195 - 205 doesn't have much of an effect on mileage, maybe 0.25 or 0.5 kilometers to the liter on the highway. A grippier tire compound would have more of an effect.
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Review: Yokohama Advan Neova AD07 - 205/45-16
I'd changed from the stock Advan A460s (nice tire, grippy, but not terrific) to a set of 215/45-17s borrowed from my brother's Mazda 626. They gripped terrifically well, but I lost some steering feel on turn-in because of the wide tires and extra-wide rims (7.5 or 8, can't remember which, they rubbed like sh*t).
I ponied up and bought another set specifically for the car, 205/45-16s on ROTA GT3s.
Upsides: Great tire and tire-size for the Proty platform, good feel on turn-in, great grip. A little more thumpy than my stock wheels over bumps, but grips so well that I'm starting to experience wheel-hop when I'm not careful. Front-end grip is terrific, and the car feels dartier than when the stock tires were new. Grip levels feel similar to the track-only A038s we had on the Silvia a few years back. Haven't flogged them on the track yet, but I will next week.
Downsides: Noisy on the wrong surface, thick sidewall and rim protector make the tires look under-inflated at times (even at 36 psi!

). If you get a set that is too wide, they'll tramline over everything They work in rain, but lose some grip when it's cold or greasy out.
Recommed? Hell yeah. This is the third car I've had them on. Just need to match the width to the car. They're marginal as daily tires, but if you want a track-day / canyon-carving tire you don't have to swap on and off, they're great!