My tyres were oversized my MPG went down!

[Hauki]

*warning - long post!*

Once aftermarket wheels/tyres go on, the door jam sticker is as much use as a one-legged man at an arse-kicking party.

But even with stock tyres on I would be inclined to run at max psi or as near to max as you deem comfortable.

Wiki'd:
"Should a low pressure tire be forced to perform an evasive maneuver, the tire wall will be more pliable than had it been of a higher pressure, and thus it will "roll" under the wheel. This increases the entire roll movement of the car, and diminishes tire contact area on the negative side of the vector. Thus only half the tire is in contact with the road, and the tire may deform to such an extent that the side wall on the positive vector side becomes in contact with the road. The probability of failing in the emergency maneuver is thus increased.

Further, with low tire pressure—due to the side wall being more pliable—the tire will absorb more of the irregular forces from normal driving, and with this constant bending of the side wall as it absorbs the contours of the road, it heats up the tire wall to possibly dangerous temperatures, as well as degrades the steel wire reinforcement; this often leads to side wall blow-outs. In an extreme case of this phenomenon, the vehicle may drive into a pot-hole, or a hard elevation in the road. Due to the low tire pressure, the side wall at the contact area will temporarily collapse, thereby wedging the tire between the wheel and road, resulting in a tire laceration and blow-out, as well as a damaged wheel.

High tire pressures are more inclined to keep its shape during any encounter, and will thus transmit the forces of the road to the suspension, rather than being damaged itself. This allows for an increased reaction speed, and "feel" the driver perceives of the road. Modern tire designs allow for minimal tire contact surface deformity during high pressures, and as a result the traditional wear on the center of the tire due to reasonably high pressures is only known to very old or poorly designed tires."

My opinion on this is,
Bridgestone, Dunlop, Pirelli etc spend a damn sight longer testing tyres tolerances and flex at varying pressures than the car manufacturer. So if the sidewall says it is safe up to 50PSI, then you can be damn sure it is safe up to 50PSI. They will have pressure tested these in multiple different temperatures, humidty & conditions. Max pressure means exactly that, the max pressure the tyre can go to whilst retaining all its design characteristics and effectiveness. IT IS NOT the max pressure the tyre will inflate too, you will probably get 100PSI into a standard tyre!

IMHO, max sidewall pressure is optimum pressure. To think your cars handling is going to be dangerous inflating your tyres above the pressure stated on the door jam sticker is quite frankly, ludicrous. As stated, this is designed more for comfort than anything else. And I would suspect in some auto manufacturers, this is to hide a multitude of sins in the cars chassis/suspension so it is purposeful set to 32 or whatever to hide these flaws.

[CapriRacer]

Originally Posted by Hauki

....Once aftermarket wheels/tyres go on, the door jam sticker is as much use as a one-legged man at an arse-kicking party.....

I am going to disagree with that. The load carrying capacity defined by the placard tire size and pressure tells you a lot about what the vehicle manufacturer was thinking - and he has a lot of expertise in this area. You can disagree with the compromises that he made, but you can't completely ignore what basic information is available.

By the same token, the load carrying capacity of the aftermarket tires can be determined as well. So you have some basic infomation to connect those 2 items. It would be better to know what each of those is.

We can discuss the merits of various inflation pressures, but without knowing the basic information about what the vehicle manufacturer was specifying, and how the aftermarket tire fits into that picture - then, indeed we would have a one legged man.

[Fat Charlie]

Originally Posted by Bill in Houston

"Yeah, the manufacturer didn't design the grille right, and the air dam is too high, and the underbody is a mess, and a lip on that hatch would drop the Cd by 0.01,
BUT
I am POSITIVE that he is EXACTLY right on the tire inflation pressure, and that that single number is correct for all drivers, all tire manufacturers, all tire styles, and all driving conditions."
Just listen to yourself.

Bill, that's the funniest thing I've read in a while.

[iskyfly]

Originally Posted by Fat Charlie

Bill, that's the funniest thing I've read in a while.

But really didnt lend much to the discussion.

I was under the impression that we are adults...



Are there any vehicle engineers here who can say that overinflating a tire past sticker value doesnt;

-decrease safety
-increase expense because of faster tire wear

?



I really would like to believe overinflating a tire past sticker is not an unwise thing to do. I don't think one can fault somebody for being cautious of claims made over the internet especially when said claims have pros and cons that could be debated ad infinitum. This is not personal, I was hoping we could exchange thoughts about this in an adult way. Thanks to those who have done so thus far.

[Fat Charlie]

Originally Posted by iskyfly

Are there any vehicle engineers here who can say that overinflating a tire past sticker value doesnt;

-decrease safety
-increase expense because of faster tire wear

?

I'm trying to figure out what pressure to use in my Firestones on the old Explorer I just picked up. Knowing that the vehicle engineers are more concerned with safety than comfort and ride characteristics, I won't use the numbers on the actual tire as a guide but will stick only with the door sill sticker.

The only engineers involved that got to act as engineers were the ones who set the sidewall pressures. They aren't talking about how much room they left above that number, but you can be pretty certain that the actual limit isn't below that. The vehicle manufacturer came up with their numbers based on how they made the ride feel. That's focus group engineering. I'll worry about my own ride feel, okay?

-Tire wear is a function of the tire, not the car.
-Safety, is your concern loss of traction or blowout risk? If it's blowout, pay attention to the sidewall. If you're worried about traction, don't outdrive your tires. If a few PSI is the difference between a safe corner and a fatal accident, I'd point fingers at the driver and ignore everything else. You shouldn't be driving that close to the edge on public roads.

And yes, I work at a dealership and we treat that sticker and other factory specs like they're laws. That's not because they are, it's because we're representing the factory and that's our schtick.

[iskyfly]

[QUOTE=Fat Charlie;244256]-Tire wear is a function of the tire, not the car.

That doesn't negate the fact that over inflated tires increase tire wear. If our goal is to decrease the expense of operating a vehicle then how does replacing tires more often because of over inflation fit that objective?

-Safety, is your concern loss of traction or blowout risk?

Huh? Safety should be everybody's concern. Should we be put at risk of getting rear ended, run over by somebody who over inflated their tires?

[Fat Charlie]

Safety is everybody's concern, and tire safety here boils down to traction and blowouts. You kind of indirectly implied that you're concerned about traction loss. Anecdotally, I can say that my extra 5 PSI haven't turned the highway into a skating rink for me, even with some vicious rainstorms lately. I'll repeat that if softer tires make the difference for traction it's the driver's fault for driving at that ragged edge of performance in the first place.

[Hauki]

iskyfly, run your tyres at whatever PSI you feel comfortable with! Never ever over-inflate your tyres for better fuel efficiency, better of with a lighter wallet than dead!

Over-inflation and max PSI are two complete separate entities, you need to realise this. If someone runs at max psi, they are not running over-inflated, they are still within the designed tolerances of the tyre.

Also, I would suggest if your car becomes unstable if you deviate from the car manufactures recommended air pressure, then either a) it is a **** car or b) you have damaged components.

Running at whatever pressure the tyres are designed for, be it at 30 psi or 50 psi, then the car should still handle adequately.

An over-inflated tyre and an underinflated tyre will bother wear incorrectly, but especially the less air in the tyre, the quicker they wear out due to heat build up. Max psi usually gives you perfect wear right across the tread pattern. Running them soft usually causes feathering at the edge and at least causes them to taper off at the sides, which come wet weather, is much more dangerous if you ask me as you have less contact with the road.

But again, it's whatever YOUR comfortable with! YOU make the decision.

[Bill in Houston]

Originally Posted by Fat Charlie

Bill, that's the funniest thing I've read in a while.

Thank Yooo! I'll be here all week!

Originally Posted by iskyfly

But really didnt lend much to the discussion.

Instead of rolling your eyes, pay attention and think about it. Try again. I know you can do it.

[NHRABill]

Originally Posted by iskyfly

But really didnt lend much to the discussion.

I was under the impression that we are adults...



Are there any vehicle engineers here who can say that overinflating a tire past sticker value doesnt;

-decrease safety
-increase expense because of faster tire wear

?



I really would like to believe overinflating a tire past sticker is not an unwise thing to do. I don't think one can fault somebody for being cautious of claims made over the internet especially when said claims have pros and cons that could be debated ad infinitum. This is not personal, I was hoping we could exchange thoughts about this in an adult way. Thanks to those who have done so thus far.

don't even bother... some people just like to argue even if they know they are wrong.