the Quailcom?

[marek4792]

Pen and paper, mileage divided by gallons used. Subtract a gallon for each hour idled for highway mpg vs total mpg. It may be accurate within 2/10ths or could be off .5-1+ mpg.

[smorgan87]

I find it to be a few seconds lagging.

[Curtisj28]

I've never had a dash mpg be anywhere close to accurate. I believe Qualcomm gets same info as dash from ecm correct?

[Waterloo]

(08-30-2019 )marek4792 Wrote:  Pen and paper, mileage divided by gallons used. Subtract a gallon for each hour idled for highway mpg vs total mpg. It may be accurate within 2/10ths or could be off .5-1+ mpg.

Ditto. Same here.

[Rawze]

(08-30-2019 )Daytripper Wrote:  How accurate is the instant mpg readout on the prostar dash? Or should I rely on the performance monitor in the Quailcom?

The engine spits out that information. Every other device simply reads what the engine is doing.

For CM870 and CM871 engines, the higher your actual fuel mileage, the more inacurate it tends to be. For example, if your measuring 6 mpg to the pump via pen and paper, then it is likely to read 6.6 to 6.8. If your getting 8 mpg, it is likely to read as high as 8.7 to 9mpg, and if your bobtailing around and getting 10 mpg, it is likely to read 12 to 13+.

CM2250 and CM2350 engines are more acurate. they are usually within a few tenths unless your getting really high fuel mileage, where they will read higher than what your getting, but not by the extremes the older engine used to do.

<<<-- ALL THIS HOWEVER is completely and totally dependent on the fact that you have the correct program for your exact ser# engine and truck installed into your engine. On top of that, nobody has made any sort of custom program (delete program) and altered the fuel quantities the ecm "thinks" is going to the injectors/cylinders. I see this problem all the time with badly written delete programs where the dash of the truck is reading 1, sometimes, 2, or even 3+ mpg higher than what someone is actually getting when they use pen and paper.

I tell everyone ... USE PEN AND PAPER to figure out your fuel mileage. Don't be lazy. Especially if you have had major repairs done to your engine or turbo, etc. and especially if you have any kind of de-mandate or anything like that. Always assume what you see it the dash of your truck is a flat-out lie and verify what it is telling you every so often at the very least.

Personally, I have never used the dash of my truck to monitor actual fuel mileage. I have every single fuel receipt + mileage at time of fueling for all 1.3 million miles of my truck. I have a program I made that uses a database and tracks things very thoroughly (ref: http://rawze.com/forums/showthread.php?t...8#pid24378 ).

Profit margins are not going up, they are only shrinking out here,.. so controlling those expenses is where that higher paycheck comes from these days. Because of this, and because FUEL is my number one expense!, my number-one thing that is tracked, monitored, and controlled very very closely is that precious substance!.

Any kind of smart business person in trucking should have their highest priorities focused on their highest operating costs. For all of trucking,. that is FUEL!. If a person is not making great efforts to conserve it, and striving to become a hyper-miler... then I have no sympathy for them at all when they complain that they are not making a high enough profit margin!. - Hence why I absolutely can't stand people who brag about big power, mods,. and all the other things they do to waste fuel like a complete idiot.

[ynot]

The 870 actually has a parameter you can adjust too make the instant mpg read out accurate to within a tenth of a gallon, it does take a few tries too nail down though. Unfortunately i was never able too find an parameters like that for the 871. An I've never played with a 2250 or above

[Chamberpains]

Ynot, Whats the name of that adjustable parameter? Insite or CTerm?

[kryten]

Every time I fill up I reset my dash readouts so I can compare what the truck is reporting to what is actually happening. If I really feather foot that tank I will take 1-3% less fuel than the truck thinks it should take. There is a little error built in for the angle the truck sits at the fuel island as I have rear fill. On the other hand if I've had to push it a bit I will pump 1-3% more fuel than the truck thinks. The harder I push it the greater the error. This is for my Swedish nightmare so I don't know if the Cummins is like this or not.

At best the dash readout is nothing more than an indicator but it still gives you an indication of how heavy your foot is if you have the instantaneous readout.