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Old 06-27-2019, 08:23 AM   #1 (permalink)
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Wheels: Smaller rims, taller sidewalls?

Good day! I'm looking for new tires as my old 155/65/r14 expire (cracks started appearing on the sidewall, yikes!). I've been reading a lot of the posts here and I've been convinced that downsizing rims (but equipping taller sidewalls to match the stock size) will be beneficial in increasing my fuel economy.

So I'm planning to go from Goodyear GT3 155/65/r14 to Michelin XM2 155/80/r13. Would this be a sensible setup? I'm looking to lose a few pounds each wheel so it would be easier on the car to rotate the wheel from a stop. Anyone mind sharing some valuable input?


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Old 06-27-2019, 09:18 AM   #2 (permalink)
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My understanding:

Narrower tires are more aerodynamic, which has a larger benefit than you'd think. Most highly economical cars use narrow tires.

Taller tires have lower rolling resistance, because there's less deflection.

In bicycles at least (and likely cars too) a wider tire has lower rolling resistance with the same pressure, because they can bear a higher load and thus there's less deflection. Or, in other words, it takes less pressure to have the same rolling resistance with a wider (or larger) tire. Wider tires are heavier though, and less aerodynamic. Although a wider tire can have lower rolling resistance, car companies making "economy supercars" use narrow and tall tires.

Weight in a wheel should have little to no impact on highway economy. They will hold more intertia - resist accelerating more, and resist slowing down more. At steady speed we don't much care about this, instead you want an aerodynamic wheel with a LRR compound.

In stop and go driving this isn't good, you want a lighter wheel because you lose more energy with heavier tires every time you have to brake, and it takes more energy to get up to the same speed.

Smaller rims should be a net win. You move the metal closer to the center of rotation, which reduces inertia, even if the weight is the same. However, be mindful that in smaller rim sizes you may not have as much tire choice. Good LRR tires help tremendously and are often worth more than a lighter and narrower tire.

Make sure if you get smaller rims that they fit over your brakes.

EDIT: BMW i3 tires: 175/55r20 - tall, narrow, use LRR compounds


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Old 06-27-2019, 09:45 AM   #3 (permalink)
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Changing tire size doesn't change RR much. The really big RR changes result in careful selection of the tire itself - meaning make and model.

Unfortunately, the tire sizes you are mentioning are not mainstream enough to have the latest LRR technology applied to them.

So don't expect a change of this type to result in a significant difference.
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Old 06-28-2019, 07:08 PM   #4 (permalink)
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Ecky, I would love a set up like that for my Ranger. 175/65/20 pretty much matches my 235/75/15's. I wish my bolt pattern was the same. I'm sure Ford 20" rims are heavy as heck.

I believe mine are 5x114mm but I'm not sure about the center hub.
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Old 06-28-2019, 10:46 PM   #5 (permalink)
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Quote:
Originally Posted by Ecky View Post
Although a wider tire can have lower rolling resistance, car companies making "economy supercars" use narrow and tall tires.

EDIT: BMW i3 tires: 175/55r20 - tall, narrow, use LRR compounds
I find it telling that (almost) no no other MPG car has gone that route. All the current day EV's have pretty wide tyres, so I wouldn't read too much into the i3's tyre spec.
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Old 06-28-2019, 11:17 PM   #6 (permalink)
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Quote:
Originally Posted by oldtamiyaphile View Post
I find it telling that (almost) no no other MPG car has gone that route. All the current day EV's have pretty wide tyres, so I wouldn't read too much into the i3's tyre spec.
Most EVs have treadwear problems from instant torque. They tear up even wide tires. I'm guessing this is primarily the reason - the i3's tires would not last long on a powerful EV.

The VW XL1, Honda Insight and BMW i3 all have very narrow tires. The Mitsubishi Mirage also has very narrow tires and is the #1 non-hybrid in fuel economy you can buy new on a lot.
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Old 08-07-2019, 07:16 AM   #7 (permalink)
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What confuses me is that the BMW tyre (bridgestone ecopia EP500) gets a fuel economy rating of only "B" (eu tyre labelling) which i was surprised about given how skinny it is..

Tyres i have on my honda are way wider (225/40 18) and are rated better for fuel economy - they are "A"; pirelli p7 blue.

How is the pirelli better than the skinny super-eco bmw bridgestone? Or am i missing something?
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Old 08-07-2019, 09:20 AM   #8 (permalink)
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I suspect these ratings are relative, not absolute.
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Old 08-07-2019, 09:31 AM   #9 (permalink)
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Quote:
Originally Posted by rob.e View Post
What confuses me is that the BMW tyre (bridgestone ecopia EP500) gets a fuel economy rating of only "B" (eu tyre labelling) which i was surprised about given how skinny it is..

Tyres i have on my honda are way wider (225/40 18) and are rated better for fuel economy - they are "A"; pirelli p7 blue.

How is the pirelli better than the skinny super-eco bmw bridgestone? Or am i missing something?
What you are missing is that the physical dimensions only play a minor role in RR - and width is just one of 3 dimensions. You didn't say, but I'll bet the BMW tire you compared to your Honda tire is actually SMALLER in load carrying capacity - and more load carrying capacity = lower RR (all other things being equal!).

Then there is the issue of the tire itself. The tread compound plays a HUGE!! role in RR. It just may be that the Pirelli is just more up to date than the BS. (Just an FYI: Once a tire has been qualified at an OEM, the OEM doesn't want it to be changed. If the BS tire was qualified a number of years ago, it might be stuck in a timewarp.)
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Old 08-07-2019, 09:58 AM   #10 (permalink)
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^ i guess that makes sense

maybe a factor also is that to get the necessary grip with such a narrow tyre, the compound they use for the i3 bridgestone is stickier, hence higher RR than can be achieved by my wider pirellis.

Obviously the bmw gets another benefit in the aero efficiency of the narrow tyre

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