I am planning on getting a set of 19x8.5, 9.5 with an offset of 40 in the front and 45 in the back for my 5at coupe, what size tires should I go for?

 

I don't have a coupe, but those ofsets do not sound good, those would better fit a sedan. I don't even think they will clear the brembos.

'03.5 Silver AT Sedan, Premium, Aero Kit/Package, Sport, Winter, Stillen Air Box, Z pipe, Polk Tweeters, Clifford RSX 3.5, Gram Light 57F Pro's (on the way), Hotchkis sways, Borla

 

I have 245/35/19's front and 275/35/19's rear.
They look just right for that width, anything wider and it looks balloonish, and I don't care for the sucked in look of a smaller width.
You might could get away with a 285. I tried a 295 and that was way too much.

My $0.02

 

I have 245/35/19's front and 275/35/19's rear.
They look just right for that width, anything wider and it looks balloonish, and I don't care for the sucked in look of a smaller width.
You might could get away with a 285. I tried a 295 and that was way too much.

My $0.02

 

You need an offset of about 25 front and 30 rear.

Black on Black 5AT Coupe, Navi
Axis Milanos 19x8.5 F 19x9.5 R
Toyo T1S 255/35 F 275/35 R
HKS Legamax Exhaust
350Z Duct
Stillen Hi Flow
7 Wire Hypergrounding Kit

 

Both Chops and GoSooners are correct on tire sizes and offsets. You are correct on the wheell widths.

Front should be 19x8.5", +25mm offset with a 245/35/19 tire
Rear should be 19.9.5", +30mm offset with a 275/35/19 tire

If you want 19s

I happen to like the look and versatility of 18s however.