Just curious...

I have a 1993 325i. The rear tires are 245/40R18's, front are 225/40R18's.

Will this setup adversly affect my speedometer by several MPH?



Reason I ask.. I honestly don't think so, but need some input on this.

I recently was cited for doing ~77MPH in a 65MPH zone. My speedo was registering 71-72mph. I was on a long stretch of highway, heading down a long hill with the CC set at 71-72.

The officer was KansasHP in a patrol unit, moving towards me in the opposite direction. He was flanked both in front and rear by other vehicles going his direction and withing 100ft or less.

Officer claimed his radar unit first registered my speed at 79MPH! Which is absolute BS!! I knew something was wrong... I seen the radar unit locked in at 77MPH and he wrote the citation as 75MPH... he admitted that the unit can be off by a couple mph's.

I truly feel I was had by what is called the "shadowing" error... the cars near the patrol car were affecting the readout (do a google on it if you want to get more info).

I'm going to fight this in court with all the info I can muster... it just don't seem right to be paying for a ticket I don't deserve.. if he wrote it for 71-72, then I would accept it, but that's not the case here...


Any help/info would be greatly appreciated. I have a feeling the speedo is very accurate when compared to other vehicles on the road/traffic groups and when passing those standalone units on the side of the road that show your speed to you... IE, the unit said I was doing 30mph and so did my speedo.

http://paspeedo.com/calculator.htm

Most E36 prior to 95 had 235/40/17s spec'd for 17" duty. I just used that as my base. I entered 245/40/18 as the rear. If you were doing 73mph on your speedo, car was really going 77mph.

Staying close to 225/40/18 front and 255/35/18 rear is the best bet. If you git 245/40s now, I would try the 255/35s as the height is much taller on the 245/40. Maybe a slight fender roll and you can have the 255/35/18 fit.

Thanks for the link.

Damn... I didn't realize my speedo could be that far off. :eek:

Wonder why for almost a year I never got a ticket until now... :confused



Strange how the faster you go, the less accurate the speedo is.. higher speeds create larger error with added mph.. at 120mph it is actually 126mph. At 35mph it is only 36mph.


Well this may change things.. I may not fight the ticket after all.. :dunno

What are you getting for the rear now? 245/35 or 225/40? :D ;)

Run the 245/35R18 if you want to stay with the wider tire. If I can help let me know.