I hope I'm posting this in the proper forum.

I need to replace 6 stair treads in a boxed-in staircase. The walls are not 100% parallel, so there is a difference in width of nearly 1 inch from the top stair to the bottom. As well, each tread is narrower at the back then at the front. Since there is no molding between the walls and the treads, and the stairs will have a clear finish, there really can't be a gap.
I currently do not own any power saws, and don't plan on doing a ton of renos (except maybe installing engineer flooring in a 120 sq. ft. room). What type of saw should I buy to make these angled cuts (or what should I rent, if this is a better option)

 

You can do it all with a framing square and a skilsaw with a nice fine toothed carbide cross-cutting blade. (you'd probably want to cut the treads upside down to avoid any potential tearout from the saw blade.)

You'll place the framing square on the stairs so that one edge of the framing square is against the riser, while the other edge will show you how much out of square the wall is. By taking careful measurements with the help of the framing square, you should be able to cut treads that fit perfectly. You might find out that one side is perfectly square, while the other side is 1" out of square. Or it might be 3/8" on one side and 5/8" on the other. The framing square will help you determine that.

You might want to make a drawing of each tread as you measure them. When you've got a lot of measurements, it makes it challenging to keep them all straight, and you usually can't afford mistakes.

If you're really hot to buy a tool (the wife has given you the green light) then you'd probably want a sliding compound miter saw. But they're pricy and not usually the sort of item a DIY'er would get. You could also cut them on a table saw using the miter gauge- a miter gauge clamp would also be helpful.

 

Another way I have done it, not to take away from XSleeper's method, is to make a rudimentary adjustable angle gauge. It will have a 10" leg and a 24" leg. That way it will fit almost to the next riser without hitting it(whereas a framing square won't always). Connect the angle with a bolt and wing nut so you can tighten it. You can, then, move directly to your tread and duplicate that angle. Taking good measurements is critical. If we all did it the same way, we wouldn't be individuals.

 

A T-bevel will give you the exact angle you need - readily transferable to the new tread. Available at better hardware stores for under $15.00

 

I need to do the same thing as oakridge, so thank you for all the replies too! Oakridge, what kind of wood are you using on the treads?

(I am not trying to or meaning to hijack the thread, but I will be doing this exact project in a couple weeks, only with an entire staircase :-) )

The sliding miter saw that Xsleeper recommended is definitely a cool toy. Our miter/chop saw is gasping through it's last rough cuts, and we will probably splurge on a sliding miter saw to do all the flooring, stairs, trim, etc. in the remodel. Having the right tool DEFINITELY makes the job easier.