Tire Pressure

[pyon2]

Hi, I'm a newbie and have a 2004 sunfire cope ,5 speed..8)
Can someone help me answer,
I put 4 of 225/40 ZR 18 92W tires on my sunfire,
they have 42 psi for front-weels and 50 psi for rear wheels.
How much air should I put ?
for now, I put 40 psi on all tires.
Thanks for any quick responses.

 

[melsg5]

There should be a sticker on the door post with the recommended tire pressure. Since you have upgraded the wheels and tires you can go higher. If the sticker says 30 go to 32 or 34.

 

[Kevlar]

The tires you have now are slightly wider than the largest factory option. Are they on OEM rims or after market?

Regardless, as they are not OEM size, the recommended pressure may not be appropriate for the best wear/performance. You will have to determine the optimum pressure based on tread wear. With wider, low profile tires you will want higher pressure, enough to ensure the tread has even traction with the road.

If the pressure is right, the wear across the tread will be even. Too much pressure will wear the center quicker, not enough will cause more wear toward the sides.

I'll give you an example from my own experience; I had a 1988 Mustang LX 5.0 HO. It had the 5 star Pony rims with 225/50/16 tires (Goodyear Eagle GT II). Max cold pressure listed on the tires was 44 psi. I filled them to that pressure. I live on the Central Coast of CA, 99% of my driving is at or near sea level. I learned that that pressure gave me even wear across the tread. I got over 40,000 miles out of 3 sets of the same brand/model tire.

I'm not saying that you should fill yours to max cold psi, I'm saying that you will have to figure it out by trial and error. The other crucial factor is your suspension, it has to be straight and true, properly aligned with good struts in order for even tire wear.

Hope this helps you.

 

[Kevlar]

Oh, one other thing. You need to have your speedometer calibrated as the tires you have now are larger in diameter than OEM. That way, you may get fewer tickets (or at least won't have an excuse) and your odometer will be accurate too.

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[melsg5]

Kevlar is lucky he lives in CA. I assume they dont see pot holes, frost heaves etc. All these horrors of the roads in the North East are the reason you cannot run your tires based on what is listed on the sidewall. You will find debates over this all over the internet. The car manufacturer spends a lot of time and money to determine the Vehicles recommended tire pressue as opposed to what the Tire manufacturer molds into the tire which is the maximum pressure at the maximum load. You are not placing the maximum load on your tire driving a Sunfire. Placing the tires at the maximum pressure will prevent the air in the tire from absorbing normal impacts it could handle at the recommend Vehicle manufacturers pressure. This additional energy from an impact would normally compress the air in the tire but if the tire is overinflated the energy from the impact will be transferred to the tire itself and the suspension components. Rather than getting into a debate over what is the correct answer I recommend you do your own research.

 

[Kevlar]

Mel, as I'm sure you know, the tire pressures listed on the factory stickers on the door posts of our vehicles are safe marginal numbers and it could be argued that they are relevant to the rim and tires provided by the factory. When we change to custom rims or use a tire make and model other than OEM we've just changed the design.

The factory numbers are calculated for ease of ride as much as safety and performance and as such, are a compromise.

The important thing that we are both saying to pyon2 and anyone else reading this thread is when you change the design of the vehicle by using rims and tires that aren't OEM you have to do your own homework on what works best.

Yes, I am very fortunate to live in California for a myriad of reasons, I wish the lack of pot holes was one of them! (haha!) We have some very rough public roads around here. Not enough money for CalTrans just like most every other state in the Union I'll wager.

The Mustang I mentioned had Eibach springs and sway bars. It was lower than stock and the suspension was tight and stiff. I handled like a go-cart. With the low profile tires, high pressure and stiff suspension, you could feel a dime when you ran over it! I learned about the best pressure for those tires on the first set and it took a while to dial it in. Once I got there, it was consistent for all three sets that I bought for the car. I imagine that another make/model of tire would be different.

Best to start conservative and work up from there.

Cheers!

 

[melsg5]

I agree! Bets on whether we ever hear from the OP again?

 

[Kevlar]

No money on that! But I think we both made an admirable CYA effort for anyone else reading this!

Nice job!

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[dayne66]

Draw a chalk line across the tread and slowly roll about 50 yards and see how the cahlk has worn.

 

[Kevlar]

Nice Dayne! Now that's thinkin! You'll want to do that on as smooth a surface as you can find. Let us know how that works out.

 

[Zimgxp]

You can do the same thing with a puddle of water. Drive through it and make sure you see the whole tread print on dry pavement.