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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

All I've been away from the forum for quite a while, and since the issues regarding the name etc , the amount of daily content has dried up 😞 We need to get it as one of the busiest forums once again, so what can we do to generate some more activity? Is it worth having a facebook or an Instagram or even a drive tribe page? Gets some meets happening any ideas would be great, then we can approach the admin with some ideas