I have a 2003 jeep Cherokee and the only tire I have for a spare is a 225 70r16 but currently using 245 70r16s. Wondering if it would be OK to use temporarily?
Just one data point: on my 2016 Toyota RAV4 hybrid all-wheel drive, the spare is T165/80R17 and the regular tires are 225/65R17.
This makes the spare 17*25.4+165*0.8*2 = 695.8 mm in diameter.
The regular tires are 17*25.4+225*0.65*2 = 724.3 mm in diameter.
The difference is 4.1%.
In your case, the difference is 3.9%.
I'm 99% certain that you can use the smaller tire as a temporary spare. Note temporary means you drive at most 50km with it. Some AWD systems don't like if the rolling radiuses of tires differ.
Of course, you'll have to consult the owner's manual to have a final, definitive answer. If it says that tires of different rolling radiuses cannot be used even temporarily, then I guess you'll have to purchase a new tire for the spare. For example, the answers to the question Will different tread depths hurt my 4 wheel drive? indicate that you shouldn't ideally mix tire sizes, but those answers are concerned with permanent mixing, not by temporary mixing.