Automotive → Tire Inflation ?

I grabbed a great deal on a set of tires a few months back. 200 bucks for 4 of them. I had them installed and am amazed at the difference in cornering and stopping. The garage inflated them to 35 PSI and they feel good, but the tire itself calls for up to 50PSI.
The downside is I can tell they wear fast----which is expected. I'm heading on a 2000 mile roadtrip tomorrow and am wondering if they should be inflated to the tire's rating, or what the sticker on the car calls for?
2005 Focus ZX4 ST.
Heres a link on the tires:
»www.tyrereviews.co.uk/Ty ··· ctra.htm

Go by the tire specs.

no! go by the car specs!

»www.tirerack.com/tires/t ··· chid=147
»www.tirerack.com/tires/t ··· techid=8

Lol---maybe I should make a poll!

Or, maybe run the tire spec down to Florida and see for yourself. You've had them on for a while, and can judge the difference.

Always go by the vehicle specs. This information is generally in the owner's manual and on a placard inside the driver's door jamb column.

Well, the problem is that the info from the door jamb and info for the tire itself contradict each other. Even reading the links from tire rack don't really say follow 1 or the other. They lean more to the tire rating.
I'll do what Jug said. I'll drive 1000 as is and 1000 at the tire rating.

I

read my second link again. It clearly says more than once to follow what is listed on the car.

The pounds per square inch (psi) pressure number branded on the tire's sidewall identifies the maximum cold inflation pressure that specific tire is rated to hold. However, the tire's maximum pressure is not necessarily the correct pressure for every vehicle upon which the tire can be used (almost all vehicle manufacturers' recommended tire inflation pressures are less than the tires' maximum pressure).

Therefore when checking and adjusting tire inflation pressures, the "right" inflation pressures are those provided by the vehicle manufacturer, not the "maximum" inflation pressure branded on the tire's sidewall. The vehicle manufacturer's pressure recommendation can be found on the vehicle's tire information placard label, as well as in the vehicle owner's manual.

To expand upon what DannyZ says, the maximum pressure on the tire is based upon what the tire will safely withstand (with obvious margin) at rated load (weight). It may be necessary to go to this pressure if the tire is subjected to the its rated weight to get the proper rolling diameter and still maintain optimum contact with the road.

The auto manufacturer bases the vehicle's recommended inflation value, in part, on the load that will be placed on a tire for that particular vehicle.

If you over-inflate a tire, you will tend to wear out the tread in the center of tire more quickly. If you under-inflate a tire, you will wear out the tread on the outer edges of the tire more quickly.

As an example. I drive a 2004 Grand Am beater for my 84 mile round trip commute everyday. The tires I have on the car now are rated for 50 PSI and ~1600 lbs. My car only weights about 3100 pounds. The recommended tire pressure by the manufacturer is 28 PSI. This reflects that each tire is only bearing about 775 pounds (distributing the weight of the vehicle across all four tires). This pressure is specified because Pontiac considers that the best overall performance (a compromise of handling, ride, tire wear, etc...) is attained with a pressure of 28 PSI.

Will it hurt your car if you go too high... probably not. But you will definitely notice a difference in ride, handling, and tire wear.

Nope, that is incorrect. You always go by the OEM vehicle specification for load and pressure.

Although this article applies to Pickups that have had larger tires installed, there are some good points here and some tests to run. I take the position that too much can be dangerous. It can can cause changes in braking distances, bounce and more.

»www.4wheelparts.com/tire ··· ker.aspx

just one part to consider:
"You can also “calculate” your tire pressure with the chalk method. This involves coloring a section of your tire with chalk to see how much tread is making contact with the ground. Start by finding a flat road surface. Concrete is actually the best choice, but you can also do this on asphalt. Make a mark with soft chalk that goes all the way across your tread. Then, gradually drive your truck forward about 50 feet and then backwards 50 feet.

Analyze the chalk on the tire. If the chalk is only worn off on the center of the tire, reduce the tire pressure slightly and go through the process again. With the adjustment, you should see the chalk wear off more broadly. Keep making tiny adjustments in the tire pressure until the chalk wears off evenly and all the way across the tread."

Throwing my mostly pointless 2 cents in, go with the vehicle specified pressures. The tires on my work van are rated for up to 80psi. But the van's specifications say 45 and 50psi maximum for maximum load. There is no reason to go to 80psi. I get about 75,000 miles out of the tires I use (BFG Commercial TA) which is pretty close to their rated tread life.

The rating on the tires is simply the safe recommended maximum pressure the tire can hold. It has nothing to do with the safe operating pressure. That pressure is recommended by the vehicle manufacturer.

The optimal pressure can be determined using one of several methoes, such as the chalk method. Unless your tire is significantly different from the OEM, you can use the recommended pressures from the door.

Overinflating the tire will make the ride harsh, results in a bad tire wear and a signifdicantly reduced traction on wet or icy roads.

Go with the vehicle manufacturer's recommended pressure. I add 2 to 4 pounds to that because the car makers go towards the low side to provide a smoother ride.

Put in too much air and you'll get a rough ride, shorter tread life, and increase the chance of tire damage due to road hazards.

That's an interesting school of thought. Where did this info come from? Perhaps they go too high to get better gas mileage rating? Or do they go right where it should be to get a middle ground on safety, load rating, and comfort?

The Exploder debacle?

In any case, I also slightly overinflate my tires (37 psi instead of 35) because:
a) slightly better mpg
b) more margin in case the tire loses air
c) I like things 'firm'

The vehicle mfg sticker recommended is for the average owner, calculated for average anticipated vehicle loading. If you always are running loaded toward the max GVW an extra couple pounds pressure is reasonable.

Run whatever pressure makes the car feel like you want it to. 35 is a fine spot for most normal cars on normal tires.

The recommended pressure on the car's sticker is what makes the OEM tires provide the ride that the factory thinks the car should have. Change the tires or want more than a soft comfy ride? You'll have to pick your own pressure.

I'm also happy at 35-36 psi on my cars and I don't care at all that it's 5 psi over the factory recommendation.

I too go 2-3 psi higher for the same reasons, although in the winter I tend to drop it down to give better traction since I use all-weather instead of snow tires

Well--I filled up today and decided to check pressure in the tires.They were all at 28PSI. I suspect that the tires were filled on a hot day---and the cold weather caused the drop in pressure.
I put 38 PSI in them and there is a big difference in the car now. It feels more sensitive than before and seems to roll easier. I'm not sure about grip because at low pressure they stuck like glue to the road. They still do but haven't pushed it hard yet. Before I couldn't lock the tires in panic stops.
I'll leave them as is for now and see how they feel when I get used to it.

You don't want to do that anyway.

No-----but with the old tires The antilock kicked in when a lady started across the intersection when I was about 30 feet away and stood on the brakes. That would have been my thrird wreck in 4 years in the exact same situation. They drive like idiots in this town.

Go by vehicle specs. PSI affects wear, braking distance, gas milage, handling etc... The manufacturer has already figured out the optimal pressure for you.

My vote is to set the pressure at the vehicle recommendations for the load you'll be carrying, being sure you do so with the tires cold and not driven any distance.

If you're going back to South Florida, check 'em when you get to warmer climes. I don't think a couple of pounds over would hurt, over five pounds or more, I'd bleed off some unless I was heading into cold weather.

You've probably already done so, but just in case; »www.tirerack.com/tires/t ··· echid=73 for temp vs. pressure discussion.

Thanks guys. Just drove the Sky Highway and Blue Ridge parkway. I had a blast and the tires perform great at 38PSI. My arms are sore from all the cornering. I DO notice a harmonic sound at 45--50MPH and think It's the tires. No big deal.
For the money---the ST suspension from 2005-2007 is hard to beat.

Don't forget that over-inflated tires are very quick to hydroplane on you while you are also accelerating the center wear out of the tires. Also the tires contact patch is smaller too from over-inflation which increases your vehicles stopping distance wet or dry.

If they are truly overinflated, yes. Just being higher than the auto manufacturer's recommendation does not necessarily qualify them as such. Ford Explorers in the Firestone tire fiasco specified 26 psi on tires that were rated for 35 (I'm guessing a 15-inch wheel). We see how well that worked out for them.

Give me 30k miles of great driving vs. 50k miles of so-so driving per set of tires any day.

I believe they did that to reduce the likelihood of a rollover. Unfortunately, it backfired because the tires overheated, blew out, and caused the vehicle to roll over. Most likely those tires would have been safe if they had been inflated to 35 psi to begin with.

Nope, they didn't blow out (if some did it was an after effect), the tread separated and after that the leftover tire carcass would sometime disintegrate (then blow out afterwards) since the protection and overall tire integrity was gone with the tread.

»en.wikipedia.org/wiki/Fi ··· troversy

quote:

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The failures all involved tread separation — the tread peeling off followed often by tire disintegration. If that happened, and the vehicle was running at speed, there was a high likelihood of the vehicle leaving the road and rolling over.

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And you should read about old rubber stock being used.

Firestone Plant in Illinois Made Many Problem Tires
»abcnews.go.com/US/story? ··· 2&page=1

quote:

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ABCNEWS has learned that eight former employees of Bridgestone/Firestone Inc., have testified or promise to testify that they used out-of-date rubber stock for their tires; that radial coils were exposed to humidity, making them vulnerable to rust; and that final inspections were done too quickly.

A report in today’s Washington Post also said that some employees punctured bubbles in tires, in order to cover up flaws in the products.

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