Spare tyre different size. OK or stupid?

My 110 is currently running BFG 265/75 AT tyres while the door-mounted spare is some ancient Avon item. I decided that I should probably replace the Avon in the event that I might one day get a puncture. While I like the chunkiness of the current tyres I think they're a bit on the large side and transfer a lot of judder through the vehicle on pot holes and the like. I'm inclined to replace them (when they wear out) with the same tyre but 235/85. I figured if that size was good enough for G4 vehicles it would certainly be good enough for me and might help reduce the unsprung mass. Now, to the crux of the issue. I've looked at the circumference of the 235 against the 265 and they're pretty much identical so I reckon that if I were to buy the 235 and then have to use it in an emergency then there would be no serious lop-sidedness of the vehicle. Also, the smaller overall size (and therefore presumably weight) of the 235/85 would be less of an issue for my rear door which, as yet, does not have a swing away carrier.
Can anyone sanity check these assumptions for me?
If advice is to keep with 265s all round would the rear door hold this? My immediate thoughts are, no, probably not but I guess that might also be true of the 235. The Avon is a right skinny little number.

if they are the same rolling radius then there shouldnt be too much of a problem although i wouldnt recommend long periods.

I have 5 x 235/70 mts for the disco but also carry a spare 205/80 in case of problems.

I'm also running 265/75 BFG AT and both my spares are the OEM General Grabber 235/86. It's fine to run this as a spare, AS LONG AS you only put it on the rear axle and not for long distances.

Thanks both of you for your responses.
Naks, what makes running the tyre acceptable on the rear axle rather than the front? I'm curious to know.

Nerdos said:

Naks, what makes running the tyre acceptable on the rear axle rather than the front? I'm curious to know.

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'cos the front axle steers, the rear axle doesn't

Running different diameters on the front axle would cause undue stress on the front diff when turning, whereas on the rear diff this is to a lesser extent.

But the radius is the same, isn't it? 235/85 gives a sidewall height of 199.75mm and 265/75 gives a sidewall height of 198.75mm. Given Land Rover's own <cough> generous tolerences I thought 1mm would be negligable in these circumstances. I'm happy to be corrected. My profile says I'm a junior member and with good reason.

with regards to weight, they will all be much the same. Get a swingaway as soon as possible. radius's may be the same on paper but some manufacturers 'actual' tyre sizes vary widely. If you do buy a 235/85 as a spare get thw same make and tread as those currently fitted.

G