Hadn't thought about this.
Just checked Oponeo website, and they say 175/70 is fine on the 5.00B 13 rims, producing a rolling diameter 0.5% smaller, so the speedo will read slightly higher. Now when I need more tyres, I'll have to think about that too.
When I first got mine, I was disappointed at the amount of understeer. (155/80 13)
A good tip is to search the tyre manufacturer's website for their recommended tyre pressures for your vehicle. Mine was originally on Continental, now have Vredestein on the front. Both these manufacturer's give higher pressures than in the Fiat handbook, and slightly different to each other. Running on these higher pressures has brought less understeer, and smoother and quieter running. I was surprised about it being smoother, as I expected higher pressures to mean a harder ride, but not so.