Ok What problems will I run into if I ran 245/35/19 front and 285/35/19 rear? Will the cars electronics freak out?

With this setup the fronts will have a diameter of 25.75" and the rear will have a diameter of 26.85" difference of 1.1"

Will this freak out DSC or set off the FTM monitor?

Also where is the speedometer / odometer read from? front wheels or rear wheels? or is it averaged out through DSC?

What I'm afraid is that the FTM will go off like mad...

 

I think the speedo reads from the rear wheels, but the set up you propose is almost standard.

Why not just run 245/35 front and 285/30 rear (standard is 275/30 rear)?

 

Right! but the wheels will have 285/35/19 on them already and I'm not willing to fork out for 4 new tires... since the rears 285/35/19 will fit I want to just use them. Otherwise I'm going to pass up on the deal and not get them.

 

This website has a calculator that will show the affect on rotational speed of the rear tyres. Looks like your speedo will read a bit fast.

http://www.chris-longhurst.com/carbi...yre_bible.html

I can't see that you will have a problem with the tyre pressure monitor so long as you initialize it to the new wheels.

My 275/30/19s are on 9 1/2 inch rims, I don't know if these are suitable for 285's or if this will affect the setup of the car.

 

Rear wheels if wheels. What you want to do should not freak any things electrical out.

 

Wouldn't have thought the FTM would complain once it has initialised and as far as I know the speedo reading is taken from the gearbox not the wheels.

 

You are so close to OEM Sport Package diameter as to not even be an issue with Speedo or Odo. BMW recommends stay within 3% and you are well within that. OEM 275/35R19 may be anywhere from 26.3 to 26.7 depending on brand. Split the difference at 26.5 and you are nominally 1.4% over. Once FTM is calibrated it does not care. Nothing else cares. However, you may find slight bias towards understeer with the slightly wider rears (around another 0.5-0.7 inch wider than a 275), but that is what the sport package does anyways. Not a big deal at all.

 

Ok sounds good... so seems like having the fronts run 1.1" smaller than the rear wont be a problem...

I see most OEM tires (obviously) have all the tires running at almost the same diameter... thats why i was a little worried at first.

From discount tires calculator it shows the fronts will be running 33.1 more revolutions than compared to the rears over a mile as where on a standard setup they would only be off at no more than 4 revolutions per mile...

 

You'll probably be fine, just remember you're going to raise the rear end of the car by 0.55" with that rear wheel combo. Maybe it's not enough to notice.. who knows.

 

Oh ya! thats right... the rear will be higher than the fronts huh? ARGH! Doh!