on my 300zx i've got 17 inch rims, with 235/45zr17 on front and 255/40zr17 on rear. what i would like to know is could i put 245/40zr17 alround please.

Why do you want to put 245's all around? The problem will arise in the wet, the larger the tyre the less chance of cutting through the water on the front. Large on the rear for traction. I had 245's on the front, but due to the ride set up on my zed, the inside tyre wall was rubbing on the plastic in arch skin. With all this in mind you might get the rear wheels done first and try the clearence with the rears, on the front. Stay lucky.

you could, but you should really have wider ones on the rear, widely accepted optimal setup is 235 up front and 265 on the rear.

Your turn in will be poor if you go too wide on the front, as well as any rubbing as mentioned above.

Why do you want to put 245's all around? The problem will arise in the wet, the larger the tyre the less chance of cutting through the water on the front. Large on the rear for traction. I had 245's on the front, but due to the ride set up on my zed, the inside tyre wall was rubbing on the plastic in arch skin. With all this in mind you might get the rear wheels done first and try the clearence with the rears, on the front. Stay lucky.

 

do you know where i can get rims to fit the tyres, which i already got please

Have you thought about putting one pair on the rear, buying some different size for the front and keeping the other 245's for when you wear the rears out?