How accurate is your trip computer and avg mpg?
02-09-2011, 06:24 PM
#1
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How accurate is your trip computer and avg mpg?
Every car that I have owned with a trip computer and estimate mpg has always been optimistic, except for the 2005 E320 CDI that I picked up in October. The computer consistently calculates 1-2 mpg less than what I get by hand calculations. The discrepancy seems worse in cold weather. The week of single digit temps that we had a couple of weeks ago, resulted in the computer predicting 29.7 mpg but I hand (Excel) calculated 31.8 mpg.
What are other folks seeing?
What are other folks seeing?
02-09-2011, 06:48 PM
#2
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04 CL55 (sold), 2012 CLS550 (sold), 2014 S550 (sold), 2015 ES300H (DAILY)
I think the calculations in my CL55 are even worse...sticker says I should be getting 14-15 city and 21 hwy, but I've been getting 12 mpg all around (includes city + hwy). I'm not a lead foot racer, and I drive very conservatively....I am envious you are getting such great gas mileage haha
02-10-2011, 08:09 AM
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1982 300D VNT, 1980 240D 3.0T, 1982 300TD
The computer display is based on the calculated amount of fuel injected into the engine. Its accurate.
What isn't accurate is the tires. Even a few psi can make a measurable difference in rolling distance. Thats why the difference is larger in the cold, the tire diameter shrinks due to the higher density of the air in the tires filling less volume with the same pressure. On top of that, a tire gets smaller as it wears. Slap on a set of new tires and your MPG will instantly jump a few points, even though actual economy doesn't change.
On top of that, the tires balloon out from centrifugal force on the highway. Which will further return worse mileage in city driving.
What isn't accurate is the tires. Even a few psi can make a measurable difference in rolling distance. Thats why the difference is larger in the cold, the tire diameter shrinks due to the higher density of the air in the tires filling less volume with the same pressure. On top of that, a tire gets smaller as it wears. Slap on a set of new tires and your MPG will instantly jump a few points, even though actual economy doesn't change.
On top of that, the tires balloon out from centrifugal force on the highway. Which will further return worse mileage in city driving.
Last edited by 240D 3.0T; 02-10-2011 at 08:13 AM.
02-14-2011, 05:38 PM
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2006 E320 CDI 235 hp 420 ft tq
Mine shows more , two MPG than I get for the most part.
I have watched and have been able to see the mileage go up when I'm going down hill with little or no input on the go pedal so I can see how its off.
I have checked with a hand held GSP to see if my speedometer and odometer where right and they where on the money.
I have watched and have been able to see the mileage go up when I'm going down hill with little or no input on the go pedal so I can see how its off.
I have checked with a hand held GSP to see if my speedometer and odometer where right and they where on the money.
Last edited by 2slowcdi; 02-14-2011 at 05:45 PM.
02-26-2011, 09:31 PM
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03-21-2011, 09:41 PM
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The instrument read out on my 2008 E320 BlueTec is usually 2MPG higher than what I actually get when comparing it with the trusted miles divided by number of gallons method.
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03-23-2011, 06:13 PM
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03-28-2011, 03:46 PM
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Just go back from a 2300 mile road trip a week or so ago, got 24.9mpg as measured after a few fill-ups. It was extremely windy for about 1/3 of that. The trip computer showed within a couple tenths of that figure. Have about 23K miles on this SUV with original tires.
08-04-2011, 10:52 AM
#11
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Revisiting this thread since we got a 2011 R350 BlueTec for the family. I was excited at the first tank mpg average of 25.3, but then hand calculations were 23.6. Long trip last weekend, computer says 28.5, hand calculations say 25.2 mpg. 
I guess the new computer over estimates and the old ones, under estimated?
I did go to 17 inch wheels on my 2006 E320 CDI with slightly smaller ride height tires. Supposedly when my speedo reads 61.2 mph, I'm now going 60 mph. Now my hand calculated mpg is within 0.3 mpg of the computer. Overall average on the 2006 E320 CDI is now 33.1 mpg.
I guess the new computer over estimates and the old ones, under estimated?I did go to 17 inch wheels on my 2006 E320 CDI with slightly smaller ride height tires. Supposedly when my speedo reads 61.2 mph, I'm now going 60 mph. Now my hand calculated mpg is within 0.3 mpg of the computer. Overall average on the 2006 E320 CDI is now 33.1 mpg.
Last edited by SnowCub; 08-04-2011 at 10:53 AM. Reason: typos
08-05-2011, 04:49 PM
#12
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My 2008 220CDI computer always reads a better MPG than the hand calculation. About 6.7 % difference!!Original 205-55-16's tyres.Speedo reads accurately over the standard 5 km measured strip.
