RRUGG
Newaygo, MI,USA
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Joined: 08/08/2003
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I compared the overhead with hand calculating over two tanks of gas. Both represented about 300 miles of driving. All highway driving was 55-60 mph. Both tanks included about 25 miles of non-highway driving, mostly cities & small towns. The first hand calc was only 1/10 of a mpg lower than the overhead. The second hand calc was 1.9 mpg lower than the overhead. Curious that there would be so much variation. Ample reason to do the hand calc and ignore the overhead.
RRUGG
2008 Dodge 2500 QC 4x4 SB Cummins 6 speed auto 3.73
1997 Holiday Rambler 29FK travel trailer
2006 Summit 22RB travel trailer
2003 Dodge Grand Caravan Sport
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donn0128
Pronounced Ore-gun
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Joined: 04/21/2005
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Since the OH probably does not take into account the actual miles driven, but rather a combination of factors it is highly unlikely that it can ever be really accurate. It was only designed to give people a feel for how good/bad of economy they are getting.
Donn
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thebat68
Spring Hill, FL
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Joined: 06/10/2005
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Mine is not very accurate, so I really don't pay much attention to it. I'll say one thing for it , though. It sure knows when I'm towing - you can see those numbers plummet! 
2000 27' Fleetwood Prowler
2000 Ford Expedition 4WD Eddie Bauer
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mowermech
Billings, MT
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Joined: 06/28/2003
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Keep in mind that the computer is giving an AVERAGE figure. That's why if you hit the reset when coasting down a hill it might say "99.9MPG", and if you hit the reset during a jack-rabbit acceleration it might say "4.5MPG".
So, if you coast down a hill to a stop light, then do a full throttle acceleration away from the light, then cruise at 55 MPH for a while, the computer is going to average that all out, and that is the figure it will show.
No, that isn't an accurate indication of overall mileage, but it is a computer, and that's the way computers work.
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'94 Dodge 3500 4X2 CTD, Std. cab, LB, 5 speed, 4.10 LS diff., Jacobs Rambrake, 273,000 Miles
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ktmrfs
Oregon
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Joined: 06/22/2005
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While the OH readout knows the exact miles driven, they calculate the amount of fuel used by looking at injector pulse width, pressure etc. (Since the fuel line has a return to the tank, it's hard to measure actual fuel flow).
The other variable is how full the tank is on each refill This will affect your hand calculated MPG, but not your OH readout MPG.
Are you topping off the tank till you can see it in the filler tube? If not differences in tank level can affect your hand calculated MPG, and this level can be signficant. You could easily see a 1-2 gallon difference in how full the tank is if you just let the pump click off. On a 20 gallon fill this is 5-10% which could have a > 1 mpg difference in your calculated MPG.
On my chevy, I always top off my tank and my readout and hand calculated seldom differ by more than 0.5mpg, usually within 0.2 mpg.
All bets are off if you have installed any kind of tuner. Then the factory cal constants for fuel used are usually way off from actual.
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Greg B
Minnesota
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Joined: 07/28/2003
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Mine seems to be pretty accurate. I'll get a tank here or there thats a little off, but its usually within .5 MPG. But my driving style is pretty constant with that truck. Its not a daily driver so almost all of the miles put on it are longer trips with the camper loaded.
04 Dodge 1 ton dually, Cummins, auto, quad cab, 4x4, SLT, Sport, Reese Titan V
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hotpepperkid
Chino Hills CA
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Mine has always been +- .2 the times I checked it with pen and paper. I dont bother anymore with compairing.
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2000 Ford F-350 SRW 4X4 PSD Jayco Super Lite 29.5RKS 5th wheel 50 gal X-ferflow in the bed tank. Banks big exhaust and Stinger kit.
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dodge guy
Chicago, western subs.
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Joined: 03/23/2004
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Yes, the OH is very accurate. it looks at several different factors that the average driver could never look at. so when someone says that their OH is off when compared to hand calculations, it is more than likely that their hand calculation is not accurate. another factor to consider is the computer can read the amount of fuel in the tank by the 100th of a/gal. something you could never do from looking at the fuel gauge.
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Tvov
CT
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Joined: 07/19/2003
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Now that I've had my truck for a few months, I have found that the computer is usually within .5 mpg when you get to about 1/2 tank of fuel used. Right after I fill up the computer mileage will jump really high, but then it levels out after awhile. I hand calculate all the time out of habit.
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crickeydog
Marietta, Ga.
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Joined: 01/29/2004
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FWIW: I don't bother checking our MPG except that once in a great while I'll look at our truck's MPG computer display. It is what it is what it is. I alway's reset one of the truck's trip odometer's at each fillup so that around 325 miles I gotta start looking for the Flying J.
Happy camping!!! See ya'll down the road!!!
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