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?

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

https://blog.consumerguide.com/wp-co...M-1024x765.png

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.

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.

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.

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?

I suspect these ratings are relative, not absolute.

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.)

^ 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

The BMW is also rear drive so it is less likely to tear up tires accelerating compared to the Spark EV. Apparently just about any speed under 40 can produce tire spin on the Spark. The BMW overall looks more like the set out similar to Tesla to do what's right to make the best EV even if it's outside the norm so I wouldn't discount the weirdness as being wrong. Tesla doesn't do it because they market their acceleration and handling performance as much as their economy.

VW's XL1 has, though.

Many EVs go "wide" because of the excess power and torque they "need" to lay down ...

New Leaf, like over 200HP ... silly

The EU rating is not absolute , but relative

"Same" tyre, but with an XL load rating will often return a better FE rating

For sensibly small tyre sizes, like 175/65/14 it's hard to find an A rated tyre - there's just one ... even though the tyres are modern.

205/55/16 ? A's aplenty ...

Ultimately it's not RR that counts, it's RR force you need to overcome
So factor in the weight
C on a lighter vehicle will cause less drag (force) than A on a heavier one


Then add extra aero drag from wider wheels

Relative to what? I don't understand that statement.

Did some research. It's absolute, not relative

https://eur-lex.europa.eu/LexUriServ...46:0058:en:PDF

So, to answer my own question, yes my 225/40 tyre with A rating has lower cooefficient of friction than a skinny bmw bridgestone that "only" has a B rating.

BMW i expect though did the maths and decided that best option for them was compromise of narrow for aero, but grippy compound for safety/putting the electric torque down even if it meant marginally higher rolling resistance.

No, I don't think so. The grade is determined by the RRC (Rolling Resistance Coefficient), which is determined by a test.

That only means that the test gets a different result if the tire is different - one of the peculiarities of trying to do RR ratings. Size and Load Range are examples of "different" tires. The EU didn't try to address this in the regulation.

Yup, larger tires get better RRC - and that is reflected in the ratings.

I think the idea is similar to comparing crash "stars" on different classes of cars. Crash a 5 star compact into a 3 star pickup and the difference in weight is going to make a loser out of the 5 star. So a big tire might have a better RE score but a smaller tire give you better feul economy because there is more to it than RR.

Load index
So it's become popular to increase LI to achieve better RR class
Your car of course has a given (max) weight.

Upside is the higher LI means less deformation, usually allows higher PSI
If your suspension can cope with it without bouncing about.

Measurements are done by manufacturers
No control tests, no penalties.


Wet braking is related to a 2􏰗􏰞􏰣􏰠􏰧􏰗􏰞􏰣􏰠􏰗􏰗􏰞􏰣􏰠􏰧25/60/16 reference tyre
Making it hard for smaller tyres to get the best results

Test conditions can vary wildly in temp and road condition.

Exactly

My C rated winter tyres return the same (even slightly better) FE than the 10mm wider summer B rated ones on larger & heavier rims.

The winter tyres' smaller size, lower weight is more than compensating for their worse RR and less favourable driving conditions.


Go for (or stick with) light (wheels) & narrow if your goal is FE.

Aerovironment had to constantly fight GM to keep them from putting wide tires on the EV-1 for styling reasons.
Unfortunately, there is no good data on belted tires in the public domain, but racing bicycles still win on narrow tires for pavement. Also unfortunately, after a century of everyone understanding the basic engineering behind rolling resistance being inversely proportional to diameter, which operates on steel wheels and ball bearings, Greenspeed published a major test of bicycle tires all based on using a small diameter roller instead of a flat road surface, completely masking the difference.
Once they lay the rubber on the road, narrow tires leave it there for support for longer than a wide tire at the same pressure. However, a belted tire does not have to distort and scrub to create a contact patch. It can have a lower sidewall, and that is important because the strength needed there is directly proportional to the radius that it bulges out with to take the pressure.

My emphasis!

Part of the problem with testing bicycle tires is that narrow tires require MORE pressure to carry the same load than wide tires. The testing needs to be done at the same load carrying capacity - ergo, different pressures.

This problem is also present in passenger car tires, but to a much lesser extent because a difference of 10 mm width is almost nothing for a passenger car tire.

Racing bicycle tyres are getting wider and wider too. The 23mm tyres of a few years ago are nowhere to be seen and the pros ride as wide as 28mm. 18mm tyres are just history now.

ok, so i've FOUND a 5x114.3 wheel that's only 3.5" wide and made of aluminium - woohoo!


https://www.ebay.co.uk/itm/Genuine-L...0/262841685517

should fit right on to the honda, bit expensive but that's not the issue, i can't find any suitable tire to fit this. Only things i can find that are narrow enough are temporary use only :(

looked at i3 tyres but these are only 19" or 20". Also researched adapters to fit some i3 wheels but the increased height of the narrow tyre wont' fit in the arches.. :(

Decent-quality 13-inch tyres with a good efficiency rating are getting harder to find in my country, but recently my mother inquired me if it was worth replacing the 14-inch rims on her Toyota Etios for 13-inch ones to use tyres with taller sidewalls. I'd go that route if it wasn't for the harder availability of 13-inch tyres back here...

A recent test at bicyclerollingresistance addresses this issue. They did tests with multiple controls, the important one here being same tire drop(probably load capacity too). The rolling resistance turned out to be extremely close.

https://www.bicyclerollingresistance...000-comparison

Thanks, but that is still not the right way to test a tire. You need a flat "road" surface. I'd use a bench model belt sander with a worn belt that produces the same friction as asphalt. The wheel rides in a frame with load weights on each side, below the belt to keep it balanced. A cage keeps the rig from going off the test area, and once it is up to speed, the operator adjusts the inclination to get the tire running in the middle. The inclination reading is the payoff.

The absolute readings may not be correct, but I don’t see why the relative readings would be incorrect.

The deformation of wider vs longer contact patches will not be the same on a drum.

Going to a taller sidewall means you are less likely to dent your rims when you hit a pothole.

Just an FYI:

There's been a lot of work about measuring RR in car tires in advance of regulations requiring publication of said RR values. One of those things is the difference between testing on a wheel vs a flat surface (rolling road). The result was that there was a very good correlation between the values. I can't seem to find the study on the internet, but I have notes from a conference I attended.

In other words, when it comes to testing RR in car tires, it doesn't matter whether the tires are tested on a flat surface or a large diameter wheel, the values are proportional.

The pothole factor is one of the reasons I went to 65 aspect tires v the OEM-supplied 60 tires on my Mazda3. I'm now riding on 20565R16 Ecopias. They're working well.

This, and the fact that 13" tyres used to be considerably cheaper than the 14" ones, were the main reasons why my mom lurked about it a few months ago.

That's where I messed up. Got lots of used tires & wheels & they all fit nicely. Saw one spare, but never used tire mounted on a 5 by 114.3 mm bolt pattern wheel & 40mm offset, just like I needed. Got it for $30. A week later, I REMEMBERED that I FORGOT to measure the center hole. Needed a 67mm center hole, but the wheel was 62mm. Oh, well. I can still use the tire, if I can't find a Lexus buyer, who needs a spare wheel/tire combo.

From another thread, but more appropriate here, with additions:
Years ago, I uprated my 175x70x14 inch short wheels/tires to used 205x65x15 wheels/tires, which are about 8% taller than the 14's. Tho the aspect ratio(middle number) of the tires is "less", the 205 tire width, coupled with the aspect ratio, still gives an 11mm longer sidewall length, which handles the potholes better. My ride is wonderful with the 205s.
However, influenced by this thread in part, I continued to look for used 14 inch tires that were as tall as my 205x65x15 inch wheels/tires, because I kept the 14inch wheels.
Hey, I finally found a used 195x75x14 inch tire that is as tall as my 205x65x15 wheel/tires. The aspect ratio gives an additional 13mm sidewall length to really help on the potholes. Hope to find another 195x75x14 inch tire, but now I can use a 15 inch wheel/tire with the 14 inch wheel/tire, if necessary. ;)

In the ID3 155/70R19 were about 4% better in efficiency than stock 215/55r18 tires. Both were with fully blocked air cooling holes. 215 were A on rolling resistance, but the narrow ones ony C.

Actually difference was higher as the tire deflectors were 60mm too wide for these tires and still the first test showed 4% improvement. After I tuned the deflector size to match the narrow wheels it ofcourse got even better results.

In this case I would say it comes from aerodynamic drag. As the RR of that tire is said to be higher than the oem tire size.

On the tests 215 were with 3,5 bar and 155 with 4,0bar tire pressures. On tire pressure tests done to the 215 size it did not help much with those tires at least to boost the pressure, but I wanted to compensate the worse RR figure with little bit higher pressure.

The 155 have 0,84% bigger diameter, that was not taken into account in the tests. Meaning savings are that much more better.
https://www.willtheyfit.com/index.ph...h=5&offset2=40

I had also spacers in the car to bring the more out where oem tire is.

OK, where did that 4% info come from?

If I use my RR by size calculator (Barry's Tire Tech), I get a 2.02% difference in favor of the 18".

I think he means 4% better and fuel economy, not 4% difference in RPMs. That's why he references blanking out the ventilation holes on the rims.

Sidewall difference between the 155/70 R19 and the 215/55 R18 being less than one inch, it may eventually be worth to consider the lower sidewall could lead to fewer deformations at the exact instant of acceleration. But I still value the enhanced smoothness a taller sidewall may lead while driving in a harsher environment.

I suspect this was a personal measurement, and I am wondering how valid it is. I asked the question to get confirmation.