If I change my wheel setup from stock, how do I determine the proper PSI for my setup

[deafmidget]

Just like the title says, I am unsure what the proper PSI would be if I swap wheel sets.

Car is a 2016 Subaru Legacy 2.5i Limited. I went from stock 225/50R18 on stock 18x7.5 at a recommended 33psi front and 32psi rear to 245/45ZR18 on larger wheels of 18x8.5. Should i be adding more air to make up for the larger wheels and tires?

 

[NutBucket]

No. Start with the stock pressure and adjust. You'll know if you go too high (harsh ride, etc.) or too low. That's a fairly minor change anyway.

 

[deafmidget]

is there some calculator tool or a formula to figure out what the "proper" psi would be?

 

[NutBucket]

No. The pressure on the door is just a balance between ride & handling anyway. It's only when you get to extremes where you'll see tire wear issues.

 

[mindless1]

The guesstimation goes like this:

Determine max weight rating of stock tire.
Determine max weight rating of replacement.

Divide the stock tire rating by the replacement, then multiply the result by the PSI rating on your door sticker.

That is roughly the proper amount to support your vehicle weight, but you will have better handling and fuel economy if you use the same PSI instead of the adjusted (usually lower PSI with wider tire and rim but you dropped in sidewall ratio) PSI.

 

[deafmidget]

The guesstimation goes like this:

Determine max weight rating of stock tire.
Determine max weight rating of replacement.

Divide the stock tire rating by the replacement, then multiply the result by the PSI rating on your door sticker.

That is roughly the proper amount to support your vehicle weight, but you will have better handling and fuel economy if you use the same PSI instead of the adjusted (usually lower PSI with wider tire and rim but you dropped in sidewall ratio) PSI.

That makes sense. Now what doesn't make sense is that when I put the stock PSI to my new wheel setup and I hit a tiny pothole it feels like i'm going to crack my rim, but it feels way more solid and manageable at around 36-37 PSI. I am guessing this is due to the reduced sidewall meaning less travel distance between ground and rim and putting more air means less tire flex?

I'm horrible at math, but from what you said I'm guessing that my "adjusted PSI" would end up somewhere around 30 or 31 PSI? Yeah no way I wouldnt crack a rim running that.

Guess I should just run something like 36 PSI and call it a day.

 

[mindless1]

I would have never picked a lower sidewall ratio in an area with potholes.

Also, the above guesstimation-calculation assumed that both tires have same max PSI rating to reach their max load rating.

 

[NutBucket]

Well he also went to a wider tire...

225/50 = 112.5mm sidewall
245/45 = 110.25mm sidewall

I'd call that a wash. Also consider what brand/type of tire you're running as a sportier tire will have a stiffer sidewall. That makes a big difference.

Oh, and just avoid potholes!

 

[mindless1]

I consider even a 50R, a bad choice for areas with potholes.

Potholes are often unavoidable unless you prefer to drive into a ditch or oncoming traffic. Best you can do is slow down and not pick tires with short sidewalls, and steel rims aren't a bad idea either but these days mostly coming on economy class or trim vehicles.

 

[NutBucket]

I just prefer to avoid potholes regardless of what I'm driving. While I like visiting my alignment shop because of the folks that there, I dislike paying the bill!

Now that you mention it the stock tires on my Accord are 215/55 and the tires on the Ascent are 245/50. Both ride quite nicely.

 

[mindless1]

I don't end up needing alignments due to potholes, but there's an unavoidable stretch of road near my home where I'm certain they were the cause for accelerated lower ball joint demise.

 

[deafmidget]

Well he also went to a wider tire...

225/50 = 112.5mm sidewall
245/45 = 110.25mm sidewall

I'd call that a wash. Also consider what brand/type of tire you're running as a sportier tire will have a stiffer sidewall. That makes a big difference.

Oh, and just avoid potholes!

I did this on purpose to keep my speedo as accurate as possible and hopefully not lose much fuel economy while being able to run my better looking and lighter STI wheels. If i remember correctly it was something like a 0.6% difference. I also went with a ZR tire in the hopes that the extra stiffness would help and it does compared to stock.

This is the tire i went with if anyone is curious, and yes they were on sale.

https://www.tirerack.com/tires/tires.jsp?tireMake=Goodyear&tireModel=Eagle+Exhilarate&partnum=445YR8EEXXL&autoMake=Subaru&autoYear=2016&autoModel=Legacy&autoModClar=2.5i+Limited

Trust me, I try and avoid potholes as much as possible, even before the wheel swap, but have you seen the Seattle streets? Its pretty rough out here.

Anyways, I guess my question has been answered. Thank you all for your input. I will just run a little more air and swerve like a drunk cab driver.

 

[NutBucket]

Yep, we ran 225/40s on our WRX for over a decade. You just learn to avoid potholes. You also learn how good your wheels are by how much abuse they can tolerate! Never busted a tire but I did put a dent in a wheel once but barely noticeable and still held air just fine.

 

[BarkingGhostar]

I would think that anything provided in terms of air pressure with the tires your vehicle originally came from was a compromise between what the tire maker suggests and what kind of ride the auto maker is trying to convey in terms of softness/firmness. Sometimes, though, this can be hard if you do not know how much deviation the automaker went from the OEM tires to whatever was on there most recently. If it were me, I would go with whatever the tire maker suggests first, determine the ride for soft/firm-ness and other handling characteristics, and then add/lose air from their to adjust.

Another thing you might wish to consider is asking the dealership service department in what rules they follow. While I am sure they do not new as many new tires sales as say the standalone shops, they do put new tires on cars and they do this with the intention it meets the vehicle's design requirements while at the same time MINIMIZES liability. But ultimately I would follow the tire maker's requirements as long as the tire loading is sufficient for the vehicle weight/4 because they could care less about the vehicle and know what it takes to get the tire to last until out of warranty.

 

[mindless1]

^ Generally, the model of tire the vehicle comes with, was a very strict dictation of design by the auto manufacturer under contract. It is optimized for low rolling resistance to improve fuel economy and of course provide a smooth, quiet ride. Except for sports cars and off-road marketed SUV/pickups.

The tire maker does not suggest an inflation rating, only an "up to" max PSI to support the max inflation limit of the tire. Otherwise it is the door sticker on the vehicle when it is the stock tire. That guarantees the tires are fit for the vehicle manufacturer rated GVWR, unless stated otherwise in the manual.

Some manufacturers do provide charts or could be contacted and find the correct PSI for a particular load per vehicle corner, but unless you have a scale to determine that, it's more info than can be used, but more info is seldom bad as long as you interpret it correctly.

A dealership service department may not be willing to install ANY size except stock. Some or more often 3rd parties will be willing to install a tire with equal or greater max load capacity, but that does not tell the owner what the correct PSI is for best tire wear at less than max capacity, nor is there any way to make the kind of subjective owner decision about softer ride versus fuel economy and the average that handles most of their driving scenarios best.

What it takes to get the tire to last the warranty period is the right PSI but that depends on the load on it, to achieve a flat footprint, barring any other warranty-breaker situations.

An old timer's trick was to take a white crayon and draw a line across the tire tread. Drive in a straight line and see if it scrubs off evenly. Make a list of PSI where it does. If so, increase PSI a little and try again. Keep doing this until it wears off unevenly, then reduce PSI a little for max fuel economy or a little more for max road traction. We're talking less than 20% total PSI difference between these two options, usually less than that.

 

[BarkingGhostar]

I was thinking of how they directed you to the retailer. But then again redneck garaging may not be the best choice.

And you cannot always rely on door stickers--what if it isn't there anymore. BTW, I have never had a problem with the BMW dealership service department installing non-original tires on a vehicle. It isn't like only one tires can be used in mass produced vehicles. As long as the product is sized correctly--along with the wheel--there are plenty of legitimate sizing options and they have to know how to inflate those options if chosen.

For instance, the last time I had the dealership service department replace my tires the original make/model tire wasn't available, but several options were. Not only was the dealership going to warranty their work, and the tire manufacturer warranty their product, they didn't necessarily follow the placard. But not this brings me to an interesting point about the door placard ...

Many new vehicles come with a host of wheel and tire options, which include larger diameter wheels as well as wider wheels. And as such, those dictate the gross options for tires placed on them. How is it one placard meets all possibly cases by the manufacturer? I actually hadn't thought about this before.

 

[mindless1]

How is it one placard meets all possibly cases by the manufacturer? I actually hadn't thought about this before.

It's not "one" placard for all. The door stickers are different, with information specific to that one vehicle including tire, wheel, other suspension variances that affect F/R GVWR, paint color codes and more.

 

[maluckey1]

Some euro cars actually spec different pressure on the placard for fully loaded or towing. When in doubt, the door placard is a good start. You can adjust as needed from there

 

[BoomerD]

How does anyone not remember?

inflate to sidewall.....

 

[mindless1]

^ If you mean inflate to pressure # molded onto sidewall, that is only the maximum pressure safe to use in order to achieve the max tire load rating.

If you're always running the max load your tires can handle and they're the stock size, you're probably already past your vehicle load rating so one or the other is going wrong.