[v3.2mc]

a change of over 4% in diameter is a large difference when comparing performance of tires in relation to engine and chassis dynamics.

as far as fuel economy is concerned, increased diameter will not only change the overall rear end gearing so that more fuel is required to get to X speed at Y time compared the smaller diameter, but it also increases rotational inertia which creates acceleration and deceleration issues. it's possible (though I believe highly unlikely) to over-lever the rear brakes on the non-///m due to the substantially smaller rotor size when paired with too tall of a tire.

if the car is being tracked, I personally would not consider this move. instead, consider a 235/40r18 front and 265/35r18 rear.