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[pauls]

Had a look at a 2004 bmw x3 when you slow down you stop then you get a slight thump from the back. Chap told me it was because he has just put on 2 cheap tyres on the back. His  mechanic agreed with. I have googled it and it does seem to be a problem if same tyres are not the same make on all 4 wheels. Just thought i would ask your opinions on this. Yes i can walk away but would just like to know.

[STEMO]

2 cheap tyres on the back. Sounds like it's been well looked after. Walk away.

[Viral_Jim]

Tell him you'll give him the asking if he puts 4 matching branded tyres on it and the thunk goes away - see what he says  .

Although as STEMO points out, if you can't afford decent rubber, a premium branded 4x4 isn't the car for you, so its probably been run on a shoestring.

[STEMO]

I like that he brought his mechanic for backup. 

[al brown]

Yeah, the diff really doesn't like odd/non BMW approved/worn tyres. You can get away with it if you've just fitted them and take them straight back off, but it doesn't seem to take long to bust the diff if you run it like that. Only way to tell is fit new, BMW approved tyres on all 4 corners and try it.
Often the knocking shows up at town speeds when coming on/off the throttle.
Al

[tunnie]

I’d walk away from that car, also X3’s in general.

[henryd]

Many 4x4 vehicles need all the rubber being the same,my Touareg  is one of them,I wouldn't class the x3 as a proper 4x4 anyway,more like a freelander really

[pauls]

He did state he had only put then on 60 miles ago and they did look brand new.

Tunnie ..why leave all x3s alone

[2boxerdogs]

Should always change all tyres at the same time on a 4x4, if it hasn't been maintained properly walk away.

[TheBoy]

The X3 in general is one to avoid no matter what.  If it must be that style of car, and must be BMW, get an X5.

[pauls]

The X3 in general is one to avoid no matter what.  If it must be that style of car, and must be BMW, get an X5.

Why?

[TheBoy]

Appalling build quality (and its not built by BMW), poor reliability.

[Doctor Gollum]

Appalling build quality (and its not built by BMW), poor reliability.

Not strictly true...

Mk1 was built in Austria by Steyr, same firm that builds European Chrysler/Dodge tat...

MK2 built in Alabama by BMW

[Andy B]

While I know why you should change all 4 tyres at the same time on a permanent 4x4 I'm not prepared to fork out a Łgrand each time. As Costco will only change 4 at a time I asked Mercedes. Merc said you didn't need to change all 4. So far I've put 2 tyres on an axle 3 times & nothing has broken.

[Kevin Wood]

While I know why you should change all 4 tyres at the same time on a permanent 4x4 I'm not prepared to fork out a Łgrand each time. As Costco will only change 4 at a time I asked Mercedes. Merc said you didn't need to change all 4. So far I've put 2 tyres on an axle 3 times & nothing has broken.

It all depends what type of system the vehicle has. If it's only part time 4x4 then clearly unless you are covering significant mileage in 4x4 mode, it's a non issue. Most modern 4x4s are full time 4x4 with open diffs all round and a traction control setup similar to the Omegas that brakes the slipping wheel and I can't really see a problem there if tyres are mismatched. So, the centre diff has a slight mismatch to cancel out, but that's what diffs are designed for. It's only when you have viscous coupled limited slip diffs or clutches directing the drive that I an see a problem. I think the X3 uses the latter.