Tyre Size Question

[The Mortgage Man]

My 1998 Rav4 is fitted with 4 alloys with 235/60 16 tyres but the steel rimmed spare has a 215/70 16 tyre.

The hand book says there are 2 sizes of tyre for my car:

235/60 16 and

215/70 16 so technically they are all within spec.

I have been on a tyre size comparison website that says that there is 19mm difference between these 2 sizes.

I also have a brand new, unused Avon 215/65 16 which the comparison site says is just 2mm smaller than the 235/60 16s.

My question is, if I need to replace a tyre or 2, do I have to replace all 4 at the same time because its a 4 wheel drive or can I replace tyres as and when each needs replacing.

And, can I have 2 slightly different tyre sizes on the same Rav4, if I make sure that the 2 on the same axle are the same size (ie 2x 235/60 on the front and 2x 215/65 on the back or visa versa).

Sorry for the rambling post.

Regards

Jonathan the confused