When changing wheel size, say originals 16" (to 17 or 18"), is the original 16" spare wheel still usable ?

Yes although not technically legal (well wouldnt pass an MOT)

Was curious what everybody else does, I'm thinking a 17/18" spare wouldn't fit in the well !

Ross, a hypothetical thought,

if the spare wheel had a sticker on it (like a space saver does) stating something along the lines of don't exceed xx mph etc, could it still technically fail an MOT??

Yes although not technically legal (well wouldnt pass an MOT)

However you could just remove it at MOT time.

Was curious what everybody else does, I'm thinking a 17/18" spare wouldn't fit in the well !

The diameter would still be the same though - the bigger the wheel, the lower the tyre profile needs to be.

The diameter would still be the same though - the bigger the wheel, the lower the tyre profile needs to be.

That's what I was thinking but didn't know there was an MOT issue.

I think it's road-legal; certainly Citroen used to supply different (narrower and higher section) spares ex-works, and I've never heard of them failing an MoT for having an odd-size spare.

Havn't the rulings on MOT's changed regarding the spare wheel. Afaik it isn't checked anymore. If the car is safe AS IS then all is fine.

I have 18's on my octy and the spare is the oem 17" affair. Recently passed MOT.

My 2nd car (Range Rover HSE) dosn't even have a spare wheel as it has an LPG tank in the wheel well. Just has one of them emergency tyre inflator things - Passed MOT.

Hope this helps.

There is no legel requirement to carry a spare wheel........However if you do carry a spare and the tyre a bald it will fail an MOT......

But, as noted, it can legally be a different size from the road wheels.

Also remember, If the rolling radius of the spare is different to the rest of the wheels fitted to the vehicle, do NOT put onto a driven axle, as the Diff will be working all the time.