Are you fitting a larger width tyre?
What's a tyre's profile?
This relates to the depth of the sidewall (the wall at the side of the tyre). The profile is expressed as a percentage of the tyre's width. Eg. Our 215/60R15 tyre has a profile or sidewall that is 60% of the 215mm (width). A low profile tyre is generally 50% or below, down to as little as 30% with ultra low profile tyres. Higher performance vehicles are usually fitted with tyres that have lower profiles.
While low profile tyres give you greater control and better handling on corners, they can give a slightly harder ride. Also, going to a lower profile on the same size wheels will alter your speedo.