getting new tires on stock 18's
 

I'm getting new tires on my stock 18's for my '04 coupe and i was wondering if 255/45/18r, 245/40/18f would be ok? will there be any issues with the vdc? and pretty much is there any reason why I should not be doing this setup? tia

The only reason would be that your diameters are really far apart. The fronts are 25.7" diameter and -4.5% circumference from stock, and the rears are 27" diameter and +5.6% circumference from stock. As far as VDC issues, you can probably expect it to come on for almost every turn you make. Literally. You're trying to do a staggered setup on identical wheels (18x8). You'd be better off getting tires closer to stock sizes.

The 18x8's are somewhat limited for tires. The only 50 series tire that comes close to stock is a 215/50, but who the hell would want that tire? The only 45 series that fit are 225, 235, or 245. And the only 40 series that fits is 245/40. A 255/40-18 would be a nice size, but it's too wide to fit properly on an 8" wheel.

So, for a simulated staggered look, 225/45-18's in the front with 245/40-18's in the rear would be a fair match. Both are smaller than stock, and close in size to each other. The problem is the 245/40's are about 4.5% smaller in circumference, which causes odo/speedo/VDC problems. 18x8's should be run with the same size tire because there just isn't a good staggered setup that works. 18x8.5's have a nice option with 225/45F and 255/40R. 18x9's have an even better option with 235/45F and 265/40R. Those both match the stock diameter.

I'm considering the following setup: 235/40 front, 245/40 rear. I'd prefer 225/45 in the front, but I have a good deal on 235/40.
With this setup rear tires will be 1.2% larger than the front ones.
I believe the difference is too small to cause any problems with VDC, right?