Hi, just noticed that although my spare looked 'at first glance' to be identical to the other four alloys on the car it actually is not !!!

So i have four 19'' alloys on the car but an 18'' as a spare. Will this work as an operational spare or will it mess the 4x4 system up? 2003 Range Rover HSE 3.0 TD6
Java Black, Cream Leather Heated seats all round, Sat Nav, Rear TV's.
2013 Range Rover Autobiography 4.4 SDV8
Black, black and piano black. Rear entertainment system. 22 inch turbines

Yes, if the diameter of the wheel and tyre is the same

i.e a 255/55r19 is 30" diameter and a 255/60r18 is also 30" so that combination would work.

get the tyre sizes and use the calculator on Difflock.com (sticky on tyre section) to check.

Thanks for advice, i will check it out 2003 Range Rover HSE 3.0 TD6
Java Black, Cream Leather Heated seats all round, Sat Nav, Rear TV's.
2013 Range Rover Autobiography 4.4 SDV8
Black, black and piano black. Rear entertainment system. 22 inch turbines

http://www.1010tires.com/tiresizecalculator.asp 
RRC 2Dr, RRC 4Dr,
P38, and 2 L322s
(wife thinks I'm nuts - prob right, too)

you'll find loads of members have upgraded from 18''/19'' to 20'' but have only brought 4 wheels so the spare is still the old size [like me]...i luckily find out in time that my 18'' spare had to use the smaller black nuts whereas my brought 20'' had to use the newer larger silver nuts...so had i had a puncture the spare would not have fitted, when i realised i put a set of smaller nuts in with the spare, just in case i needed to use it..

In theory you can run the car on 18s, 19s and 20s all at the same time as the rolling circumference is the same (within tolerances)

Having a different size spare is no problem Discovery 4 HSE
1998 110 TUM HS FFR Hard Top XD WOLF
1982 Series 3 Hard Top
1998 90 TUL FFR Hard Top XD Wolf
1993 90 Tdi

*Gone: L405 4.4 VSE, L462 D5 HSE LUX, L663 Defender 110 HSE, Discovery 3 HSE, 2014MY Range Rover Sport 5.0 Supercharged AB Dynamic; L405 Exec Vogue SE 4.4, 5.0 Supercharged Autobiography, Defender TDCi XS CSW, Defender TD5 HT, Vogue SE TDV8, Vogue TD6, RRSport SC 4.2V8, Classic 3.9 Vogue Auto, Land Rover Series 3 SWB