Right see what you make of this question?

Current wheel / tyre combination (275/45R20):
Diameter = 29.8 inch
width = 10.8 inch
Rim = 20 inch
side wall height = 4.9 inch
Circumference = 93.6 inch

Spare wheel (255/60R18)
Diameter = 30 inch
Width = 10 inch
Rim = 18 inch
Side wall height = 6 inch
Circumference = 94.2

as the internet tells me the spare is only 1% larger than the normal tyre do you think that this is an okay wheel/tyre combo to continue to use as an get me out of trouble unit? (I'm lead to believe that you can get away with up to 1.5%) but does this still hold true for a full time four wheel drive set up?

Thoughts....

For the cost, I'd be trying to get hold of a proper matching spare - or even a part worn tyre in a slightly larger size with the same circumference.

Can it be used ? probably - on the same basis as a space saver - 50 miles @ 50mph - but for that, you may as well just carry a tin of tyre weld.

I have a full size matching spare so that I can carry on any journey, at full speed, over any distance.

Keeping that as a spare wouldn't worry me at all tbh

IMHO that will be no bother at all....  Pete
__________________________________________________
2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-

Thanks for your replies, I will in the fullness of time invest in a new 20" rim however as the consensus is it'll be okay for now I can concentrate on slightly more important purchases (in no particular order suspension parts and the wife's birthday )

Full size spare is much better IME. Even if the size of the space saver is the same you can't drive far or long on it which means your journey is interrupted by a trip to the nearest tyre fitter who likley won't have the same model tyre to match the one you want so you have to hope they have a cheap replacement to get you on your way which you will have to replace when you get time. All a lot of time and £££. With a same size matching tyre you just change it and you are on your way. 2012 TDV8 Vogue (L322)

+1 for full size spare.

Makes things much more straight forward and you can take your time when getting the flat replaced. I would not want to be stuck halfway down to the Alps using a space saver. Tom

Current Drive
2011 4.4 TDV8 Autobiography - Santorini Black - Ivory Leather

Previous Drives
2004 Model Vogue Td6 (Touchscreen) - Java Black - Parchment Leather
1994 RR Classic 3.9 V8 Soft dash - Niagara Grey - Grey Leather
1972 Series III SWB Safari - Green (Hand Painted) - Black Plastic

Yes I've just co opted one of my Turbines to be a full size spare (I still have 4 left) obviously It'll get me home at full speed regardless of distance but I'll still need to swap it because it won't match the rest of the wheels. Geoff

The OP wasn't asking about space savers, he is using an 18" full size wheel as a spare when he has 20" wheels on the fatty.

Their overall sizes are so close as to be insignificant, no harm will come to the diffs or electronics etc.

There would be no mileage restriction on that wheel.

I totally agree a matching wheel and tyre would be preferable but his question was asking if the 18" wheel and tyre would be OK to "get him out of trouble"  Pete
__________________________________________________
2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-

The tyres that are on your 20” wheels aren’t the original size either.

David. 2002 4.6 Vogue SE - Alveston Red with Lightstone Leather
2007 Range Rover Supercharged in Java Black with Ivory Leather
2012 Range Rover 5.0 SC Autobiography in Indus Silver with Jet/Ivory Interior
2012 Range Rover 4.4 TDV8 Autobiography in Baltic Blue with Sand Interior