The temperature needs to be warm enough to make heat(a 160* will do that anywhere except below zero), and to let the system go into closed loop. A 160* thermostat will do that also. Therefore a 180* unit is fine for almost everywhere, only at sub zero temps would the heat inside be compromised.
Fuel economy is virtually unaffected by any choice, the only measurable difference will be when the PCM is running in open loop. All engines start in open loop when cold, they heat up quickly and at some point they go into closed loop, say after a couple of minutes. Once in closed loop, the A/F ratio is closely controlled to optimum, you guys like those special words.
Measuring fuel economy is not accurate by looking at the dash trip computer(any make or model or year). It can only be done accurately over a long time, many fuel tank fill ups, by writing down the actual amount pumped in, and the mileage. Followed by simple easy math over time, multiple fillups, then you get an idea of the actual fuel efficiency.
The engine does not care what the coolant temperature is, it'll run hotter or colder than the stock levels. But heat is bad for most things, bad for longevity, fluid condition etc. So the hotter you make it, the worse the fluids and parts will wear. Cooler is better up until any greater friction occurs, which is all in the first minute or two of operation. If you could get it to run at 120*, that would not wear any parts out faster than at 220*, expert engine builders know this. The myth of everyone is that an engine needs 200* to make best power, and the efficiency. BS, the lower the better, with knowledge that if you did get it to run at say 150*, you need to be sure the system is in closed loop, and the A/F ratio is very good(tune if needed). There's no magic bad temp, low or high, but I prefer to aim lower for a comfort zone to allow room for rise in temps due to conditions.
I use a 180* unit, but more important than that, a coolant additive which increases heat transfer(Water Weter is the easiest to buy anywhere). Also don't go overboard with anti-freeze, less is better. You only need enough for your climate, not enough to see minus 40* at night. I only need about 1/3 anti-freeze here, and we get single digits every year regularly. Anti-freeze does not transfer heat as well as water, the more you put in, the higher the temperatures will be.
Add the Water Weter, and see if the OEM gauge doesn't go down by half a letter or so. Mine typically run in the "R-M" range.
