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[SJ]

I was just playing with the TireRack conversion calculator and I noticed something about the standard vs. optional BMW tire configurations (2009 spec.).

The standard 135i tires are:

R: (8.5") 245/35/18 ... summer performance RFTs
F: (7.5") 215/40/18

The optional tire package is:

R: (7.5") 225/45/17 ... all season RFTs
F: (7.0") 205/50/17

Within each option, the front and rear are virtually identical in diameter, and both produce 60 mph at a speed of 60 mph.

However, if you had the car with the standard 18" wheel package, and then put on the standard 17" wheel package, according to the tire calculator the car would show 61 mph at the same speed it formerly showed 60 mph. And of course over the long term the odo would be off as well. So, 60 mph vs. 61 mph, maybe that's just acceptable limits within tolerances? Or, if you order the 17" wheel package, do they tweak the odo/speedo calibration w.r.t. where it's set for the 18" wheels?

What if you buy aftermarket tires? Do you, can you, tweak the odo/speedo to maintain exact calibration, if possible?

[JimD]

I don't know the answer to your question but I have noticed that the speedometer in my 128i vert with Sport is not very accurate. I use a Garmin GPS and when you do not set a destination it shows the actual MPH. It disagrees with the speedometer by up to about 3mph but it is not a constant amount. It seems to be as little as 1 mph at slower speeds. I haven't figured it out exactly but it may be the biggest error at about 60 mph.

Given my observation, I doubt BMW tries to make a 1mph adjustment. I was surprised when I noticed the speedometer error. My Suzuki SUV has a much more accurate speedometer (that's nearly the only thing the bimmer is not better at, however).

Jim

[oneintheory]

Originally Posted by JimD

I don't know the answer to your question but I have noticed that the speedometer in my 128i vert with Sport is not very accurate. I use a Garmin GPS and when you do not set a destination it shows the actual MPH. It disagrees with the speedometer by up to about 3mph but it is not a constant amount. It seems to be as little as 1 mph at slower speeds. I haven't figured it out exactly but it may be the biggest error at about 60 mph.

Given my observation, I doubt BMW tries to make a 1mph adjustment. I was surprised when I noticed the speedometer error. My Suzuki SUV has a much more accurate speedometer (that's nearly the only thing the bimmer is not better at, however).

Jim

BMW speedos are programmed to read high. I think it's up to 6% at top speed.

[SJ]

Originally Posted by oneintheory

BMW speedos are programmed to read high. I think it's up to 6% at top speed.

That raises a second question: if the speedo reads high, then the odo will read high. I guess you could say it accelerates your depreciation. But why on earth do they want the speedo to read high? Seems quite unGerman as well.

What do people do when they buy after market wheels/tires? You could specifically get ones that slow down your odo over the life of the car. Doesn't seem like there is any recalibration going on ...

[oneintheory]

the speedo reads high, but the ECU knows the actual speed of the car, and the ECU controls the odometer, resulting in accurate mileage. You also get accurate MPG data in spite of the speedo.

[Tom K.]

By law in some countries, speedos cannot read low. So most BMW speedos (like other Euro cars) read from about 1 to 5% high - but each car seems to vary, so there seems to be no evidence that BMW programs them high. My experience has been that the OBC will provide an accurate speed reading which which you can calibrate the speedo.

And re the odometer, while the readings are not tied to the speedo as in older cars, my 128i odo actually reads a bit over 1% low. Applying this correction, the speedo is accurate within 1%.

Regarding re-calibration, different tires create only a minimal speedo/odo difference, although I have heard of some dealers willing to try to correct large errors by physically bending the needle! (I sincerely hope this was tongue-in-cheek.)

Tom

[SJ]

Originally Posted by oneintheory

the speedo reads high, but the ECU knows the actual speed of the car, and the ECU controls the odometer, resulting in accurate mileage. You also get accurate MPG data in spite of the speedo.

How does the ECU know the actual speed? I mean, it can know the actual number of axle revolutions, but it doesn't know the circumference of the actual tires. That is, if I had 75" cir. tires, and switched them to 77" cir., how would the ECU know about that? Since the ECU is programmable, is that a parameter you can input if you change your tires, perhaps?