Sorry I don't get this. You bought a FL1 Sport, presumably because you like the look of it on its smart 18" wheels. Then you want to change the wheels to smaller ones, simply to save a few £ on replacing tyres. That's nuts in my opinion.
You're better off selling the Sport, which fetch a premium price over lesser models. Then buy said lesser model with smaller wheels.
I bought Sport rims for my SE because the wheels look the best of all standard FL1, the cost of tyres is just incidental to having those wheels. If you think outside the box, the 18" tyres aren't much more expensive then 16" tyres.
Don't forget that you'll massively reduce the value your Sport by removing the standard 18" wheels and fitting 15" or 16" wheels instead.
Like I said, sell the Sport and buy an SE instead for 1/2 the price and save a few £ on tyres.
