Hey guys, I'm a diesel newbie. How come the onboard computer economy numbers don't match up with paper and pencil numbers. Any input would be helpfull. I just went on a long hwy trip ( 250miles ) no load at approx 80 mph. Computer was reading 15.0 and paper and pencil showed 14.47 mpg, all readings done as the fuel station.

 

You are going to catch some real slack for that title. You might want to edit it to read diesel milage.

Brad

 

I wouldn't worry about that small of a discrepancy. Especially if thats an 80 mph average. Last tank I had, the overhead said 19.4 and my calculator said 19.2.

Heck, it seems that lately, lots of folks would kill for that kind of fuel mileage.

Topping off the tank can result in slight differences too in calculated numbers.

 

I've been tracking my fuel mileage religiously for the last 10,000 miles and my overhead reads a "reliable" 8% lower mileage than calculated. This includes a combination of empty and pulling.

100 Proof

 

Thanks for the tips... Told yall I was new to this!!!!

 

Welcome to the site and yes I'm going to give a little bit of "What the Heck" of course you will have a problem if you are putting GAS in a diesel .

i just didn't want to let you think tht you got away with that fopah

 

With my Edge box and the 315s and no recalibration of the speedo yet, mine is showing exactly 1 mpg more than I am really getting....

 

mine is getting 19.6 right now, thats with 285s so it will read a little off but not much

 

I guess I need to switch to gas and maybe mine will increase also

 

The answer to the question is that the computer must guess and the hand calculated method is exact (plus/minus variations in how much you top off). Per the manual it says the computer is combining the historical mileage since it was last reset and the instantaneous mileage. This is nice since it stops you from seeing 0.5mpg while idling parked and 99.9 while coasting down hill. Still, it's a guess.

My computer seems to calculate high for unloaded city driving and low for towing and unloaded highway driving. However, it is a good relative indicator.

 

Thanks for that info Dave. This site and everyone that uses it is 100x more helpfull than the service department at the dealership!!!!!

 

Just put my first tank of fuel in my new Dodge that i got last week, got 14.8 mpg, thats with 4.10's & Auto trans. BTW: at 65mph i'm 2000RPM seems high, 70mph 2300-2400.