MPG/Distance to empty on trip - K-Bikes.com - Excellence in Motion
 
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post #1 of 10 (permalink) Old Jul 28th, 2009, 9:12 pm Thread Starter
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MPG/Distance to empty on trip

I just got back from a 3 day trip to the Michigan UP (the new rain suit works fine).

On the last day in to the trip, I noticed my trip computer was showing up over 50 mpg, which it had never read that high before. My distance to empty, after a fill up was way higher than I thought it should read. Initial calculation was around 302mi, and went up a little even, up to around 308. Took around 100 miles before it went below 300! It did eventually go down to what I would call a reasonable reading. Sort of what I've noticed before, the distance to empty falls rather quickly towards the end. I am just wondering if this was caused by the highway miles, which I am not used to doing strictly highway stuff. Or maybe the engine is getting broken in? I rolled 5K on this trip.
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post #2 of 10 (permalink) Old Jul 29th, 2009, 9:46 am
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Did you fill up after that and do a MPG yourself to see what you actually got.

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post #3 of 10 (permalink) Old Jul 29th, 2009, 4:15 pm Thread Starter
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No, you got me there. Maybe the tank I am on now, which will return to the more mundane commuting/running around driving I will check it.
2 main questions are;
does 300+ miles on the tank (computer reading anyways) sound even possible on a 1200GT?

Has anyone noticed a fairly dramatic rise in MPG due to engine getting broken in? My early checks (using the traditional method for checking mileage) were getting me in the 40-43 mpg range. But I did get tired of checking it. Since my range was about what I was getting with my old K75 I didn't worry too much about it, although the K75 only had about 5.5 gallons of capacity.

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post #4 of 10 (permalink) Old Jul 29th, 2009, 6:31 pm
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How often do you reset the MPG reading? My 1300GT normally gets about 43 mpg, but highway cruising at steady 75 can exceed 45 mpg. Especially if you started from a higher altitude from where you ended, 50 mpg seems within the realm of possibility, but you'd have to be pretty gentle with the throttle.

OTOTH, maybe you accidentally switched the computer from US to imperial gallons, which would probably get you 50+ mpg from now on [i doubt this is something that could be done accidently]

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post #5 of 10 (permalink) Old Jul 29th, 2009, 7:13 pm
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Quote:
Originally Posted by GillyWI
On the last day in to the trip, I noticed my trip computer was showing up over 50 mpg, which it had never read that high before. My distance to empty, after a fill up was way higher than I thought it should read. Initial calculation was around 302mi, and went up a little even, up to around 308. Took around 100 miles before it went below 300! It did eventually go down to what I would call a reasonable reading. Sort of what I've noticed before, the distance to empty falls rather quickly towards the end. I am just wondering if this was caused by the highway miles, which I am not used to doing strictly highway stuff. Or maybe the engine is getting broken in? I rolled 5K on this trip.
I had something similar happen on my 2007 GT. After a fill-up I saw a very high (>50 mpg) mileage readout and a similarly absurdly high miles-to-empty figure. This very slowly normalized to proper readings by the end of the tank, was normal on the subsequent fill-up, and hasn't happened since. I have no idea what got crossed.
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post #6 of 10 (permalink) Old Jul 29th, 2009, 9:58 pm
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Quote:
Originally Posted by smiller
I had something similar happen on my 2007 GT. After a fill-up I saw a very high (>50 mpg) mileage readout and a similarly absurdly high miles-to-empty figure. This very slowly normalized to proper readings by the end of the tank, was normal on the subsequent fill-up, and hasn't happened since. I have no idea what got crossed.
I have had it happen on long trips and especially in the mountains. I didn't believe it either until if filled it up with 260 on the clock and it only held a little over three gallons. It did the same thing several days in a row in the mountains, but I only get 43 around home.

James
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post #7 of 10 (permalink) Old Jul 29th, 2009, 11:34 pm
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Quote:
Originally Posted by GillyWI
I just got back from a 3 day trip to the Michigan UP (the new rain suit works fine).

On the last day in to the trip, I noticed my trip computer was showing up over 50 mpg, which it had never read that high before. My distance to empty, after a fill up was way higher than I thought it should read. Initial calculation was around 302mi, and went up a little even, up to around 308. Took around 100 miles before it went below 300! It did eventually go down to what I would call a reasonable reading. Sort of what I've noticed before, the distance to empty falls rather quickly towards the end. I am just wondering if this was caused by the highway miles, which I am not used to doing strictly highway stuff. Or maybe the engine is getting broken in? I rolled 5K on this trip.
Gilly
The slow-at-first, faster-near-the-end rates are commonly reported for both the fuel remaining "gauge" and the distance-to-empty computation. I have not seen published information on how distance-to-empty is computed. Based on information I have seen from others, my guess is that the computer keeps a rolling "mpg" window, and multiples fuel remaining times the windowed mpg. Because the estimate of fuel remaining is somewhat flawed, the distance to empty is equally flawed, and thus appears to drop faster near the bottom of the tank.

The computed mpg is almost always optimistic. For my 2007 K1200GT, the computer computes 2-3 mpg too high in mixed city/highway, but is somewhat closer (still 1-2 mpg too high) on long slab highway rides averaging 70 mph or so.

You can find a two year history showing gas mileage (reported and actual) and other fuel usage and maintenance information posted at:

http://www.k-bikes.com/forums/showthread.php?t=13211

The latest data is at post #47.

...

Regarding your other question about whether a K1200GT can get 300 miles on a tank:

With a 6.3 gallon tank, you would have to average a true 48 mpg. Note that the filler neck may prevent you from getting a full 6.3 gallons into the tank. The longest range I have achieved on one tank is 255 miles. The best actual mileage that I have seen is 46.4 mpg. (The OBC reported 49.0 mpg.)

Bottom line: I believe it is possible to squeeze 300 miles from a KGT tank, under optimum conditions of fuel quality, altitude, speed, wind, etc. If you deliberately set out to do it, you could plan a ride that would achieve 300 miles on a tank to prove the point, but don't count on it for an ordinary trip.

(Note: Some people have auxiliary fuel cells attached to their bikes, or carry Sigg fuel bottles on rallies to extend their range.)

Last edited by XMagnaRider; Jul 29th, 2009 at 11:51 pm.
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post #8 of 10 (permalink) Old Jul 30th, 2009, 1:55 pm
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Yes, it is possible to go 300 miles on a GT tank, but you have to ride very conservatively to do so. No quick starts or stops, accelerate very slowly, maintain the speed limit, windshield down, etc. Basically, be bored out of your skull.

And yes, the reported MPG and Distance to Empty is very inaccurate. The DTE will often show way over 300 after filling, then drop slowly as you ride, then drop much faster as you get closer to empty. You can use it as a very rough guide, but don't count on it unless you like walking . . .

Basically, the gas tank isn't a uniform box or cylinder. It's wider at the top, and narrower at the bottom. So a 2" depth of fuel up top may be a full gallon, while a 2" depth near the bottom may only be 1/4 gallon. So you may drive 40 miles when full and drop 2" of fuel and a tiny bit on the gas gauge, but driving 10 miles when low will also use 2" of fuel and thus show the same gas gauge drop. That basically boils down to the BMW engineers setting the fuel strip as uniform all along its length, when clearly the volume isn't uniform throughout the tank.

My 40 mpg average means I can get 240-250 miles on a full tank, so I use the trip meter to monitor my progress and then plan gas accordingly. Unless I have the fuel cell installed, which means I can get 450 miles between fill ups.

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post #9 of 10 (permalink) Old Jul 30th, 2009, 3:11 pm
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I ran out of petrol on my K1200 GT on 267 miles , i filled up 24.5 litre of fuel. I'd done the same route the day before but filled up before i got home , about half a mile further.

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post #10 of 10 (permalink) Old Jul 30th, 2009, 3:14 pm
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Yes, the fuel gauging system isn't very linear and since the 'Miles to Empty' calculation requires knowing how much fuel is left the whole system isn't very accurate as a result. The MPG readout however only needs to know engine rpm, fuel pressure, and injector duty cycle, all of which can be measured fairly precisely and as a result the MPG display is actually pretty consistent. It could be accurate as well if only BMW would calibrate it properly, but currently it seems to read around 2 mpg high on most bikes. But subtract that out and the MPG data actually is pretty accurate most of the time.

Kind of like the TPM system which seems to read about 3 psi high on most bikes. But it also is consistent so if you back out the error the readout is pretty useful.

Since the magnitude of the error on the MPG and TPM readouts seems to be pretty common among all bikes one wonders why BMW can't simply make a slight calibration adjustment and bring them closer to spot-on, but who knows why BMW does what it does...
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