12 inch vs. 13 inch wheels for best efficiency / MPG?
 

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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.

I would think that, with city driving and frequent accel/decel events, lower unsprung weight would come into play more than rolling resistance.

Tire Rack only has only one 12" tire but fortunately it has a 15" counterpart. The 12" is 13 lbs, the 15" is 16 lbs. It stands to reason that a 15" steel wheel will be heavier than its 12" counterpart. So the 12" combination should give livelier acceleration at less energy cost, even discounting any overall diameter discrepancy -- which would also favor the 12" combination.

Potholes are another matter! Also there might be fitment issues, particularly whether the brakes would fit inside a 12" rim.

Good point about unsprung mass.

I suppose the real answer to the question is: It Depends (TM)

- It depends on the difference in RR between the available tires;
- depends on the weight difference between the wheel/tire combination;
- depends on whether the driver will be using top gear enough that the effective gearing difference will be an issue

Sure the points raised are relevant, including the unsprung weight issue. So, it makes me remind the Brazilian version of the Volkswagen Polo BlueMotion which featured tyres that were not just skinnier but also had a smaller diameter (while the differential ratio was substantially higher)...

Correct me if I'm wrong but I think the stock size is 155/80R13. The 165/50R15's are actually a 5.5% smaller diameter than stock. Since the question is posted by a fellow Albertan, I'm going to sidetrack a little. It's winter here for the next 6 months and the 12" winter tires don't fit. Forget changing brakes. Sell the 12's and the 15's. Buy a set of GOOD WINTER tires that fit. Traction trumps all until spring. There are a few winter tires that are LLR like the Nokian hakkapeliitta and have really good traction. It's a win win. Just don't go too big and heavy or you'll pay at the pump. As someone else who also drives in the city all the time, I think tire weight and LLR is abut 50/50. I haven't had a Firefly since I've been hypermiling but my Echo is in the same class. My winter tires are terribly inefficient but they have good traction and they came with the car. The summers are old and almost worn out. They're not LLR but they roll really well pumped up hard. They're 2.5% oversize but lighter because they're worn. In my opinion spend the money on good LLR winter tires and get some cheap summers. Try to keep the weight down and make some custom aero Ecomodder hubcaps to cover up the steelies. You can compare weights of tire and wheel combinations on the tire sites.

I'm with Puddleglum. When the snow flies, everything takes a back seat to safety. Fit snows and go on.

When things dry out you could go back to the stock size, or get weird. You've already got 15" wheels fitted, Coker Tire has sizes all the way down to 125 width to fit 15" wheels. That's a bit taller than the stock height but over an inch narrower. Might help a bit in rain, fight hydroplaning, and will probably help if you get a lot of highway miles.

Stay with the 12 inchers.

Rolling resistance is peanuts compared to rotational inertia, especially for city driving.

Every time you accelerate or decelerate (regardless of how gently) you actually accelerate the wheels and tires which them transfer motion to the road. That acceleration is resisted by rotational inertia.

Rotational inertia increases with the square of the diameter. Less inertia means less energy goes into accelerating those four "flywheels" that hold you off the road.

Double the diameter and the inertia goes up four times.

From a practical standpoint it is getting harder to find 12, 13, and 14 inch tires these days.

12-inch tyres in my country had been quite hard to find in the last 20 years, and the only applications which I can remember right now are the rear axles in the DRW versions of those Korean cab-forward compact trucks, even though many had been converted from to SRW and fitted with bigger rear tyres that match the front ones in size.
http://cdn.automovelbrasil.com/800/2...25914caea2.jpg
http://cdn.automovelbrasil.com/800/2...5b6a713306.jpg

13-inch is slowly becoming harder to find here too.