Bought a used x5 to tow a light travel trailer. Tire store websites say the tire size is 235/65r17, which is the same as the Michelin spare. But the tires on the vehicle are 255/55r18. Both sizes are listed on the door sticker. I want to get new and have all same size and brand. Anyone know if one size is better than the other?

Morning Kevin

Welcome to the Forum

Take care when buying tyres for your X5 mis-matched tyres will cause damage to the transfer box and diffs.

The X5 E53 was available with 17" 235x65 tyres square set up 18"  255x55  tyres square set up 19" tyres 255x50 Front 285x45 Rear Staggered set up. 20" tyres 275x40 Front  315x35 Rear Staggered setup. Your car has the standard 17" spare wheel which with the correct tyre size matches the rolling radius of the 18" wheels fitted.

17 and 18 square set up tend to be a quieter/softer ride due to the sidewall depth also the range and price of tyres available, 19 and 20's staggered need more care as when buying as a difference in rolling radius front to rear will cause damage to the drive train. Tyres in these sizes also tend to be fewer and more expensive and due to the stiffer side wall a harsher ride.

As the old saying we make our choice and pay our money!! Obviously the bigger wheels were not just aesthetics they allow larger brakes as well 

My 4.6is runs 20" staggered wheels with summer tyres and I have a set of 18" winter wheels and tyres 

Later X5 E70 models were available with 21" wheel sets but E70 wheels have different wheel centre bore sizes so require spigot rings for correct fitting.

Dave