Approx 2.8% reduction according to tyre size comparison site.
http://www.tyresave.co.uk/tyresize.html
http://www.miata.net/garage/tirecalcold.html

My rough calc on the diff indicates 7.24% reduction, so nowhere near close enough to give a true feel.

Although no guarantees the maths are right!!


DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4