2nd Gen Non-Engine/Transmission MPG: Trip computer vs hand calculation

[TimBrunett]

I'm new to TDR having my 24v for about a month and I've been keep records on fuel consumption. So now the Mini-trip computer, MTC, is telling me an avg fuel eco of 17. 7 mpg on a tank, but a hand calculation shows about 16. 3 mpg. I think 1. 4 mpg difference is significant and I’m curious if anyone else is getting a variation?

I reset the MTC at each fill up but, it continues to indicate a higher avg eco than a hand calculation based on the miles driven and gals used. Math is math - a calculator and the MTC should theoretically show the same, shouldn’t they? There’s got to be something that I’m doing wrong or not doing. Can someone please help me out here? What’s the right way????? I’m the anal type and this is going to bug me until I get it resolved. Thanks Any comments would be helpful.

 

[Timbeaux38]

the MTc is notorious for being incorrect- are you runnig stock tires?- if not, that could throw hand calcs waayyyy off-

 

[TimBrunett]

the MTc is notorious for being incorrect- are you runnig stock tires?- if not, that could throw hand calcs waayyyy off-

The tires are Michelin LTX M/S, LT265/75 R16. Not the original tires but I dont know what was orgiginal. I can measure the circumference but I dont know what original was anyway. I could run it between mileposts to see what my odometer says. What would you suggest? Thanks



Tim

 

[Gary - K7GLD]

1. 4 mpg error for these trucks is actually pretty good! Those numbers CAN get even further from reality as power mods are added, since the controlling computer has no idea what those mods are, or how they potentially affect mpg. A pity in my case, because with my current mods, MY mpg readout CLAIMS I am getting 25. 5 mpg over the last 300 mile sampling period...



I gotta top off the tank and hand-calculate again - earlier comparisons showed my readout was only about 1 mpg off - I seriously doubt that is still true...

 

[WBusa]

The tires are Michelin LTX M/S, LT265/75 R16. Not the original tires but I dont know what was orgiginal. I can measure the circumference but I dont know what original was anyway. I could run it between mileposts to see what my odometer says.

The original should be on your door pillar sticker.

Occasionally checking for accuracy between reliable mileposts is always a good idea, IMHO.

Once you've confirmed that a non-standard tire size isn't screwing up your odometer readings (or you've determined the correction factor to employ), I would always trust hand-calculated mileage over what a computer thinks... but then, I'm a cussed old fogey.

 

[GSatler]

when i got my pynon factor set up properly & matched it with my gps, mpg came out within fuel tank fill error.

dont beleve your tir cir in in. unless u measure.

my was 5in. off.



gregg

 

[DHawthorne]

Mine is also off when running lite above 70mph. When running at 68mph, which is my best mpg speed, it runs real close, within a . 5 gallons/mile. When towing it really gets close as well. I use a excel spread sheet to track mine.



Dave

 

[TimBrunett]

I took a 20 mile trip on the interstate and checked my odometer against the mileposts. Return trip was in the opposite direction for a 40 mile total. I used a GPS to check the speedometer. Results: The odometer appears to be within about 2% accurate against the mileposts and assuming 20 miles of mileposts are in fact 20 miles (questionable) then my odometer shows about 2% less distance traveled. The GPS showed 61. 2 mph when the cruise was set on 60 and that’s about a 2% error. Since both were about 2% off I wonder if there is a corelation ?



Anyway, all this hasn’t really explained why the MTC is coming up with 17. 7 when a hand calc shows 16. 3 but I have learned that the MTC is generally high and I should keep that in mind. I am somewhat disappointed that my overall mpg is only 16 (mostly in town driving) when I was expecting a little more. Maybe a road trip is in order here to boost my mileage numbers and my moral. The MTC consistently shows 20-25 at 60-65 mph which is what led me to think 16 was a tad low. A little disappointment today but in the scheme of things - a very small thing.

Thanks for the replies. It’s good to have TDR for getting answers.



TB

 

[McCRam]

Its hard to get consistant calculations by hand as well. Its just not possible to get the exact same amount of fuel into the tank each time. Temperature, the accuracy of the pump meter, odometer, and quantity of fuel can all effect your numbers.

 

[WBusa]

Its hard to get consistant calculations by hand as well. Its just not possible to get the exact same amount of fuel into the tank each time. Temperature, the accuracy of the pump meter, odometer, and quantity of fuel can all effect your numbers.

Every measurement has an associated uncertainty, sure nuff... it always cracks me up when I see people expressing their fuel economy to two decimal places ("21. 37 MPG"), because the measurement errors involved probably create at least a 5% uncertainty... i. e. , about 1 MPG.



I don't believe temperature has a big effect, since the fuel is coming out of the ground where temperatures don't vary too widely, so the fuel's density shouldn't change much with air temperature. Ditto the pump meter, since these are (in principle, at least) tested for accuracy annually. Odometer can be a big source of error, but with mileposts, GPS, etc. you can check it, and apply an appropriate correction factor. Filling your tank to the exact same level every time has to be the biggest source of error. I keep threatening to put a 'sight glass' on my tank to solve this problem. As I envision it, this would be a clear plastic hose plumbed on one end into the bottom of the tank, with a valve on the other (free) end. When filling, I'd fish it out from under the truck, hook the valved end onto the fuel door, open the valve, and fill the tank till the fuel level in the hose came up to a mark. This would work great (provided there's a diesel-rated clear hose out there somewhere), but you'd have to be mighty anal to go to all the trouble of installing and using it :-laf and I don't think I'm there yet.

 

[KBalzuweit]

Heck my trip computer over lets say 5000 miles shows an average of 17. 7 and if I hand calculate I am 14. 8 or so. Thats 50/50 towing and empty and almost all highway.

 

[DHawthorne]

You need to reset the mileage and average fuel mileage each time you fill up in order for it to get close.



Dave

 

[jkalchik]

And on my 99, the trip computer ONLY sees the last 30 miles.

 

[rweis]

What does the MTC use as its input? I have the impression it is the TPS on a 2002.



I have DD2's and would expect some error there consistantly. Less TPS travel for a given MPH.



Turn on the Comp box (1x5) and the MTC gains about 4 MPG. Turn off the Comp, MPG drops 4 MPG. You KNOW it is BS.



The only thing I can figuer is it is based on the TPS. It is fun though to take an uninformed person and turn on the Comp and watch them marvel at 32 MPG on a 1 ton CTD. I was filling up at a truck stop and the trucker said he had a Furd and got 14 MPG. I can make the MTC show 27 MPG with Comp off @55 mph level. I figuer it has 20 - 30% error (stock tires etc) and really get about 16 (guesstimate, I reset the odo each fill and do a quick mental 400 miles / 25 gallons ).



Bob Weis

 

[DThelen]

Is there a way to calibrate this thing. Mine is way off.



Dan

 

[azpete]

I love it when you take your foot off the peddle going down hill and it shows 99mpg It's the same hill I used to walk to school and back 5 miles in the snow uphill both ways years ago.



I use the overhead readout as a gauge to what the roadway and outside conditions are. By knowing over a period of time what mpg it displays on level ground and no wind you can see how hard the engine is having to work in head winds and hills or towing a load and figure you'll be stopping for fuel alot sooner. It's also neat to see if you can accellorate with a light peddle and keep the mpg at the higher level, although it's alot more fun when it shows 2 to 3 mpg :-laf As far as accuracey of the average reading, mine is way off also. Thats why I have my log book at the fuel stops.

 

[B.G. Smith]

What does the MTC use as its input? I have the impression it is the TPS on a 2002.

I believe the MTC uses input from the APPS (TPS) and the speed sensor in the rear end to calculate MPG. Sometime when going down a long grade with the foot off the accelerator the MPG will go up pretty fast. It uses the current MPG calculations and the input from the fuel level sender to calculate DTE (distance till empty). I can fill my main tank while driving and watch the DTE climb. My MTC is off anywhere from 2 to more than 4 MPG depending on driving conditions. bg