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Question Of The Week

106 Hand-Calculated MPG Confessions

We asked our readership if they, “Trust the Dash, or Do the Math” to calculate their truck camper rig miles per gallon.  Over one-hundred readers answered, most with a resounding “Do the Math”.  A few said, “Who cares?  I just want to go truck camping, and have fun!”

We’re in both camps now.  We hand-calculate, but will keep on truck camping no matter what the results.  Besides, you can’t put a 5,000 pound truck camper on a Prius, at least not for very long.

“Why does it matter?  You already own the truck and camper.  It burns what it burns.  There’s not really a whole lot you can do about it except slow down a little.  I say, just forget about what mileage you get and enjoy the trip.  If you feel like it burns too much fuel, sell the rig and find something more economical.  Worrying about a constant is only going to upset you at how low it really is.” – Brian Rushing, 2014 Ram 3500 diesel, 2013 Lance 855

“We have a log book in the truck where we keep every fuel purchase.  We notate the gallons, price, and miles on every tank, calculating by hand the mileage on each tank.  At the end of the trip we have an overall average to see.  It is interesting to see how the terrain, speed, and especially the wind affect the miles per gallon.  We get between eleven and twelve plus mpg at 65 mph speed.” – Dewey Lackey, 2003 Silverado 3500HD diesel, 2014 Lance 1172

“I hand calculate my fuel mileage using a calculator and recording at each fill up.  At the end of a trip, I divide the total miles driven by the total fuel used always starting with a full tank and ending with a full tank.  I do have an onboard computer as part of some Banks aftermarket equipment I installed and find that it is very accurate, but I still hand calculate.

After driving almost 12,000 miles on a trip to Alaska this summer, I averaged 12.49 mpg overall and the Banks onboard computer showed 12.5.  From tank to tank, the onboard computer seems to be off as much as .2 mpg +/-.” – Eldon Rhodes, 2008 Chevy 3500 HD, 2011 Lance 1050

“I hand calculate and it’s about 10 percent off all the time from the dashboard.  We pretty much have the same truck as TCM, except I have 4.10 gears and it stays in MDS (cylinder deactivation) a little longer due to less effort due to torque multiplication when loaded.  Loaded I average about 12 mpg hand calculated and unloaded about 18 mpg hand calculated.  The display says 13 and 20 on average.” – Roy Del Pozo, 2014 Ram 3500 gas, 2013 CampLite 8.6

“I never trust the dashboard fuel mileage other than the current estimate.  I use an application called Gas Cubby to track all vehicle expenses and mileage calculations.” – Timothy VonWolffradt, 2014 Ford F350, 2011 Lance 850

“Rule number 1: Don’t believe the lie-o-meter.  That said, I’ve found mine is actually relatively accurate over the long haul.  For example, loaded with camper and boat in tow running 1,700 miles to the Florida Keys, I found that my dash readout was pretty accurate based mostly on the DTE and what I was seeing on the odometer, per tankful.  At 35 gallons and 12 mpg that’s 420 miles on a tankful.  Resetting the ODO, I was easily seeing in the mid to mid-upper 300s before pulling off for a refuel, dumping in 26 to 28 gallons.  So, 420 miles was definitely in the mix, which would indicate 12 mpg.  And that’s a 3,600 pound camper wet and 4,000 pound boat and trailer in tow.  Without the boat in 2011, on the way home, my dash was reading almost 14 mpg.” – Nik Rende, 2011 Ram 3500, 2005 Lance 1010

“I check by hand.  I use a Scan Gauge to watch mileage.  It is accurate.  Driving by the Scan Gauge really helps keep the mileage up.” – Bill Harr, 2005 Toyota Tundra, 2013 Four Wheel Camper Hawk

“I hand calculate the fuel mileage.  I don’t have the computer to do it for me.  I use the phone calculator.  The truck mostly gets 10 mpg, so I can pretty much figure it in my head.  If it looks different, I’ll use the calculator.” – Jeff Hagberg, 2002 Ford F250 SD, 2006 Travel Lite 800SBX

“I run a pen and paper continuous mileage log on all truck expenses, maintenance, and fuel.  I occasionally do hand calculations after a number of fill-ups to even out spot variations and to compare to the dashboard numbers, which optimistically tend to run a little over 10% high.  I still find the dash numbers useful as a trend guide, like if I do an extended run at say 63 mph versus 71 – do I see a mpg improvement, and how much?  Or, if I adjust my Smarty from 99% down to 93%?” – Wil Painter, 2007 Dodge diesel, 2010 Host Cascade

“The dash board results on our truck is usually very, very close to hand calculations.  We’ve had this truck since new in December of 2003.  Loaded with the camper and everything, it gets 15 mpg.  I just had a check engine light issue related to transmission electric supply.  I don’t know if they reset any computers to different settings or not.  Our upcoming three week trip next week will tell the tale.  Funny though is that the truck ran and shifted just fine while the issue was ongoing before we were able to get it fixed.  Thankfully, it was only a $200 one day fix!” – Cheryl Nelson, 2004 Chevy 3500 diesel, 1990 Shadow Cruiser 9.5

“Whenever we buy a car or pickup, we test various grades of fuel to find the best cost to milage ratio.  Hand calculation only!” – Philip Tron, 2009 Chevy 3500, 2012 Lance 1050

“I calculate the MPG by resetting the trip miles each fill-up and dividing that number by the fill up gallons.  With or with out the camper, I get between 10 and 12 mpg depending on whether we’re doing stop-and-go city driving or 59 mph, 2,000 rpm, cruise control across the central plains of Nebraska.  My miles per gallon are accurate, but the tenths are off on what the dashboard says.” – Jesse Taylor, 2005 GMC Sierra 2500HD gas, 2006 Lance 815

“I’ve done it doth ways but, since the dashboard data agrees with hand calculations within 1%, there’s no reason to do the math by hand.  Mileage with the camper varies quite a bit depending on how fast we drive.  We get right at 13.5 mpg at 65 mph, dropping to around 11 mpg at 70 mph, and a little under 10 mpg at 75 mph.  A  little extra speed really costs.” – Phil Rodacy, 2012 GMC 3500 diesel, 2006 Okanagan 90W

“I have cross checked on my F350 and have found it to be very accurate.  Lots of things can affect mpg calculations such as a different tire size from OEM and over/under inflation pressures.  These can throw the computer off and, if you drive a diesel with longer distances between fill ups, the effect becomes accumulated +/- in the on board computer.” – Pryor Donald, 2011 F350, 2009 Arctic Fox 1150

“The actual mileage we experience is about 10% lower than the amount computed by the truck.  On our recent 15,000 mile trip around the US and Canada, the truck said that we averaged around 12 mpg.  In reality we got a bit over 10 mpg.  Since putting in the 56-gallon fuel tank, we noticed that the gallons used indicator is off by the same 10%.  I conclude that the fuel flow sensor is not reporting the correct fuel flow.  Maybe it’s mis-calibrated, or by design?  The cynic in me leans toward the latter.” – Jim Goodrich, 2006 Chevy Silverado 3500 diesel, 2008 Lance 1191

“Depending on wind direction and speed, versus ideal conditions with no wind, I get eleven to the high twelves.  Hand calculated seems to always agree with my dash data.  I simply divide total miles by gallons pumped.” – Dennis Glow, 2003 Ford F350 gas, 2014 Travel Lite

“My wife, Cheryl, has kept track of all of our truck camper trips using pen and paper.  With over 13,000 miles, we have averaged 14 miles to the gallon.  As soon as I noticed that we put in at least one more gallon than the truck said we used, I quit believing the dash numbers.” – Matt Reinker, 2006 Chevy 1500 gas, 2007 Northstar TC650

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