So, took the plunge and bought a 2008 2.0 with 174 hp. Car is quite tidy though the performance is quite dull I think a lot of this being down to the mismatch of tyres fitted. It would appear that the recommended tyres are different size font to back which strikes me as being a little strange for a car which is sensitive to having its wheels all the same rolling size. What size are people here fitting to their 18 inch rims?

As far as I can determine, it should have the same rolling diameter on all four wheels but could have different width tyres on the front to rear as long as they are the same on each axle set

Hi Ian

Bmw have for years fitted either "square" wheels sets ie: same rim size all round and a "staggered" set up wider rear rims narrower fronts which started with the "M" cars. X3's were offered with both set ups.

The rim size is stamped on the inside of each wheel your hand book should give wheel/tyre sizes and inflation information. As an example my X5 runs 20" rims running 275x40 front and 315x35 rear most of the "advantages" are cosmetic however it is essential that when replacing tyres the front and rear rolling radius is within a few mm if not damage to the transfer case is almost certain. BMW have agreements with "approved" manufacturers who produce star* rated tyres to ensure that when staggered set up is used the tyres are within tolerance. Non rated tyres can be used but you should take care that they match as closely as possible. First signs of transfer case damage will be jerking when on full lock particularly when reversing. I had this I put a chalk mark on the front tyre where it met the road same on the rear then got my son to roll the car forward one revolution while I watched there was 20mm difference in distance front and rear, may not seem much but when you multiply that by a few thousand revolutions the reason the transfer case gears get screwed up is clear.A square set up is more forgiving particularly if the tyres are all from the same manufacturer. 

All 4x4's have the same issues regardless of the Manufacturer needing correct tyres that match front and rear, if your current tyres are mismatched across axles or front to rear it will end in tear for your wallet.

Normally tyre size if they are correctly inflated will have little effect on performance, your engine should feel quite lively as it is a diesel clogged EGR valves are common which strangle performance

Dave

 

Cheers, yes I didn't think the performance was all down to tyres so will have a clean out of the EGR, presumably find details on here. I have a poor tyre on front and rear and a nearly new one on the rear so not good. If I put a new one on the front, would each diff front and rear compensate and be the same "speed" for each axle?

The Front and rear differentials will cope with a variation new and worn across that axle as long as tyre sizes are correct and preferably from the same manufacture the transfer case on the back of the gearbox though is another story. The transfer case is not a differential it is a chain driven system to get drive from the back of the gearbox to the front differential some (like yours) have a motor driven gearset to adjust torque between front and rear. My older version is fixed (roughly 60/40 split) both systems will suffer badly if front and rear tyres are badly mismatched, there is a guy up in Rotherham who has built a business repairing them.

Have a look at www.realoem.com put the last 7 digits of your vin into the search box, it will bring up your model it is a BMW online parts list but also has exploded drawings which will help. Also look at www.newtis.info it is a BMW technical information site used by Dealers you should find help there on cleaning the EGR system