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Did some data logging tonight with the scanner for speed and rpm as well used a gps at the same time. Set speed points with the cruise control, and checked it against the trip computer.
The results are interesting because the calculated speed off the rpm pickup using the transmission gearing is actually pretty close to the cruise speed with cold tires at minimum pressure, and the speed reading off the scanner was almost identical to the gps based speed. The trip computer speed is dead on the cruise control speed.
Trying to figure out what the percentage difference is and then correct it with tire diameter.
For example at 120km/hr my RPM calculated speed is approximately 117.5km/hr. And the gps and scanned speed measured is 112-114. Quite a difference. The one I am inclined to trust the most is the one I can calculate but it is interesting. I would be curious how much temperature and initial pressure will vary the results.
The reason I have been doing this is more of a continuation of my testing of tires. My fiancé has a 323i with 17's and I have the M package with 18's so I have driven now with both the 18's and the 17's in a run flat tire on the 335d. I also was having a long discussion with the BMW sales guy who on first getting his car dumped the run flats in favour of the non-runflats. I was curious about the normal tires, so he let me take his car for a drive to see the difference, and what a difference in ride and control.
So before I take the plunge and buy the tires, I just wanted to make sure I am getting enough rubber between the rim and the road and as well to make sure I don't have too much an error in my speedo since there has been quite a lot of discussion on the topic.
So that was my whole reasoning to do all this which I think I'm pretty close to making the purchase. I need to only do one or two more comparison's and then I think I'm good.
Sorry for the long winded reply.
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2011 335D
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