Hi, in terms of the efficiency gain I don't have exact figures as I switched quite quickly after getting the car. We then went into winter so comparisons are quite tricky. My subjective feeling is that it was probably worth 10 - 15 miles, possibly a bit more.
Ride is noticably smoother. To the point where my other half was quite scathing of the ride on the OEM 20s and now seems quite a lot happier with the 18s. She'll get a shock soon as I'm putting the 20s back on for a few months. I was going to keep the 18s on over summer as we had a France trip planned, but clearly not now. The 20s do look nicer and even though the pot-hole risk is a lot higher, I'll be close to home!
All that said, I'm still pleased with the investment in the 18s and would do it again
