Winter Tires for a Canadian 2012 Prius

I'm relatively uninformed about cars, but I am an aerospace engineer who just went overboard on research for winter tires and wheels for my 2012 Prius. All this information is available out there in pieces, but I figured I might save others some work by summing it all up in one place. My goal was to sort out the marketing hype and misinformation from the things that truly improve winter safety, fuel efficiency, and cost efficiency, and try to trace everything back to solid engineering principles. I never race my car and have no intention of making it look like I do, so a lot of the “performance” recommendations I found were of no relevance to me.

THE QUICK ANSWER
If you're a busy person who doesn't want to worry about the details, just get a dealer package: Ask for four Michelin X-Ice tires on 15” Corolla steel rims with TPMS and nitrogen fill. That comes to $1285 plus tax minus a $70 rebate, and they'll take care of you with free bolting and TPMS registration in future years. You might save about $200 over the life of your car if you're a do-it-yourselfer willing to read up and buy some tools, but then you shouldn't be looking for quick answers.

WHY THE FUSS?
The Prius tends to have more trouble in snow than most cars, for two fundamental reasons. First, it has less clearance between the ground and the car. That helps fuel efficiency, but means you can get stuck on smaller lumps of snow. (Or lumps of anything for that matter.) The second problem is that the hybrid synergy drive locks up when you lose traction. That feature is needed to protect the electric motors essential to hybrids, but it also prevents you from using some traditional snow techniques. (Spinning down to pavement, and rocking back and forth out of a rut.) So if you must drive a Prius through snow, it's a good idea to try to compensate with the best snow tires you can get.

TIRES
The most important difference between winter and all-season tires is the hardness of the rubber. Rubber gets stiffer at colder temperatures, which is good for fuel efficiency and tire wear, but bad for traction. Winter tires are made of softer rubber so they will still be soft enough in cold weather, regardless of whether there is any snow or ice on the ground. I found some advertising that recommends switching your tires when the average temperature during your drives reaches 7ºC. Based on historical climate records for Toronto, that roughly means from November 1st to April 1st. Other locations, or people who do most of their driving at night or in the afternoon, may want to adjust those dates, but basically just don't wait for the first snowfall. The safest option might be to use winter tires year round, and I know someone who does that, but you would pay a price in noise, fuel, and faster tire wear. Plus, those faster-wearing winter tires were more expensive to begin with. Still, it's something you could consider on a car near the end of its life, to avoid buying one last set of summer tires. For completeness, I should note that some high-end summer and all-season tires do have a marginal safety advantage at warmer temperatures, especially in heavy rain.

Michelin X-ice tires have consistently been in the top three winter tires across a variety of tests by different professional reviewers for many years. Mind you, I question the impartiality of professional auto reviewers; this might just indicate that Michelin buys the most advertising space. You can certainly find a range of personal opinions out there, as with any product. But as an engineer I'm drawn to the experimental data, dubious as it may be. The X-ice have low rolling resistance and long life. Depending on where you live, you may want to consider studs, which would rule out the X-ice.

You'll need to know your tire size when shopping. The stock sizes that Canadian Prii came with are listed below, but check what's written on your own tires, and then consider if you want a different wheel size. I found the best price on Xi2 from 1010tires.com, and the best price on Xi3 from pmctire.com. (Free shipping to Quebec and Ontario from PMC!) Dealers and other shops may accept to have the tires shipped directly to them for installation, but then you'll be missing out on important package benefits.
Gen III 15”: 195/65R15
Gen III 17”: 215/45R17
Gen II 15”: 185/65R15
Gen II 16”: 195/55R16

You ought to check the load rating and speed rating, although this usually isn't a problem unless you fill your car with cargo and take it onto the autobahn. (BTW, much credit goes to Chap's posts on this site to help me understand load ratings.) To figure out what you need, start with the front and rear gross axle weight ratings (GAWR) printed on another sticker inside your door jamb. Front 1025 kg / rear 985 kg in my case. Divide by two to find the maximum load you might place on your tires with a fully loaded vehicle. Then you would ideally subtract out some cargo weight to arrive at your normal load, taking into account weight distribution on the axles, but that's a lot of work. I would just conservatively assume that my GAWR is my normal load. Now you divide your normal load by 0.94 to get the minimum load capacity your tires must have by US law, OR divide by 0.88 to get the minimum load capacity recommended by the tire industry. Do not skip this division even if you took the GAWR shortcut in lieu of normal load; on the Prius, this is not enough to compensate! However, it's a reasonable excuse to ignore the 0.88 recommendation and just stick to the 0.94 legal requirement.

To figure out the load rating of the tires you're buying, look at the service description embossed on the sidewall. Online vendors often misreport these details, so look for a picture of the sidewall, or go to the manufacturer's website. The code will look something like this: P195/65-15 89S XL. The 89 is the load index and the S is the speed index. The sidewall may also show the load capacity in kg or lb, but that's only for a particular tire pressure which should also be marked. If there's a “P” in front of the size, that means you're looking at a “P-metric” tire whose load capacity must be adjusted for tire pressure using the TRA inflation tables. If there's no “P”, then you're looking at a “euro-metric” tire and would use the ETRTO inflation tables. If it's euro-metric, then pay particular attention to presence or absence of an “XL”, “EXTRA LOAD”, or “REINFORCED” marking, because those will point you to a different table with lower load capacities. If there's an “LT” in front of the size, you're in the wrong forum entirely. These load-inflation tables are hard to find online; start by googling the Bridgestone/Firestone Replacement Tire Selection Manual, which contains a sizeable excerpt of the real tables.

My car came with 15” P-metric Ecopias with an 89S service description, which is pretty marginal. The Michelin X-Ice is a euro-metric tire, so I need a slightly higher rating. The Xi2 in my size is 91T standard, while the Xi3 is 95T XL, both of which are fine at door jamb pressures of 35f/33r. You'll have to do your own homework if you have anything other than an identical copy of my car.

INSTALLATION
The next question is whether to change the tires on your existing rims, or buy a dedicated set of winter rims. That used to be a no-brainer. Swapping tires on and off costs at least $100 per year, ($172/year at the dealer) whereas swapping full wheels is $0 to $48/year depending on your relationship with your shop. A brand new set of steel rims costs $280 at most, so in a worst-case scenario, your investment still pays for itself in five years. Put another way, that's at least 18% return per annum for the life of your car, better than investing in the stock market. But new technology has made the math a bit more complicated.

Tire pressure monitoring sensors, (TPMS) are now required by US law on all new cars. They set off a dashboard indicator when your tire pressure has dropped 25% below the setpoint, which may seem a bit insensitive. A 25% under-pressure will hurt your fuel efficiency by 2-6%, shorten tire life, and may cause the car to pull to one side during acceleration or braking. The TPMS sensors are installed on the rim, inside the tire. (Now why couldn't they be on the outside, attached to the valve? Or have their own hole through the rim? Can someone answer that for me?) So there's no way to move the TPMS to the new winter rims without removing the tires from your old rims. Most shops will suggest foregoing TPMS on your winter wheels, even though this is illegal in the US if your car originally came with them. (I know what you're thinking, but the fact that everybody pays cash to avoid taxes, or that everybody speeds, does not make it legal to do so.) A red indicator light shows up on your dashboard while you run without TPMS. The system will be temporarily fooled each time you park next to the garage where you store your TPMS-equipped wheels.

There are ways to stop the red light: placing a piece of black electrical tape over it, disabling the system with a jumper behind the glovebox, (Gen III only,) or installing the sensors on a piece of pressurized pipe stuck under the seat. But all of those options offend my perfectionist nature: cold winter days are just the time when your tire pressure gets low, and when I feel least interested to get out and check the pressure. I would feel compelled to buy four more sensors at $52 apiece (price has been dropping fast) to put on the new rims. And then you have to go back to your dealer to re-register the TPMS twice a year, $54 each time, or buy your own programming tool for $150. And then the batteries on your typical TPMS will go bad after 3-5 years, and you've got to replace the whole sensor, not just the batteries. Toyota and Dill Air Control Products claim that their TPMS sensors will last 10 years, at a marginally higher price of $55 each, but then I found reports of failure after 6 years. You can get $30 sensors on ebay, but I have no faith in their battery life. Buying TPMS on pmctire.com is possible but confusing: you have to click a checkbox when selecting your wheels, and then add an extra comment on your final order.

If you get the full package of tires, rims, and TPMS sensors from your dealer, you'll pay more up front, but then the biannual swap and registration is free for the life of your tires. Outside shops have their own packages, and so do the online shops. Now you need a spreadsheet to figure it all out, but the bottom line is that TPMS wipes out most of the traditional advantage of winter rims. Some poor combination of choices will see you losing money, especially if you have to pay to register your TPMS twice a year. If you want to keep the TPMS working through the winter, it may be simpler to just find a cheap shop to take the tires on and off the rims twice a year. Or you can always live with the light and check your tire pressures by hand, in the freezing cold.

(Why can't they just upgrade the ECU software to accept two sets of TPMS like a Lexus? Or for that matter, why am I not allowed to examine and rewrite the ECU software on my own car? I can mod the mechanics however I want, but not the software? More questions that I will not get answers to.)

Having a separate set of winter rims may also avoid some wear and tear on the rims and tires from the biannual changeover, but that isn't normally how wheels fail. It also allows you the flexibility of doing the changeover yourself at home, but make sure you read up on how to torque the bolts correctly. If you were planning on getting different summer wheels anyway for vanity, then you might as well keep your stock wheels for winter. If your car came with 17” rims, you might have extra incentive to get a second set of rims, even if it doesn't save you money, just so you can minus-size to 15” rims for winter – more on that in the next section.

RIMS
Suppose you decided to get a second set of rims. Now you have two big decisions to make: steel or aluminium, and what size. Aluminium alloy wheels are more expensive, lighter, and allow more ventilation and possibly more snow and salt to get to the brakes. The importance of weight is a little tricky to evaluate. The addition to overall vehicle weight is negligible, but the fact that wheels are below the suspension and rotating makes it a bit more important. Reducing the unsprung weight will help traction during hard high-speed manoeuvres. So if you're forced to make a quantum lane change on the highway in winter, alloy wheels might just be what keeps you from spinning out. And then there's this business of “rotating weight” which is usually incorrectly explained.

Roughly speaking, lighter wheels can be accelerated and braked with less force, which should save you fuel and brake wear, as well as reduce braking distance and acceleration time. But it's not really the weight that matters, it's the actually the moment of inertia of the wheels. In a nutshell, that means that weight matters more as you get further away from the centre. A lot more. That one ounce balancing weight on the edge of your 15” rim adds about as much moment of inertia to your wheel as a full set of 5 nuts totalling 15 ounces. And that pebble in your treads weighing just a third of an ounce matters as much as either of those. Now the trouble is that wheel specifications usually don't include the moment of inertia, so how do you know if you're getting value for your money? A lighter alloy wheel would definitely have a lower moment of inertia than a steel wheel of identical design, but those fancy snowflake spokes might actually be making it worse than a heavier steel rim! And no matter what, the weight of your tires greatly overshadows the weight of your wheels. I'm guessing that's why people see a fuel efficiency improvement as the tread wears down.

But wait – the Prius's regenerative braking adds another twist. Suppose you had a heavier wheel with more moment of inertia. It takes a bit more fuel to accelerate up to speed, but then you can also get more energy back when you brake! Because of inefficiencies each way, the heavier wheel will still hurt your fuel efficiency, but not as much as on a regular car. So how much does this all matter? Well, on a regular car, tests show that thousand-dollar ultralight wheels can improve your fuel efficiency by 2% relative to a heavier wheel of the same size. On a Prius, it would make less of a difference. Toyota has told some people that the Prius's stock wheel has a weight carefully matched to the car, though no one can figure out why that would be, or how that squares with 15”/17” options. The stock 15” alloy wheel has been weighed at around 14-15 lb (6.5 kg) by various people. (The weight range might be from different years.) Probably Toyota just means that wheel weight doesn't matter much on the Prius. In the end, brake ventilation / protection is probably the more important factor for most of us, which explains why people recommend alloy in the summer and steel in the winter.

Rim size has more effect than weight. A larger rim pushes the weight outwards, so it will definitely have a higher moment of inertia. That's bad for acceleration, braking, fuel efficiency, and brake wear. And then that smaller air cushion makes for a rougher ride. But crucially for winter, a larger diameter rim also requires a wider tire, which cannot cut as deeply through snow. So why are up-sized wheels so popular? On racecars, the more rigid wheel helps them take corners at higher speeds, and they leave more room for bigger brakes. On road cars, it helps them look like racecars, and that's about all there is to it. Now I did personally see an eye-popping demonstration that 20” alloy rims can make a sudden highway lane-change that would surely be fatal on 15” rims, so there is that advantage if you need it. A popular choice for the Prius is to use 17” rims in the summer and 15” in the winter. It's possible to put 14” wheels on the back wheels, but that will not fit over the front brakes of a Gen II/III Prius. I think there might be a load capacity issue too. Mismatched wheels are not recommended, though I'm not sure what the consequence would be.

Wheel size specifications are more complicated than tire sizing. The crucial values are diameter, (sat 15” for winter) number of bolts and bolt circle diameter, (5 X 100mm,) but then there's also width, centrebore (54.1 mm) and offset. (45 mm on the Gen II/III 15”) The width and offset can forgive a bit of mismatch, but I would draw your attention to the centrebore, another important specification that is typically not published. The stock Prius wheels are hubcentric, which means the 54.1 mm centrebore is closely matched to the hub diameter, allowing it to take shear stress off the lugs. You can still install rims with a larger centrebore, and get a plastic spacer to fill the gap. That plastic ring will help centre the wheel during installation, but it's dubious that the plastic can carry the weight effectively. So even with the ring, a mismatched centrebore will hang off the lugs, vibrate more than a hubcentric wheel, and wear out your wheel bearings. I've done that on another car, and didn't feel a thing until the bearings needed to be replaced. Corolla 15” steel wheels, part #42611-02471-SW have the same centrebore as the Prius, so those can be a good choice. They are $70 each from the dealer, plus you need different lugnuts for them, part #90942-01007. The online store pmctire.com has a replica of the Prius's stock alloy wheel for $80 with the correct centrebore. They also have a $46 steel “Toyotal GM” wheel with the right centrebore, but it doesn't have an offset measurement, so it doesn't show up if you search by car and there are no guarantees it won't increase your turn radius or hurt your aerodynamics.

PRESSURE
Toyota's recommended tire pressure (35 psi in front / 33 psi rear) can be found on a sticker in the driver door jamb. My Prius shipped from the factory with 35 psi in all tires, but my dealer let this down to 32 psi during my first oil change. (He overfilled the oil too...) The Bridgestone Ecopia tires that it comes with have a maximum pressure limit of 44 psi written on the sidewall of the tire, while the Michelin X-ice has 50 psi. There is no standard safety factor for tire pressure, but prudent engineering practice would be to use at least 1.5. Therefore we can guess the true burst pressure is likely above 66 psi, but I would not stake my life on that. There is one report of a Prius Tire Explosion | PriusChat at 93-95 psi. The metal wheel itself has its own maximum pressure as well, which is typically not published or written anywhere. (It's a surprise!) So which pressure should you use?

The most comfortable ride can be had with the 35f/33r recommendations because that's what the suspension is tuned to. This will result in the least suspension repairs and need for re-alignment over time. The safest pressure is whatever gives the flattest profile on your particular tire. That will often be 38f/36r or higher on the Ecopia's. The Gen II Integrity tires seemed to need even higher pressure. This flatest-profile pressure will also give the most even tire wear, and therefore longest tire life. The optimum pressure for fuel efficiency is reported by users to be around 42f/40r at highway speed, or 50f/48r at city speeds, but they often don't give enough data about their car. In theory, rolling resistance always decreases with increasing pressure, but there comes a point where you start to lose traction and get diminishing returns. All of the above optimum pressures will vary if your roads are of poor quality (lower pressure) or your car is heavily loaded (higher pressure) or you're using different wheels sizes or tire brands. (wild card.) XL tires may need the higher pressures to meet load requirements.

At this point, it is common for someone to claim that friction is independent of contact area, and only depends on the amount of weight on the tires. This is what is taught in most high schools and appears in many physics textbooks. However, this rule defies common sense, can be proven false by simple experiments, and is not used by real engineers for anything more than back-of-the-envelope calculations. Certainly, the force normal matters much more than the contact area. But in this forum where we're looking for 2% improvements here and there, contact area certainly matters. It's also common to cite some 1960's NASA studies about hydroplaning, without considering that tires are designed very differently today.

As a professional engineer, I must recommend that you comply with the Toyota recommended tire pressure marked on the door sticker. Having said that, the negative consequence of exceeding Toyota's recommendation just seems to be reduced ride quality, so long as you stay well within the tire's pressure limit. I am not aware of any tire blowouts attributed to pressures between the door jamb tag and tire sidewall limit, though that might just be because I haven't looked hard enough. If pressure was a factor in the infamous Firestone blowouts, (and it's not clear it was,) then the problem was that the pressures were too low.

My Prius came with nitrogen-filled wheels, which is available as an option in some shops. The main advantage is just pressure stability. For comparison, air is normally 78% nitrogen, 21% oxygen, and 1% water, so it's almost the same. Oxygen is undesirable because it leaks through tire rubber four times faster than nitrogen. (Nothing to do with molecule size, though; oxygen and nitrogen molecules are the same size to within 1%) Water vapour is undesirable because it will condense onto your rims when the weather gets cold, dropping your tire pressure and initiating rust. That condensation may be a more important problem than it first seems.

Air compression increases the partial pressure of water without changing the vapour pressure, so a compressor output is close to 100% humidity no matter what the ambient level was. Moisture separators only remove liquid water that condensed during compression, without reducing humidity. You can get dry air by allowing it to cool down in a high pressure bottle (or a 150 psi compressor tank) before using it, but the little 25 cent compressors at the gas station just put hot humid air in your tires. Dry nitrogen will give you much more stable pressure through the year and will protect your expensive alloy rims and TPMS sensors to boot. I found one forum post who saw his hypersensitive TPMS go off every night just from the daily temperature cycles, and he fixed it by replacing air with nitrogen.

Nitrogen and other gas mixtures have some other mild advantages that have to do with weight and heat capacity, but those features are only valuable on the racetrack. Other than winter/summer pressure stability and corrosion prevention, nitrogen won't make a measurable performance difference below 200 km/hr. Nitrogen is just a subtle perk that's worthwhile if available, but not worth paying much for. It's $3.50 per tire from pmctire. You can get part of the benefit by using compressor with a tank, or by waiting for a cool dry day to fill your tires.

I've seen several warnings not to mix nitrogen and air, but I can't see any sensible reason behind them. For sure, if you're making the switch from air to nitrogen, you will want to heat up your wheel and purge it a few times to get rid of any water left in there. But after that, I don't see anything wrong with topping off a nitrogen wheel with a bit of wet air from the garage compressor when it gets low. As long as the dew point stays below ambient temperatures, there will be no condensation. Going the other way, topping off an air wheel with dry nitrogen, will lower your dew point and reduce condensation, even though it doesn't give you the full benefits.

 

Whoops, I accidentaly put this in the Prius v forums instead of the Gen III one. Is there a way to fix that?