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Old 11-11-2017, 11:40 AM   #1 (permalink)
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12 inch vs. 13 inch wheels for best efficiency / MPG?

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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
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.

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Old 11-11-2017, 01:43 PM   #2 (permalink)
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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.
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Old 11-11-2017, 01:48 PM   #3 (permalink)
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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
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Old 11-12-2017, 08:47 AM   #4 (permalink)
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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)...
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Old 11-13-2017, 11:42 PM   #5 (permalink)
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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.
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Old 11-14-2017, 02:00 PM   #6 (permalink)
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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.

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