Adding a 5th gear
 

Is there any possibility of adding a 5th gear to a 1975 xlch 4 speed transmission?

Not even a remote chance...

5 speed gear set is bigger than the transmission case. Just go to a larger trans sprocket to reduce your cruising rpm.

I believe it is true [but I do not know with the certainty of an expert] that the IronHead range from 1 to 4 is the same as the Evo range from 1 to 5.
For your transmission you can get a different gear known as close ratio gearset. Again I am not expert in this but if I have the correct part it will reduce that long space between 3rd and 4th. Research it, you will probably get what you want.
:)

Correct mick - the hi gear in both is 1 to 1 — 5 th gear is not a 6th gear as overdrive .086 to 1

Thanks Mick. Good to see you still around. I will def look into that. Never have ridden an evo sport.. From what you said though cruising at 80-90mph on the highway in 5th on an evo doesn't feel anything comparable to an ironhead doing same speed in 4th gear? I know they also have rubber mounts on the motor, so excluding that factor. Same rpm range? My main objective is to be able to cruise around 85-90mph and not rattle my teeth out of my head. But also not suffer to badly performance wise in the other gears. I realize the bike is older than me, so maybe just expecting too much.. I've never noticed space between any gears.. 🤔 Kinda the opposite.. hah. Possibly archive this more easily with a sprocket swap?

Not going to happen on an iron head, nature of the beast.

You wanna play those games go find a cheap evo sporty, plenty around and very upgradable performance wise on the cheap.

I owned a 1979 XLH four speed and then followed that one up with a 1991 XLH five speed. Night and day at highway speeds. Ironhead would do 65 MPH, but it was not a pleasant experience. Evo will do 70 MPH all day long and not kill you.

Hate to say it here, but the Evo was a huge improvement to the Sportster lineup and the 5 Speed was a godsend. Even if I was just going to build a bar hopper, I would start with a 1991 and up to get the 5 speed, it is just that much better.

Ironheads are great bikes, but like my Flathead, they are not highway stars, not their intended environment. Good luck.