jmnieves

Hello all, I'm scheduled to pick up my car tomorrow afternoon. My car comes with the standard 525 16" rims/tires (which I do not like) so I went ahead an bought some Style 124 Rims with tires (18").

Being that I have the Nav option in my car, will I have an issue with the different size rim/tire? Will I have inaccurate readings from NAV and or false mileage reading?

If so what do I have to do?

FYI, I?ll be swapping out rims/tires for the winter/summer seasons.

steve545

Should not make any difference as the tyre profile will be lower by 2 inches as well, unless your are retaining the same tyre type (just 18" instead of 16").

spec33

Don't worry as long as you keep changing from summer to winter every year it will equal out

das

As long as you keep the same overall diameter (or very close), it doesn't matter.

Tire sizes are in the format:

XXX/YY-ZZ

where XXX is sidewall width in mm, YY is sidewall height as a percentage of sidewall width in mm, and ZZ is inner diameter in inches.

So the formula for figuring out overall diameter is:

2*(XXX * YY * 0.03937) + ZZ = overall diameter in inches

For example, a 235/40-18 comes out as:

2*(235 * 0.40 * 0.03937) + 18 = 25.40"

I moved from a 225/60-15 (on an old 535i):

2*(225 * 0.60 * 0.03937) + 15 = 25.62"

for a difference of only 0.22", or less than 1%, which is acceptable.

Also, here's a couple neat calculators:

http://www.miata.net/garage/tirecalc.html
http://www.1010tires.com/TireSizeCalculator.asp

More detail on tire sizes and sidewall markings:

http://www.tirerack.com/tires/tiretech/general/size.jsp
http://www.tirerack.com/tires/tiretech/gen...information.jsp
http://www.tirerack.com/tires/tirete...l/sidewall.jsp

Rizbel

I noticed that my style 124 18in (chromed) rims has BBS stamp on it at the back. I removed each rim so I can clean and apply Rejex on areas not accessible during car washes. I'm thinking of using tire spacers. Anyone has spacers thicker than 10 mm? Thanks.

ch7656

Hmmm this brings a question to mind...Isn't all the Navi figured off of GPS? And if yes...even if your tires were different than the orginal size (thus throwing off your odometer/speedometer)...your Navi wouldn't be affected.

The only way the tire size would effect the Navi is if there was a tie-in between the speedo and the Navi (and maybe there is??)...but why would BMW utilize something as variable as tires to determine speed when they could use the GPS satellite ??

Rudy

In the earlier versions of sat/nav, before the government removed the error factor of the GPS satellites for use by civilians, BMW (and M-B, among others) used a combination of GPS signal and dead-reckoning to determine the exact location of the vehicle.

In other words, the accuracy of the satellites was diluted on purpose for civilians so we couldn't use them for pinpoint accuracy of targets, etc. Because of this error, the car used sensors to figure out where you were. For example, if the map showed a right turn ahead, the sat data may not have known exactly where you were on a particular stretch of road but as soon as you made the right turn, it could update your location precisely since it was able to see you turned the wheel, etc.

Now, civilians have the full accuracy of the satellites. I'm not sure if the dead-reckoning component is still there or not. If it is still there, then outside diameter of the tires can be important.

Speaking of altering the tire sizes, here's a link to my opinion in another thread on why speedo and nav should be the least of your worries....

JohanDC

I think it is still used since the map stays up to date when driving through a tunnel where no GPS signal is present.

ch7656

Makes sense...and I think you're right.

I can only draw on experience from boating where my GPS is used for both navigation and speed...but of course I'm not driving my boat through tunnels...so I didn't think of that

JDN

Which brings to mind another advantage I had when I ran the Garmin Street Pilot. I could always set my true speed with the GPS. I have no idea what my error factor is in the 545.