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08-12-2011, 10:36 PM | #1 |
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Drives: '11 Z4is AW/Walnut
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'11 speedo 3mph high, how accurate is the mpg?
At 70-75mph on interstate I average 29-31mpg and it is only rated at 24 so wondered what others are getting and if many have actually checked its accuracy. I checked the speedo against the Garmin.
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08-13-2011, 02:16 AM | #2 |
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Speedo is 3% to 5% off, sometimes as much as 8% depending on your speed
MPG is about 1 MPG off, give or take Best way to measure is to fill up, drive, then fill up full and do the math to see what you MPG is My Mpg is about the same as what the Z4 and the M3 read, +/- 1MPG
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08-13-2011, 10:54 AM | #3 | |
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Note that the rated MPG is based on a certain test, and doesn't necessarily correspond to the instantaneous MPG you'd get cruising at a single speed. You can look up the actual test procedure from the EPA if you're interested. |
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08-13-2011, 12:08 PM | #4 | |
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08-13-2011, 07:28 PM | #6 |
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On my first interstate trip, I noticed that I was being passed more than usual so I checked the speedometer against mile markers multiple times in 2 different states and my speedometer read fast by an average of 2.3 mph. I forgot to take my Garmin on that trip so I later checked it and it reads between 2 and 3 mph (no decimal readout) slower than the speedometer so that jives with the mile markers.
On that trip, the computer read about 29 mpg when I got off the interstate after about 130 miles of 75-80 mph driving on a hilly interstate. It was reading 25.0 mpg by the time I filled up (couple of days of city driving) and I calculated 23.7 mpg. On the way back, it was reading about 30 mpg. Much better than the 24 mpg EPA rating but I kind of expected that. I also have the dct. |
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08-13-2011, 07:53 PM | #7 | |
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08-13-2011, 11:30 PM | #8 | |
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I'd like to see the speeds associated with the 27-29 mpg claims on the thread. |
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08-14-2011, 07:57 AM | #9 |
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I get the 29 mpg when running 75 mph. It would be 30 except for a little in town driving mixed in.
Check your tire pressures. I always set mine to 2 over the recommended settings. A darn firm ride for the Z4 but it helps mpg and could save a wheel hitting a pot hole. I think my life time average is > 24 mpg and I don't baby it for sure.
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08-14-2011, 10:36 AM | #10 |
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You ever pass by one of those "you're going too fast" speed readers set up by the police on local streets? they always read 2-3 mph slower than your speedometer.
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08-14-2011, 11:05 PM | #11 |
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The speedo is designed to be optimistic, like on all cars, by about 3%. And it gets worse as tires wear out.
Don't trust the GPS either. It measures average speed over a few seconds based on a location that's only accurate within +- 100 ft. As for mpg, I can get 30 mpg easily at 70-75 mph on my '35i so you're in the range.
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08-15-2011, 12:29 PM | #12 |
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LOL i have the same exact logic!
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08-15-2011, 01:41 PM | #13 |
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My speedometer always reads about 3 mph faster that whatever flashes on the roadside radar sign. We have those in AZ.
The speedometer has no effect on the odometer. The odometer is accurate, so the MPG read out is accurate as well. I have a 2009 with DCT and get 30 mpg if I cruise at 70 mph in 7th gear. So, the BMW estimate of 24 MPG highway are way too conservative......OR, BMW meant it to reflect the highway MPG on an typical trip, where there is some passing and gear shifting. 24 MPG is much more realistic when you factor in passing and gear changes. |
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08-15-2011, 02:29 PM | #14 | |
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08-15-2011, 03:11 PM | #15 |
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08-16-2011, 12:26 PM | #16 |
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On both my former 335 and my current Z4, indicated 70 mph is actually 66 mph, based on GPS readings. In response to another reply, the GPS is accurate to within a few meters, so the MPH is very accurate. My understanding from what I read on the e90 boards is that this overstatement of MPH is intentional because of BMWs concerns over the speedo reading slower than actual (considering the possibility for a change in tire sidewall when tires are replaced).
On MPG, I have calculated a few times and each time my calculation was within one MPG of the indicated MPG. Its been both higher and lower. Seems like MPG reading is reasonably accurate. Notwithstanding the speedo issue, I think the odometer is accurate. |
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08-16-2011, 12:56 PM | #17 |
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By law, in most countries:
* the speedometer must be accurate within +10% and -0% (can never indicate a speed less than actual or more than 110% of actual) * the odometer must be accurate (also in the U.S. that's not specified in the law.) Typically expect +- 2.5% accuracy Tire size and tire inflation account for most of the variations. As for GPS, it's about 15-20 meters accurate for civilian receivers - that's about 1% of a mile so we should expect 1% speed accuracy over a mile, better over a longer distance if speed is constant.
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