Originally Posted by DrLev99' post='465517' date='Sep 3 2007, 08:54 PM

Has anyone considered the fact that radar guns aren't exactly accurate? Here in the states, my speedometer always reads about 3 mph more than the radar gun. We have those static signs on the highway that tell you how fast you are going. No matter what kind of car I am in, the radar is always low by a few MPH. One day on a flight back from Las Vegas, I had the luxury of sitting next to a PA state trooper. Aside from some interesting, insider info & stories re: flight United 93 that crashed in Shanksville on 9/11 (where he was on duty at the time) we talked about radar and speeding on the interstate. He confirmed what I always knew, which is that the radar gun is always low by 2 or 3 mph. So, I doubt it's your car's speedo....I am sure your BMW is more sophisticated that a stupid radar gun!

Originally Posted by iversonm' post='465522' date='Sep 3 2007, 09:16 PM

I used to work for a professor that was in charge of testing, calibration, and safety of various speed control implements for the State of Michigan.
They are more accurate than you think.

However, your BMW's speedometer is not, as dictated by ECE-R 39.

Here is one link that has some facts, and here is another

Originally Posted by Deano' post='465538' date='Sep 3 2007, 09:51 PM

Great article!

Could anyone running 285/35/18's please chime in with fit and handling input?

thank you,

Originally Posted by Deano' post='465292' date='Sep 3 2007, 07:11 AM

Hello all,

I'm looking forward to when the runflats on my 04 545 w/sport stetronic wear out.

There have been a lot of those cop radar trailers around last week (end of the month). They they flash a number to show motorists how fast they are actually going.

I noticed that my speedo was always approx. 3-5 mph faster than the radar number. My tire pressures are fine. This made me wonder about tire diameter as a possible cause.

A non-sport 545i runs on 225/50/17's all around. A sport 545i uses 245/40/18 F and 275/35/18 R. Wheel diaameters as follows:

TIRE WIDTH ASPECT RATIO WHEEL DIA TIRE DIA

225 50 17 25.86

245 40 18 25.72

275 35 18 25.58

285 35 18 25.85
It's interesting that the 285's have the same diameter as the non-sport 225's, and the extra 10mm of width would look so cool!

Based on my radar accounts, I wonder if the speedo is monitoring the rear wheels? I know I'd be giving up a little advantage off the line, but I think the wider rubber in the rear would help with cornering, and did I mention, the extra width would look so cool!

Is anyone using 285's? Are there any known fit problems with this size?

Thanks,

Dean

Originally Posted by jsal' post='465744' date='Sep 4 2007, 02:41 PM

I am running 285/25/19 on the rear with no clearance problems. You want to look at the changes in width also. Here is a tire calculator that can give you all the specs as well as speedo differrences:

http://www.discounttire.com/dtcs/infoTireMath.dos