Tallest 17" tire for the CX-5?

[forties]

Hopefully the Mazda chassis can do something with those stiff sidewalls. Read up on minimum safe air pressures people are running in those things. I have done a lot of unloaded miles on LT's and they ride a lot better when they are doing what they were meant for. Being loaded.

 

[leeharvey418]

I want to let it go, but my personal code of professional ethics demanded that I do the math.

The placard pressure on a CX-5 with 17” wheels is 34 psi. If you consult a load-inflation table it will give you a load of 1746 lbs at that pressure. Following the P-metric to LT-metric conversion on page 11 of that document, you remove the 10% de-rating from running a P-metric tire on a “tall” vehicle, and you get 1587 lbs load requirement for each tire. Look up the pressure/load numbers for a LT225/70R17 tire on page 25, and you see that for single-tire applications, the next higher load than 1587 is 1640, and you get that at 40 psi inflation pressure.

I can go into why the R speed rating is also grossly inadequate, but I can already see “I’m never going to go 106…” on the screen so I won’t bother.

 

[Saskabush]

I want to let it go, but my personal code of professional ethics demanded that I do the math.

The placard pressure on a CX-5 with 17” wheels is 34 psi. If you consult a load-inflation table it will give you a load of 1746 lbs at that pressure. Following the P-metric to LT-metric conversion on page 11 of that document, you remove the 10% de-rating from running a P-metric tire on a “tall” vehicle, and you get 1587 lbs load requirement for each tire. Look up the pressure/load numbers for a LT225/70R17 tire on page 25, and you see that for single-tire applications, the next higher load than 1587 is 1640, and you get that at 40 psi inflation pressure.

I can go into why the R speed rating is also grossly inadequate, but I can already see “I’m never going to go 106…” on the screen so I won’t bother.

Have you ever ran LT tires on a >5000lb vehicle? You are absolutely correct in terms of achieving the same engineered load capabilities... But that doesn't mean the tires will suddenly blow if you run lower PSI. In fact its quite common to run 20-30 PSI or even lower (in certain conditions) on LT tires. Some guys even do it with 7000+ lb rigs. The tires will be just fine.

A lot of what goes into to those load ratings is lateral forces at that weight, the risk being the tire popping off the bead if not inflated enough. But as long as you aren't trying to suddenly turn sharply at highway speeds while overloaded its really not an issue to run lower PSI. I've been running LT tires at ~25-30 PSI on mid size trucks (4000-4500lb) for 15 years without a single issue. They wear perfectly fine and the ride is vastly more comfortable than at 40-42 PSI like the math would suggest.

That being said, LT tires are absolutely unnecessary for vehicles that light. Some guys will argue they need the extra sidewall durability for puncture resistance when airing down to like 5 PSI, but that's an extreme use case. There's plenty of SL or XL options out there now in the same tire types. I've switched to SL rated AT tires on my Tacoma and the ride is just so much better. After all, tires are basically what absorb small bumps like cracks in the road, not shocks. You just sacrifice that on road comfort for no good reason by running LT, or situational saftey if you running them at a lower PSI. Not to mention the MPG hit from the extra weight.

But it is important to know the risks and limitations when running what are essentially the wrong tires for your vehicle type though. Even though it may be a rare case, there is still a risk of the tire failing if, for example, you have to dodge a dear or something on the highway and are running too low of PSI. You just need to decide if its worth the sacrifice to ride quality running at the "correct" PSI all the time. There is always the option of running lower and airing up a bit before highway trips too. Or you know, just don't run LT tires on light cars/trucks lol.

 

[Nessmuk]

I honestly don't understand what you are saying.

 

[Saskabush]

I honestly don't understand what you are saying.

I think leeharvey was pointing out that to be at the same performance/safety stats, the tires you just bought need to be ran at 40 PSI. And that combined with the stiff sidewall will feel like you are running wooden wagon wheels lol.

I'm saying you can run those at lower PSI for better comfort. Google the chalk test and do that, it will make sure you get even wear. You may find that 28-30 PSI works best when doing that test. But just know that there is technically a risk in doing that. Yes you get a lower load rating, but what that really means is you risk the tire coming off the bead if you put too much lateral force (sideways) on the tires at too low of PSI. So no drifting at highway speeds! And I would recommend airing up at bit when doing highway trips just to make sure the tire isn't flexing too much and heating up. You'll get a bit better MPG too, just a slightly rougher ride.

 

[Nessmuk]

That I understand. Thank you. That is how I was thinking it should be.