Tyre change

With a greater difference between front and rear circumferences, I would be concerned that the ABS computer may interpret that as wheel slip and do something bad. I have never run into this problem myself, and so have no 1st hand experience, but perhaps someone else does. That is the only thing I can think might be effected.

REAR TIRES The proposed new tire is less than 0.75 inches larger diameter than the factory tires. They will fit fine.

FRONT TIRES The proposed tire is 0.90 inches larger than the fac tory tires. They will also fit fine.

The result will be a speedometer that runs slower. When 70 is indicated, the actual speed will be faster than you are going today. You can do the math, but the speed variance is not going to cause any problems for the car. A cop might want a short meeting to explain that you were doing 73 when you though you were only doing 70, but that's the only problem that comes to mind. The restj of the systems (ABS and Traction Control, for example) look at the relative differences in speed of one or more tires over the others, and it takes a tire being stopped while the others are turning, or turning while the others are stopped, to trigger the system. The changes you want to do will not upset these functions.

However, I'm not sure the tires will solve the issue you are trying to address. Indeed, I think the issue you are wanting to address is a feature of the car that people pay extra for. If other car makers offered the road handling abilities of a BMW, there would not be so many BMWs sold in America. Obviously, there are other features that attract BMW owners, but handling is very high on the list. One function of a sports car is feeling the road, and this feeling is what you are trying to soften. I do not think you can acheive the goal through the tires. I could be wrong. If you could muster a test drive of another 3 Series (probably not in 330 trim) with different tires, you would find that the ride is very close to the same.

There is a chance that the 330 gets stiffer rated shocks and springs, and this is where you could soften the ride.

ABS - possible, but unlikely. DSC - very unlikely You may get slightly further between services, but IIRC distance is not a factor in newer cars. Sat Nav may [slightly] miscalculate position.

The nav system positions the car on a map via GPS, not through tire rotation. The biggest problem with the nav system will be driving in relatively new areas where the streets on the ground and the homes are not in the "right" places. Sometimes a subdivision will be drawn and mapped, then when the tractors start moving dirt around, something changes. The original maps get onto the geocoding system but they do not match precisely where the streets and homes actually are. In my experience, any error from this sort of thing is small, typically an error of less than 100 ft, and usually less than 20 ft. (The geocoding for my house does not match the GPS, so my house does not come up in the right place, the error is about 50 ft as I recall.)

In any case, the nav system does not know where to turn because of tire rotations, so there is no issue on this particular front.

Or you could abandon run-flats and get summer tires which would probably solve your problem. It also creates another,

R / John

Thanks (all) for the info. I do enjoy the handling and the ride is perfectly good on well maintained surfaces, but wish to alter the compromise to save wear and tear on suspension components and reduce undesirable bumps, rattles and tramlining over the less than perfect roads I need to drive on. I think that careful choice of tyre and a little more rubber might just do it. A small reduction in tyre pressure from the recommended (hard) settings has a beneficial effect on the damping balance I am trying to achieve so hopefully the tyre changes will assist in this. I was concerned at about the ABS etc so thankyou for your advice on that. If this does not provide a solution I will look to the dampers and springs.

Another option I looked at was 225/45R17 front and rear but this reduces the aesthetic appeal of the car and would mean some new rims.

Nearly right. The BMW uses the GPS to fix the car's position to a first approximation. As BMW developed their sat nav before Pres. Clinton released the p-code, it also uses the wheel rotation to calculate dead reckoning and turning information. This means that BMW GPS is much more accurate than pure radio ranging alone.

When I lost an ABS sensor the ABS and DSC stopped working and the sat nav also went bannanas, and recovered when it was replaced. See my thread around the new year on precisely this issue.

You may have map errors, which will be hard.

If you live in California your house might have really moved.

See above.

Back in the days of old, tire rotations might have been useful, but the GPS systems they have today are able to plot the location of your car to an accuracy that is smaller that the footprint of your car.

They _might_ use tire rotations to set the location of something, but the GPS system all by itself does not need any additional information. I do not know what they would use tire rotations for but I know that the GPS does not need the information. Any error in plotting the location of your car on a map is due to the error on the mapping software, not the GPS, and any vehicle-based solution will not correct the problem of having a bad map.

Ah, well, not quite.

If the GPS has even momentary sight of less than the minimum number of satellites needed (i.e. three) to fix the car's position, then you can get wacky position errors that put you somewhere on the map, but not necessarily on a road. Recently, I was briefly about a quarter of a mile offshore in the Delaware bay, though to give the GPS credit, it did have my direction of travel and rate of progress pretty much correct.

They need 4 satellites to fix your position and speed. Normally, your GPS will be able to see upwards of 6 of them, but my GPS always saw at least 4. Perhaps those in southern latitudes see more with greater regularity, I live at about 33 North, and very near the edge of the continent, so perhaps my experience is an anomoly. The only time my car was ever plotted wrong was when it was in my garage. My car was in the wrong place on the map, but the map was not accurate. There was no obstruction to the sky, and I could drive a block or two and the map would line up with my car again. I only know a little bit of geocoding, this is how they overlay a map on GPS coordinates, and I know it has errors. These errors are generally in the range of less than 50-ish feet, maybe 75 feet. I do not know the exact error, not do I care, my point is that the mapping software has more to do with plotting a car in the wrong place than the GPS not giving the right coordinates.

Yes, there are lapses that can place your car a half mile offshore or more, but they are rare as a practical matter, and a vehicle based system of counting pulses from the ABS system are not going to fix those errors.

A good GPS must have a lapse of several minutes to lose track of where you are, otherwise everybody that drives through a tunnel would be screwed. My GPS system -- I used to sell GPS-based fleet tracking equipment -- could keep track of me driving in downtown Los Angeles. The trouble with my system was rooted in the cell phone technology. It would try to send a location update via cell, but the packet would not go out and be bufffered to be sent later. After it buffered a few times, the Real Time data was late by whatever the buffer timing was. Since I was tracking armored cars, having them get lost was problematic. My GPS data was always accurate, only the real time update via cell was wrong.

There's also a lot of variation is sidewall stiffness between different tire manufacturers. The Avons currently on my wife's 2002 325i yield a much more comfortable and quieter ride than the two previous sets of tires did (Michelin and Dunlop).

-- Larry

Yes and no. If you're in a location served by W.A.A.S., the accuracy is approx 1.5M horizontal position error or better (more or less equal to the accuracy you state). The nominal accuracy (no tweaks) with 4 satellites is

16M or better, which may or may not fall into the car's footprint.

R / John

"Jeff Strickland" wrote

Jeff, I would like to correct a couple of inaccuracies in your postings.

Garmin (and others) quote 15-meter accuracy; that's bigger than your car's footprint. However, other data I see shows 5-8M accuracy, which would be *close* to a car's size.

Also, 3 satellites are needed for 2D (lat/long) mapping, 4 for altitude in addition. Speed is derived from averaging and is available in both 2D & 3D modes.

I don't believe that any car-based GPS units use the WAAS, which would give 3-meter accuracy.

FloydR

Yeah, and it would actually improve the handling if you did that by reducing the understeer. Wouldn't want that now would we?

Mine does. Garmin Streetpilot 550. I believe many others do now-a-days too.

Groovy. Too bad it won't work in Europe where you might really need it.

FloydR

Far out! But I won't be visiting yurop anytime soon. Plenty to keep me entertained over here in the "New World". And I know exactly where I am within 10 feet. ;-)

Walked round a whole yard full of E46's. They had a variety of different rims on. To me the 18" just look right for the car. 17's look right on the E36.

FWIW the M3 has 255/40 rear and 225/45 front and the ride is still terrible. The biggest improvement would be aftermarket shocks and springs and/or more "comfort" tyres than sports especially if you currently have run flats.

As for the satnav, the Mk1 system (from the mid '90s) has a wheel rotation input and you have to calibrate it every time you change tyres (so the manual says) but the later systems do not have this. I'd guess though that it still uses your speed in some way to estimate where you are if e.g. you're in a tunnel so there might be something in it. I doubt it'll make any difference though.