Deano

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

DD694

The speedo is always about 3 MPH optimistic by design.

Deano

Why?

If the car thinks wheels are turning faster than actual, Is the odometer affected?

Does the sport model w/ 275's rack up more mileage that a non-sport?

nyrz

Just sharing some feedback I received, I was told that 285 is a hit or miss, it seems. Some mentioned no rubbing, while others had to roll the fender. Im also assuming this might be because of the wheel and tire combo as well

M5 wheels are 285, and many members have those wheels..

iversonm

No, the odometer is not affected.

European speedometers read 3-5% high by law. (more specifically, they cannot ever read low, even due to improper tire inflation, tire wear, or slight size differences between tires.)

D_Sport

I am running 20's with 285's on the rear - no suspension mods as yet and no rubbing.

E60BEEMER

285s would be best on the back with 20s

heres some pics of

of 285 / 25 on the back and on the front is 245 / 30

no rubbing probs at all

E60BEEMER

EBMCS03

BMWs are always about 2mph off at around 70mph... the faster you go the more off it'll be. Its not your tires. Every BMW my dfamil;y and I've owned is always off. Inlcuding my 2003 MINI. all off the the same percentage.

Deano

Does anybody know exactly where the speedo gets its information?

RPM from one of the rotors? Which one?
ABS Sensor? Which one?
Is it calculated from the engine RPM?

Inquiring minds want to know!

DrLev99

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!