[Jadar]

I usually never pay attention, but yesterday at the gas pump I was scrolling through my settings, and I said what the hell I want to see if this calculator is accurate. And its of by .5 gal wich probably is not that bad considering the inconsistant speeds at wich the car travels.

But here it is, 26.7 mpg. About 75% highway travel I added 14.18 us gallons and traveled 385 miles, so that puts me at 27.2 mpg. Not too bad for cruising at 75-80 mph.

But onboard calc reads this

[R608]

Interesting. Mine's about 1 mpg off, and consistently on the optimistic side relative to my calculated fuel economy.

[335e92tx]

Iv never had the two methods come up with less than .5-.8 variation. The onboard method is always at least .6mpg high. Not sure why that would be the case.

[Wires]

It could depend on how much fuel you put in. Not all pumps pump the same rate or have the same kick off threshold. There a couple around where I am that pump so ungodly slow that it'll kick off when it's almost running out the neck, others when there is a 1/2 gallon left to go...

At the end of the day, the calc should be calculating down to the 1/10 of a mile and a sub fraction of a gallon (since it would be measuring injector pulses in the 1/10000 of a second.)

[95325i]

The MPG reading also calculates from the last time the reader was reset. Let say you drive 1,000 miles all highway then drive 250 mile in all city the mile will read higher when driving in city because of the average number of highway miles driven off sets and city miles driven.

Resetting after each fill up will give you a closer average reading.

[Falafel Combo]

Same here




I'm usually closer to 420 or 440 per tank. Also, OBC usually reads 28mpg (combined driving), when it's actually more like 26mpg (according to manual calculations).

[tibra1]

My MPG gauge has been inaccuate from day 1..i simply divide the trip odometer by the number of gallons to come up w the real #

[listerone]

Mine seems to be off (optimistic) by close to 4mpg.However,I haven't done a careful analysis yet (fewer than 5K miles run so far) partially because diesels improve greatly when broken in so it's early to do it now.However,I once did a semi-careful check and the computer said 39mpg and the fill-up calculation was almost 36mpg (mostly highway).At about 10K miles I'll do a careful analysis and see what happens.

[lucius]

I've never seen anything over 19.5.

[CPWINCH]

Originally Posted by nubian

I've never seen anything over 19.5.

i never see anything over 17 lol usually around 15-16. I drive like a douchebag and mostly city

[dcat151]

Originally Posted by R608

Interesting. Mine's about 1 mpg off, and consistently on the optimistic side relative to my calculated fuel economy.

That's the same thing that happens to me. The OBC is always about 1 mpg higher than what I calculate.

[witeagle]

On my 2002 325ci the gauge was about .8 to 1 mile optimistic. Usually reading around 25.5 when in fact I was getting 24.7 based on trip odometer / fuel consumption.

On my 335i I am getting about 1mpg better than the cars MPG calculator. In my normal daily drive the car has been stuck on 21.1 mpg however I got 22.1 based on my trip odometer / fuel consumption calculation. I sit in a lot of traffic on the way to work which just kills the mpg.

On a long road trip i've gotten 29+ mpg

Wires mentions above that not all fuel pumps have the same kick off threshold - The true way to test this is to reset everything and fill up at the same exact pump for 3 tanks of fuel.

Keep track of your MPG for each tank load doing a manual calculation. Take the average of the 3 tanks of fuel and then compare it with cars MPG calculator and see how close they are.

This should give you a much better average over a longer period of time and through different types of driving, unless of course you live in the city and never see the highway!

[J3NGOe90]

Originally Posted by nubian

I've never seen anything over 19.5.

ihave never seen anything over 15.6, right now im getting 14.4

[listerone]

Originally Posted by nubian

I've never seen anything over 19.5.

Buy a diesel and 35+ will become routine.

[Jadar]

Originally Posted by listerone

Buy a diesel and 35+ will become routine.

I would love the 335d as my daily driver. I wouldlmt kind a 335i either. Ill see what i land 1st.

But yeah i think filling at the same pump a few times would prob be more accurate like someone stated. Non the less if im averaging 27.2 mpg in a 6 cyl thats not too bad!

[Eights-n-Aces]

I see two issues with your method:

1. Did you reset your MPG when you filled-up prior to your test run? If not, the MPG is an average fuel economy since your last reset. It doesn't reset automatically when you fill up.

2. I think you're still within the systematic error of your measurement method. The difference between 26.7 mpg and 27.2 mpg, over 385 miles is 0.24 gallons. That's not much. Full is not full, it depends on when the pump handle clicks off and whether you squeeze in a few extra drops to round up to the next dollar. After the first click, I'll bet you can squeeze in a whole additional gallon. So, if your previous fill-up stopped at the high end of this range, and your ending fill-up stopped at the low end, you'll see much better gas mileage.

The best way to get rid of this error is to spread it out over multiple fill-ups.

1. Fill-up (stop at the first time the handle clicks off)
2. Reset your MPG and Note your starting odometer
3. Keep track of the volume pumped at each subsequent fill-up. Go for at least 2,000 miles.
4. On your final fill-up (stop at the first time the handle clicks off). Add this final volume to the previous volumes and note your final odometer reading.
5. Do the math and compare it to the MPG reading on the OBC.

At the end of the day, the difference between the MPG reading and your calculated MPG is an accumulation of all error in your method. There is error in the pump readings, error in the odometer and the differences between "Full" between the first and final fill-ups.

If this isn't anal enough, try running a separate GPS that monitors your miles traveled separately from the odometer.

[Magic-Man]

These calculators have never been close to accurate in any of my cars.

[lucius]

Originally Posted by listerone

Buy a diesel and 35+ will become routine.

Shortly after this thread I filled my car up. Mind you I just got this car and the more and more I drive it the more I'm learning about it. So I've never done a reset to the averages by holding the BC button. Now it's showing what I'm averaging for mpg.