Deadly is not a term i would use. They are as accurate the calibration
of the fuel metering system of vehicle and like anything there is
production tolerance and some with be mostly on and some will be off
some. Also since it is relative to speedo accuracy as well until they
all use GPS for speed and distance verification (and it is indeed
deadly accurate) it will never be right one unless your car is a
fluke. (change your tire size and accuracy can go right out the
Actually they suck! inaccurate as hell. You should get around 17-21 mpg mt
on the highway. Yours isn't broken in yet, after about 15K you should be at
about peak. You can hasten it by working the hell out of it. Go find sombody
with a trailer and a back hoe and run it up and down hills for a weekend.
Actually, mine is accurate to within about .7 mpg. I only have 1600
miles on my truck but I've been hand calculating mpg and monitoring
the overhead and so far my overhead mpg indicator is only .7 mpg
overly optomistic. That seems OK to me. FWIW, my 2006 2500 Cummins 4x4
Reg cab SLT is averaging 16.3 hand calculated, 17.0 on the overhead
for mixed city/highway driving. About 65% city/heavy traffic 35% open
hiway. I only cruise at 70 mph when on the open hiway.
This is my 5th Dodge truck, first Cummins. So far, absolutely no
issues whatsoever and I love it!
I've never read anything about why the computer is inaccurate, but I
would imagine that it is because it takes an average of an average, a
mathematical no-no. So, the inaccuracy could go either direction
depending what the driving style is since last Reset.
That was either an instantaneous reading (some years have both average
economy and instant economy displays), or you had just reset it. But - at
that point in time, with you running downhill, you probably were getting
If everything is left stock (tires, wheels, fuel system), I'd say the
overhead is accurate to within 5% or so, as long as you've got over 100
miles since it's last reset (again, different model years use different
algorithms to average fuel economy).
My '99 Ram gas burner shows about 10% higher than actual MPG. That also
means that the Distance To Empty (DTE) is off by about the same. I
regularly check the odometer by comparing it to highway mileage marks, which
is within 1%.
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