As far as I can gather, Volvo XC60 wheel and tyre sizes are: 235/65 R17 235/60 R18 235/55 R19 235/45 R20, so the 17-inch tyres have a slightly deeper profile than the 18s. They will roll over a bit more when cornering, but will cushion the bumps slightly better. That said, the 60 profile R18s should not be bad. The 45 profile R20s will be terrible. But as Matt Prior recently pointed out in ‘Autocar’, low rolling resistance high profile ‘energy’ tyres have stiffer sidewalls, so go for something like Pirelli P Zero Rossos that seem to suit the XC60 very well.