@storm (and all interested in ideal tyre size for vitz)
exactly! so here are all the bad things in 185/70:
All contributing to slower acceleration and higher shift speeds! Fuel economy is badly affected by only the higher shift speed here. Other than that, slower acceleration does not directly affect fuel economy as the engine doesn't rev more than needed (atleast in vitz).
And ofcourse the good is that more tyre circumference per engine rotation simply makes you cover more ground as you said... directly increasing fuel economy!
The net affect is my speedo's giving around 5-10 more kms on every cng cylinder now in city driving (within speed limits, etc.). Add to it the fact that due to change in tyre size, the speedo will show about 1.5% less kms.
So fuel economy is definitely better now. But my recommendation would still be to go for 175/70s, as the loss in acceleration, especially from a stop while climbing even slight inclines with cng and a/c, is not worth the couple of rupees u save every day. Don't forget it's not even a full 1000cc engine we're talking about here. So if u've got one of those 1.3 or 1.5 litre versions, maybe 185/70 won't be as bad.
Btw, Yokohama's Aspec A300 are very good overall and safe too. Low sound but don't expect them to be as "super quiet" as some internet literature makes them sound. Don't use them if you're into racing and all - they're soft and don't seem to be made for this purpose at all.