Offset 40 front 23 rear,...
 

front-19x9-40 offset/ rear-19x10-23 offset. Will this fit a GS400?

The front should fit just fine, the rear will most likely need a stretched tire set-up and rolled fenders. IMO, those offsets are not ideally matched, the front is somewhat conservative while the rear is quite aggressive. Just my 2 cents :)

Oh hold on, those are postive offsets, right?

If they're negative 40 and negative 23 then it's definitely not going to work.

lol i also hope they are positive offsets. if they are negative? man, that's like truck wheels :D

yeah, i don't think they fit well on the car, front and rear not proportion

front will work no problem, rear needs to be a much higher offset somewhere around a 38 to 45 depending on rear tire size might have to roll the fender a little,
check out www.sizeitup.com for more tire & wheel plus sizing and fitment data

Ok the guy doesn't know if the offset is positive or negative, If it helps they were on a M3.

Ah, if they're off an M3 then they're not going to fit at all. The bolt patterns are different.

Well,.. that does it. I can't buy them. I totally forgot to check the bolt pattern. Thanks Seize.