Question regarding changing tire size

[buschman]

Hello all, new forum member here. I drive a 2018 Subaru WRX. I know, not a traditionally hypermile type car, but I like it cause it does everything and is an engaging car to drive. That being said I do a lot of highway driving, at least 90% of my miles are highway miles. I have a set of summer tires and winter tires. I swap them on my stock 18"x 8.5" rims.

I want to get a new set of rims so that I can swap them in my garage. Most of the aftermarket rims for the WRX come in the 8.5" width and the 9.5" width. I'm debating on which one to get.

My current tires are 245/40R18. If I got the 8.5" rim that is what I would stay with. I'm getting decent gas milage with these. 31.2 mpg over 36K miles. I can usually get 32-34mpg per tank.

If I go with the 9.5" rim I would probably step up to a 265/35R18 tire. I've been reading about the pros and cons of doing this. I'm interested in hearing thoughts of those that focus on gas mileage. Obviously there are some performance and appearance gains by going with a slightly wider tire. Also keep in mind the rims I will be getting will be lighter than my stock rims. So I don't think the weight gain of the wider tire will be relevant. Overall I will be losing rotational mass even with the wider wheels and tires. However, with the wider tire I would think I will be increasing my frictional loses or rotational resistance.

I also read that wider tires can be a benefit on the highway, where I spend most of my time. I'm not sure I understand the concept on this theory.

I know it's not a huge difference going from a 245 to a 265. But once I plunk down this money, there's no turning back. Wanted to hear some perspective before going this route. Your thoughts about this subject are appreciated.

[mpg_numbers_guy]

Welcome to ecomodder.

245/40R18 are giant tires/rims. You would likely see a larger benefit going to 15" or 16" rims and narrower tires. 245/40R18 could be replaced with, say, 215/65R15 tires. 1.2% taller gearing, but more importantly a far lighter and more eco setup.

Wider tires almost never help economy unless you are comparing a wider, low rolling resistance tire to a narrower, non-eco tire. The heavier weight of a taller tire may hurt economy in the long run unless you do only highway driving.

[buschman]

Originally Posted by mpg_numbers_guy

Welcome to ecomodder.

245/40R18 are giant tires/rims. You would likely see a larger benefit going to 15" or 16" rims and narrower tires. 245/40R18 could be replaced with, say, 215/65R15 tires. 1.2% taller gearing, but more importantly a far lighter and more eco setup.

Wider tires almost never help economy unless you are comparing a wider, low rolling resistance tire to a narrower, non-eco tire. The heavier weight of a taller tire may hurt economy in the long run unless you do only highway driving.

I know most would think I'm silly for purchasing a higher performance car and giving a hoot about it's gas mileage. But sadly, I'm an odd person with odd tastes. With that said, thanks for your suggestions. However switching to a different rim size is not an option for me for two reasons. First, I would lose cornering grip which I do not want to do. Secondly, I have larger brakes than average, thus shrinking the barrel size of the rim will not work. I plan to stick with 18" wheels.

With that said I won't go smaller than the 245mm width I currently have. If I get the wider rims(9.5") I'll eventually move up to a 255mm or 265mm tread width. I'm really just trying to gauge how costly the wider tread will be to my MPGs.

I recently got some Michelin X-ice tires for my winter set. I thought they would tank my MPGs. However I was wrong. They maintained a similar, or at least non perceivable, difference in MPGs. The cold weather had far more impact than the tires. I'm wondering if my tread width will be a similar difference. Too small to notice. Or if lightening the wheels 4 lbs. each will have such a positive gain, that the increased rolling resistance of the tires will be over shadowed.

Thanks again for your suggestions.

[Daschicken]

Originally Posted by buschman

I recently got some Michelin X-ice tires for my winter set. I thought they would tank my MPGs. However I was wrong. They maintained a similar, or at least non perceivable, difference in MPGs. The cold weather had far more impact than the tires. I'm wondering if my tread width will be a similar difference. Too small to notice. Or if lightening the wheels 4 lbs. each will have such a positive gain, that the increased rolling resistance of the tires will be over shadowed.

Thanks again for your suggestions.

The difference in rolling resistance if sticking with the same tire model is probably negligible, but the increased width won't do your aero any favors. I'm guessing you might see a 1 MPG drop, probably nothing crazy.

What width are your winter tires?

[mpg_numbers_guy]

Originally Posted by buschman

With that said I won't go smaller than the 245mm width I currently have. If I get the wider rims(9.5") I'll eventually move up to a 255mm or 265mm tread width. I'm really just trying to gauge how costly the wider tread will be to my MPGs.

Anything wider will hurt your mpgs, but since your stock tires are already wide and grippier than a typical low rolling resistance tire, you likely won't see much loss from going wider. Probably in the neighborhood of 1-2 mpg, like Daschicken said. Likely not enough of a difference that you would notice it when filling up unless you monitor your fuel economy closely.

[CapriRacer]

This sort of change has more to do with what tire you chose rather than difference in size. RR will be just one of many factors that should impact your decision.

[Ecky]

Some things to consider:

Wider tires will hurt aerodynamics, especially if they begin to protrude from your wheel wells.

Wider tires in theory have lower rolling resistance, all else being equal, because they have higher load ratings and at the same pressure your tire is deforming less over road irregularities. HOWEVER the flip side to this is that wider tires do not actually have more grip at the same tire pressures either - because your contact patch is essentially the same. You have to drop tire pressure to get any more grip out of a wider set of rubber, because otherwise your contact patch stays roughly the same - it's a function of vehicle weight and air pressure.

(this ignores sidewall stiffness as a factor and assumes rubber is perfectly pliable, which it is not, but the above is *mostly* true in practice)

If you go wider and leave the pressure the same, your contact patch (and thus overall grip) will be about the same, but it will be shaped differently. The contact patch will be wider, and you'll get slightly improved lateral grip at the expense of braking and accelerating grip.

Although your rims will be lighter if you go with the the 265's, your wheel's overall rotational inertia will still likely go up, resulting in doubly reduced accelerating and braking performance. This is because although the rim in the center is lighter, all else being equal the part of the wheel that's farthest away from center (the tread), the part that affects rotational inertia the most, will be wider and heavier. Tire weight matters a lot more than rim weight.

What impacts grip the most is tire compound. The largest reason I can see to change tire size is to have access to a different set of tire compounds. If you want the fastest car possible, with the best acceleration, braking and handling, you'd get the smallest rims that still fit over your brakes and go with a narrow tire with a very sticky tread compound.

Why do supercars have super wide tires, then, if not for grip? It's actually heat dissipation and tire wear with the super sticky compounds their tires need. Wider tires can dissipate heat better, and wider tires at the same pressure will last longer, because you have more tread without increasing contact patch. Everything else (acceleration, braking, handling) gets worse.

[CapriRacer]

Ecky,

I'm going to disagree with some of the things you said - but before I do, allow me to state that I just don't have the data to back up everything I am about to write. I'll present what data does exist, but to my knowledge, there's a lot of data that doesn't exist. That means we are all working off our own ideas about how tires work.

1) Wider tires do have a larger footprint - all other things being equal - except not all things are equal.

When we talk about wider tires AND having the same load carrying capacity, it means the aspect ratio is lower AND the overall tire diameter is smaller.

Let's take the 2 tire sizes the OP mentioned:

245/40R18 XL: Load Index: TRA = 93 OD= 25.7

265/35R18 XL: Load Index: TRA = 93 OD = 25.3

Please note: I have chosen to use the US standardizing organization's loads and dimensions (The Tire and Rim Association = TRA) That choice is strictly for convenience. Other tire standardizing organizations have slightly different takes on load carrying capacity, but each is consistent within itself - and the dimensions should be the same regardless.

Back on topic: The footprint size for the 265 will be wider and shorter, but larger - meaning it will have a lower average footprint pressure.

So how do I know this?

https://web.archive.org/web/20080817...on-tires-1.htm

Please note: This webpage is in the archive, not on the web today. If you dig beyond the surface, the conclusion is that wider tires have a lower average footprint pressure under the same conditions.

Are they close? Yeah - and it's close enough that one could argue that they could be considered the same - but we have to be careful because that could lead us to some erroneous conclusions.

And one such conclusion is that the rolling resistance would be higher for a wider tire (all other things being equal) - and that doesn't appear to be the case.

I developed a formula based on the only data I know of where the tires were the same, but the tire size was different. I talk about it here (about 2/3 the way down the page): Barry's Tire Tech

Again, the numbers are close - and, again, close enough that they could be considered the same. But it is consistent with the idea that the smaller diameter causes the tread to deflect less and even though the tread is wider, the RR is smaller.

2) Wider tires have more grip, because they have a larger footprint.

Tires don't follow Amontons' Laws (that friction is NOT proportional to contact area). That's because the tread rubber penetrates the surface texture and adds additional grip.

That's why racing tires are wider. Please note: Many racing organizations dictate the tire size, so pointing out that - say - Formula 1 doesn't use the widest tires available, doesn't support the counter argument.

Why do supercars have wide tires? Mostly because of the brakes. The larger diameter wheels preclude the use of narrow, tall tires.

But a side benefit is that for the same load carrying capacity, a tire with a shorter sidewall is more responsive (quicker reacting) - which is the same reason behind using low aspect ratios in racing.

And, I don't think wider tires dissipate heat better, nor wear better. I see no reason why 2 tires designed the same but of different size would have different heat dissipation or wear properties (Keeping in mind that all other things are equal)

So I'm going to stop here. Please be aware that bring this up merely as a discussion point. I wish we had data to fill in our gaps of knowledge, but we don't.

[Ecky]

Consider the point of heat dissipation:

In track and hard driving conditions, tires can and do overheat. Normal driving habits will rarely see this unless a tire is severely underinflated, so it's not really a concern for most of us.

If two tires have the same overall rolling resistance, they are producing the same friction and heat.

A larger tire has more total rubber and more surface area and will have proportionally larger heat capacity and ability to dissipate heat. Ditto with wear. In order to wear a tire down which has 10% more tread, there would need to be proportionately larger friction.

As for rolling resistance vs size, I've read conflicting things. I know that manufacturers tend to put very tall, narrow tires on their super economy cars. Taking for example the BMW i3, BMW and Bridgestone published information on their choice of a very tall, very narrow tire:

BMW has achieved a genuine milestone with the i3 electric car. Doing justice to its unprecedented levels of both efficiency and performance demanded more than just a new tyre but an entire new tyre concept. Bridgestone’s answer came in the form of the ologic technology, which capitalises on the synergies of a large diameter coupled with a narrow tread design. The result is a tyre that delivers significant improvements in aerodynamics and rolling resistance, while still offering outstanding grip in wet weather conditions.

Combining a large diameter with a narrow tread pattern has several advantages. While the tread on smaller diameter tyres is typically inclined to excessive movement or “deformation” during driving, the larger diameter and higher belt tension significantly reduce tyre deformation and therefore conserve energy that is otherwise lost through internal friction which helps to reduce rolling resistance. By the same token, the narrow tread concept improves aerodynamics. The most spectacular achievement, however is that these improvements do not involve a trade-off in terms of safety. The tyre’s long contact patch (relative to its narrow width), revolutionary tread design and compound still ensure outstanding grip in both wet and dry conditions.

Source

I think there are a few takeaways from that. Worth noting that VW did the same thing with the XL1.

[CapriRacer]

As a followup, I researched the tires on the BMW i3 - 155/70R19 84Q / 175/60R19 86Q for the base model, inflated to 33 psi / 41 psi.

5 things jump out at me.

1) These are conventionally sized tires - just unusual in their configuration (large wheel diameter, but tall and narrow.) A more usual sizing would be 175/70R13 with a load index of 82 and a 185/70R13 with a load index of 86.

2) They are Q speed rated. Q is a common speed rating for winter tires, but summer and all season tires have at least an S speed rating. I wonder if the tread compound can't stand the higher speed (heat dissipation), but this car doesn't generate enough speed to warrant an S rated tire. I wonder if the top speed is restricted in some way.

3) They are large in diameter. A 155/70R19 = 27.5" vs 22.7" for a 175/70R13. The important take away would be that the vehicle would have to be designed to accommodate this larger diameter.

4) When I put those 2 sizes into the RR calculator, I get a 24% improvement in RR!!

5) The 41 psi for the rear tires is highly unusual for a SL tire. That ought to result in a HUGE RR improvement. (For those who wonder, the maximum load carrying capacity of an SL is rated at 36 psi (35 for English units), so specifying more than that not only does NOT increase the load carrying capacity of the tire, but should make the tire prone to ride in the center of the tread. I wonder if they did that for handling reasons.)