Good catch. Was that a front or rear tire? You wouldn't want to drive very long with different size front tires (a little harder on the transmission). Different rear tires wouldn't be a big deal.
Calgary: I've only been a few times, so I can't really comment on the heat. I stayed a couple of days once, and have passed right through a bunch of times (BC road trips). The last time was a couple of years ago, helping my cousin move from Kelowna back to Ontario.
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View my fuel log 2015 Micra S manual: 5.0 L/100 km ... 56.5 mpg (Imp) ... 20.0 km/L ... 47.0 mpg (US) ...