It disturbed me that one of the AH drivers IMO is posting false MPG figures

"34.9 MPG on NAVI, 33.8 MPG 'Trip A', 32.46 'at the pump"

he is posting the NAVI #s instead of gas pump figures.

Why would anyone rely or post what the computer guesses at instead of true miles/gallons ???


Will post this also in general but is this common reporting by AH owners??

 

NAV figures should actually be more accurate as satellites can pinpoint your true position. Your car's figures are based on the diameter of the wheel and revolutions -- which may not be 100% accurate. For all practical purposes, it's all gravy. Don't worry too much about it.

 

not worried too much just like comparing apples to apples.

yes tire dia changes and using the std. 10/32" tread thickness - 2/32 which is considered worn out = 8/32*2 = 1/2" in overall difference from BEST to WORST case in tire size. This difference on a 215/60/15 which is 26.157" using that as a center line for the tread would make the worst case inaccuracy of 25.907/26.407 = 1.93% difference.

32.46 (pump) * 1.0193=33.08 (worst case %) vs navi of 34.9

would find it very interesting if the driver on his next long trip (200+ miles) & compare his odom vs the mileage markers then use that difference and compare to his NAVI info. My Jetta, strange as it may seem, travels 1.025 miles more when I compared the odom vs mile markers on a 700 mile trip I took.

also if you traveled in a 100 ft circle for 1 mile how far would the NAVI report your distance traveled???

just wondering how good satellites and computers are getting at tracking us.

Thanks for your time and hopefully future info from AH owners.

 

I've got some issues with the whole reporting method business as well. I guess the best thing to do is just not worry about it but sometimes that is easier said than done. I always post using pump figures because that seems the most fair in the long run to me. Still the car and navi displays are better as far as compairing one tank to another. Tank fill issues make the at the pump method worthless for compairing one tank to another.

Now wouldn't it be cool if you could graph more than one method...........Hello J, how about a major upgrade allowing for more than one method to be input into the Gen 2 database? Too much work?

 

I forgot about one tank being different from another. my TDI has the vent mod thus when I fill up at the pump my tank is 100% full and there is no difference tank to tank. Also when you fill up 5-10 times your avg. tank and milage will level out and be very accurate.

ever since my C5 vette said I had 60 miles to empty and then ran out of gas I don't take much stock into computer guesses.

 

I think you should only use the numbers to compare against yourself. Maybe you can use others numbers to set goals for yourself. It's realy not a competition so who realy cares what one person posts vs another.

 

Tom you are absolutely correct, your attitude shows your maturity. I'm not quite as mature and do view the database as a competition. There is no way I'd be driving in 98 degrees heat with no air if there was no competition.

 

Hi Iboomalot:

___I use topped off tanks to topped off tanks via the legal cal’ed pump and miles traveled per the legal odometer. There is no better solution for consistency although you can argue accuracy until the cows come home due to the OEM’s non-exact odometer’s and tire wear. NAV units boot up way to slowly to take into account any movement before at least 3 birds are locked up and a continual distance track can be measured. I am not even considering when they lose lock and show you ˝ or more miles away before they get educated and jump back on course Heaven forbid when they tell you to take one route with their “Locked to Route” feature enabled (most OEM NAVI solutions do not allow you to change this ) and you take the alternate. That jump is always worth a tenth or two and I see it every day at the I-355 and I-55 junction just north of Joliet, Illinois. Another … I should upload a screenshot of the bread crumbs when I take a non-digitized road into my work location. You would think a drunk was driving though a 20 acre field instead of a single dual lane road into the exact parking lot and many times the exact parking spot each and every day.

___Jason, they should be more accurate but they are not in many instances unfortunately Another aspect that will never be accounted for by an OEM or non-OEM NAVI solution: The change in distance between the far right and far left lanes of an interstate through a turn. I have measured this more then once while on night shifts in the Corolla. Moving from lane to lane to cut off the corners (Inside lane at all times) shows a .5 - .6 mile difference over 93 miles then just taking the right lane all the way home. NAVI’s haven’t a clue what to do with that difference?

___Good Luck

___Wayne R. Gerdes
___Waynegerdes@earthlink.net

 

There was a similar issue a few weeks ago re: greenandblue's odd tank. Someone there also brought up the point that over multiple tanks, any fluctuations in how full the tank is get worked out- the deficit from one tank is made up in the next. Unfortunatley that makes it hard to look at short-term subtlies.

IMO, I'd take the odometer mileage over GPS. Even if its not as accurate in absolute terms, its more repeatable in its error, and that can be reliably accounted for. GPS has many variables out of the operator's control- satellite lock, time between fixes, algorithm used to calculate distance between fixes, yadda yadda. The 'bread crums' mentioned before allude to the non-repeatable nature of GPS fix errors. Up to 10 meters (?) for EACH fix...compound that and BAM- you've got an issue (of course, the errors can be considered to be largely statistical in nature and can average to ~0).

That's why DOT built differential GPS for coastal mariners- to find the local instantaneous error and send out a signal that corrects for it.

When the Navy switched from 80s vintage radar-and-imagery guided Tomahawks to GPS guided, there was a few meters' worth of dropoff in precision...it hit the 1st or 3rd window from the left instead of always the 2nd. But the change was there.

 

There are always going to be issues with accuracy no matter what way you measure it and for what purpose. I honestly don't think it's something to be worried about.