Tyre diameters should be the same???

[aaron68nz]

Is it true that ideally all 4 diameters should be exactly the same for a 4 wheel drive car? If this is true has anybody given thought to this as opposed to simply thinking about tyre pressure? By this I mean has anybody noticed the width of tread left behind on the ground at parking speeds. By this I mean for example coming out of the wet onto dry concrete and noticing the width of tread rear vs front and how smudged the tread marks left by the shoulder of the tyre are, or driving over dust with clean wet wheels and noticing the width of tread that picked up the dust. Maybee at normal driving speeds the diameters change or something. I have a sedan with stock wheels and eagle F1 GS-D3s 205/55 16. Mitsubisi recommends 32/29 front/rear but based on above tread mark observations yes maybee 29 for rear but 39 would be at least necessary for front. 32psi on the rear is definitely overinflated. Just seems to me that a 39/29 front/rear difference is too large for some reason. The Legnums are 50kg heavier which I guess is all in the rear so this might make a difference I guess?

[KiwiTT]

I wouldn't think different pressures would affect tyre diametre too much, if they are within 10-15% of each other. So long as they are the same size, you should be Ok

However 205/55 and 195/55 is a definate no-no. I have heard of a tyre installer who wrecked a subaru drivetrain because the tyres were actually different sizes.

[aaron68nz]

But 39/29 is a 34% higher pressure in the front than back though. I'm just thinking along the lines of max. tread to the ground and max. grip. If I have the tyres at 10-15% difference one tyre will be underinflated and the other over inflated. Maybee I should go back to closer pressure differences to be on the safe side. When I bought my tyres from Discount tyres in Glenfield they said they find it's best to have the same pressures front and rear!
By the way have you checked out the following website which is exellent as you simply type in 2 different tyres specs and it shows the difference the change will make to the speedo and so on. The above Subaru example means there is a 1.7% speed difference between front and rear wheels.
http://www.miata.net/garage/tirecalc.html

[KiwiTT]

Mine are 36 Front / 34 Rear, albeit @ 17"