FYI a 235-85 15 is about the same size diameter and width of a 245-75 16.
Not sure how common or rare a 235-85r15 tire is.
I'm not sure a 235/85R15 even exists
10.5x31R15 is almost exactly same diameter as 245/75R16, so that is what I would order for spare.
10.5x31R15 is really identical to 265/70R16, so a little wider than 245/75R16.
Putting a 235/75R15 on one side with 245/75R16 on the other on a limited slip axle will destroy a clutch type differential within about 20 miles. After which you will have an open axle until you replace the clutches. You would also have a large amount of abrasive in the oil.
If you have a Torsen or other gear differential, they will accommodate the difference in size, with just a lot of motions = heat/wear - drive slow and replace as soon as possible
For an open axle, not much of an issue, other than the differential gears would be spinning at much higher rate than normal - heed the compact spare rule and drive slow 80km/h or less until replaced.
In 4x4, you will get severe binding between front and rear axle (like turning a sharp corner). It will work to get you along in poor tractions situation, but don't do it on pavement.
I recommend 10.5x31R15 spare or get a 16" rim and same tires as you're running.