Received this question via private message ...
Quote:
Would like to ask you for some advice on 12" vs 13" wheels.
For some time I was fooling with the idea of getting the smallest engine car I can get my hands on.
I do inner city small stuff delivery so needed something smaller than my Ford which started giving me a servo fever
Google told me about your MPG achievements so I decided to for a Firefly. Searched for one in Alberta and within two weeks this one came up.
Rebuilt engine at 100 002 KM
2000 KM on rebuilt engine, new hoses, belts, wheels, seats and so on.
Bought my Firefly a month ago with 165/50/R15 wheels.
They look just fine but aren't suited for ecomodding.
Most of my driving is in the inner city - aprox 95%
Already have a 12" wheel set (rims + 145/80/R12 new winter tires)
I would like to know if I can install these 12" wheels on my Firefly. My gut feeling tells me to go on 12" instead of 13" 
It looks like the 12" rims won't fit the car due to larger rotors I have right now...
What is your take on this one, please?
Do I have to change the rotors and lugnuts to fit the 12" rims ... the front brake system, too?
Any advice would be appreciated here.
Thank you,
JR
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Since you drive almost exclusively in the city, my thought is you should be using the wheel/tire combo with the lowest rolling resistance.
I doubt the 12's are going to win in that regard.
I was going to mention diameter and effect on gearing as well -- those small 12's would have the engine spinning faster. Stud sizing issue aside, that might not be a problem - it just means you'll be able to use 4th or 5th gear where otherwise you'd be in 3rd or 4th. If you're rarely going on the highway, high highway RPM not be an issue.
Personally, I think I'd avoid the hassle of fitting the 12's and go with 13's or get a set of 14" wheels. You can still get LRR tires in 14" size: Dunlop Enasave 165/65R14 (OEM on the Mitsubishi Mirage); Potenza RE92 165/65R14 (OEM on the 1st gen Honda Insight). They're not cheap though (unless you wait to find a Mirage owner upsized their wheels... I got a nearly new set of 4 for $80.)
Then the whole issue of cost comes into play: it might make the most sense financially to just use up what you have now. Unless you think you can sell it and put the funds toward getting an LRR set of tires on wheels that fit.