I'm replacing my trailer tires which are load range D's and Discount tires will have to order them but they have E's in stock. Price difference is negligible. Will E's that have a max cold of 80psi vs 65psi for my D's ride rougher?

Chances are if you air them up to 80 psi yes they will ride a little rougher. On the other hand if you don't need the load range E for its full capacity you don't have to run it 80 psi. However for trailer tires I always run them at the max. I would think the ride difference between E's and D's is not much.

What are these tires going on?

Ranger Trail boat trailer.

Get the E rated tires. Thank me later.

Actually, get the Kumho 857 Radials that are E-rated. Then thank me later twice.

Im not gonna say E's aren't better, but if its a tandem trailer he's overgunned with D's already. There is such a thing as going overboard.

Single axle with a Z119 on it.

You are well within the weight requirement no mater which way you go. In my mind tires are the weakest point on a triler and I would be inclined to go with the E's, especially if the price dif is marginal.

I'd get the E's

E for sure. been real happy with Kumho

D is 8 ply tire. E is 10 ply tire. Lots of difference in load rating. Biggest issue IMHO is speed rating. Most trailer tires are speed rated at 62 to 70 MPH. Get the E and Kumho 857! Think the Kumho 857 are rated to 99 MPH. No more tire problems.