Tyre Sizes for 159 JTDM 1.9l

[cdscons]

I am currently running 234/45 18 in and need to replace the 4 tyres. The Pirelli/Michelin prices are horrific.

Is there any option of running say 245/40's which are much cheaper, as they are more normal.

Also after after market brake pads and disks.  Genuine quote $1400. Any suggestions?

[Fylnn]

It all comes down to what you are comfortable with.  the tyre supplier should be able to tell you what is allowable because it will alter your gearing and speedo. 
The 245/40 will have a section height of about 98mm, the 235/45 will be about 105mm.  That is just the width (235) x the aspect ratio (0.45)
If you have an 18" rim, nominally 457mm diameter, your overall rolling diameter is something like:
235/45 = 105mm x 2 + 457 = 667mm
245/40 = 98mm x 2 + 457 = 653m
That gives you a difference of about 2% SMALLER.  So with the speedo reading 100 km/hr, with the new tyres you will be about 2km/hr slower, and your gearing will be slightly lower.

[cdscons]

Thank you for your assistance. Much appreciated.