Just been checking out this website following a link provided by Keith Farrow on this forum:

http://www.carbibles.com/tyre_bible.html#construction

There is a tyre size calculator about half way down which lets you calculate differences between tyres.

Out of interest I plugged in some figures and it showed that if your speedo was accurate with standard 17inch rims, then changing to 19inch rims would cause it to read 3-4mph slower !

Is this right and do BMW do anything to adjust this - also what's the affect with people sticking aftermarket wheels on the car?

My GPS suggests that my car reads about 2mph slow - just like my last BMW, which suggests that my speedo IS set up for 19inch rims.

Confused ?

 

I think most new BMW's read about 2 MPH fast. If you don't want to throw things off when getting new wheels/tires, then you need to make sure that the rear-wheel revs per mile do not change much (spec for 17's and 18's is about 807 revs per mile). Doing so is no problem when going to 19" wheels. To keep things from changing with 19's, I suggest that you use the BMW spec of 275 X 30-19's on the rear (revs of 814 per mile). I forget exactly what the BMW spec is for the fronts (245 X35-19???), but you should be able to find this data easily--e.g., on tirerack.com on at BMWNA.

 

Already got 19's on my car and the MPH seems to be accurate (+approx2mph as expected) - was just curious to see if that had been adjusted from the 'factory' position of 16 or 17 inch rims.

From your comment do I take it that the e60 speedo is linked to the rotational speed of the rear wheels?

 

It is linked to real wheel revs per unit of distance. If your model is like mine, then you have a true/accurate digital speedo hidden away "in" the iDrive hidden menu. You can assess this menu and compare your "analog" speedo with the digital one. Here is how I get to things in the hidden menu on my car--a 545i with a build date of 4/05 and running 20.01.00.

1. Start your car. I don't know if you have to start your car at this point, but I wanted to because my purpose is to compare the readings of my car's instrument cluster speedo and tach with those of digital speedo and tach available through the menus.
2. Hold down the mileage reset button (or plunger) on the instrument cluster for about 8 seconds.
3. When 01.-- appears, let up and wait.
4. Then, 01.01 will appear.
5. At this point, you can scroll through menus numbers 01.01 to 01.08 by repeatedly pressing the reset button; however, you can't access any of the menu items at this time, and the complete set of items of interest is not yet viewable.

Short cut: Go from the appearance of 01.-- in step 2 to step 7.

6. Next, when on any of these items (e.g., 01.06), press and hold the resst button down.
7. As soon as you see 01.--, start scrolling and scroll immediately to item 18.
8. Next, scroll to 19, but this time continue to hold down until you see 19.-- Lock On (on two lines).
9. Once you see 19.-- Lock on, release and wait until you see 19.00 Lock On Code (on three lines).
10. At this point, immediately scroll until you see the sum of the five digits at the end of your VIN--which can't be greater than 5 X 9 = 45. Obviously, you should determine this sum before you start.
11. Once your sum, say, 25, appears, wait until you see 01.--, and then start scrolling immediately to the menu item you want--up to 21 as I recall.
12. If, for example, you want menu item 07, then scroll from 01.-- to 07 being sure to hold down when moving from 6 to 7.
13. After stopping on 7 and holding down briefly, let up.
14. Now, you can scroll through menu items 07.01, 07.02, 7.03, etc., etc.
15. Once you get to this point, you can press and hold down to get out of the current menu and to scroll to another. Hold down, etc., when moving to the one you are interested in.

If you make a mistake somewhere during this process, then use your intuition. But, regardless, you can always start over. The menu will be locked again once you turn your car off and restart it.

I was interested in getting to only the k/ph (kilometers per hour) menu and to the tachometer menu (U-rpm as I recall). k/ph is at 08.00 and U-rpm is at 07.03.