Messing about with tyre sizes

[AB]

Just in the process of scouting around websites looking for decent prices on tyres. I need to replace 2x 225/40/18's on the Z4 and also 2x 215/45/17's on the Focus.

What difference will it make going for 225/45 or 225/35 for example? Will it make the car look a little weird with narrower/thicker tyres on the front than the back?

I don't know a lot about tyres!

Probably going to go for Michelin Pilot Sports following recommendations on here.

[s3fella]

The figure after the / is the aspect ratio, basically the % of the width that the sidewall height it. So if you have a 45series tyre, ie 225/45 and change to a 225/50, the sidewall will be higher and the whole tyre taller and longer rolling circumference. Similarly, go for a /40 and it will be lower height.
I would not mess about with it TBH if I were you.

[Crusoe]

http://www.willtheyfit.com/
have a play, going from a 45 to a 35 section means the side wall of the tyre will go down by about 10mm so look more like a skinny rubber band. Unless you put a larger sized alloy wheel on the drop in size will make your wheel circumference smaller and make your speedometer reading change.

Edited by Crusoe on Monday 7th September 14:04

[HellDiver]

http://www.carbibles.com/tyre_bible_pg2.html

There's a size calculator there. You need to check rim width so you know what size tyres you can fit. For example, the 18x7 rims on my car will only take the factory fit 215-45-18, 225 are too wide for those rims. There's also a calculator on that page for that.

[AB]

I don't want to mess about with the width, I was thinking more of the effects on every day driving with a +/- 5 on the sidewall height.

I guess going for a 45 over a 40 would fill the arch every so slightly more? I gather a 45 is cheaper than a 40 too as less low profile?

[prand]

My gimp of a brother in law is always strying to stretch unlikely sized tyres on his mk1 golf BBS alloys for the perfect euro look. And with all the stance, suspension height and track tweaks he does to it I'm always surprised to actually see it driving up the road without the car dragging sideways along the ground or the wheels falling off.

[Superhoop]

It also depends on whether you are planning to change all 4 tyres or not.

If your car has DSC/ESP/ or whatever it happens to be called by BMW, then only changing2 tyres (Front axle for example) could cause havoc.

The ABS control module will see a difference in road speed, thinking that the fronts are going quicker (if you were to fit a lower profile tyre) and will start having a fit (ABS might also get a tad confused too)

[Mattt]

I went from 35 to 40 on my old car, and ride was improved a lot - sidewalls had slightly more 'give' in them.