One of my Bridgestone 175/70 R13 tyres developed a bit of a bulge, so I've got my Chen shin 155 (aspect ratio unspecified but I've been told its 82) truck tyre spare on the back. Thats a better tyre, but its a mismatch, and I might not get it through even a Taiwan inspection like that.

The Bridgestone tread is probably 3-4 mm. I have three "ditchfinder" SONAR 165/70 R13's with about 6-7 mm tread which, if I can revive my lost tyre mounting skills, should fit my rims.

I'm fairly sure the Bridgestones qualify as my "best" tyres in the dry season, so should go on the back, but this may not be true in the wet season. A seasonal end swap of the tyres in use is OK, since I do that anyway when I remember, but the spare should really match the driven wheels, and swapping that on and off the rim is likely too much hassle.

How harmful would a temporary 175 - 165 mismatch be on the front driven wheels, if I have to do that in an emergency?

I may not be here much longer and I'm reluctant to buy new tyres, but may get used ones if cheap enough.

I see no reason why not as people here drive on space savers on the front (apart from speed limit) they seem ok but they are temporary, I have Bridgstones on mine front and rear without any problems and are quieter than the original Michelins

I remember doing that on a MK3 Cortina as at the time I didn't have the right size tyres and they were fine but temporary, the sidewall percentage size maybe a problem (aspect ratio to some) as the width may give differently to the other tyre?

Why not use two of your 6-7mm SONARs on the driven wheels, and the third SONAR plus a Bridgestone on the others? I would think a 6-7mm 165 tyre might grip almost as well as a 3-4mm 175 tyre, especially in the wet, even if a so-called 'ditchfinder'.