I have a 2011 Nissan X-Trail Tekna 2.0DCI 173 manual that I bought for towing a caravan. I realise now that they do not supply a full size spare wheel, and the 'get you home' wheel stipulates in the owners handbook that you cannot tow with this type of wheel. The local dealer has quoted a figure of £800 for a full size spare wheel. The owners handbook lists three sizes of tyres: 215/65R16, 225/60R17 and 225/55R18, which is the size fitted to my vehicle. What are the disadvantage/advantages of the 225/55R18 over the other two sizes, and could the other sizes of tyres fit my vehicle with the appropriate wheel rims (which are:- 16 x 6.5J, offset 45mm (steel and aluminium); 17 x 7J, offset 40mm (aluminium); and 18 x 7J, offset 40mm (aluminium).