15 more PSI = 7% better MPG

[qx4dude]

Originally Posted by Frank Lee

Unless we're in the middle of a gas price spike, MPG barely makes the Top Ten List.

Not sure about that, Frank. I'm pretty certain that MPG is #1 in the minds of ecomodder members, and most people looking to find ways to lower their operating costs. I went from a 17MPG SUV to a 45+MPG VW TDI wagon. I think a lot of people do the same math when considering a new car purchase.

Perhaps MPG isn't the #1 consideration, but it's easily in the top 3.

[qx4dude]

Originally Posted by cbaber

Which is easier, designing a suspension to work better, possibly requiring more expensive parts, or simply keeping the tire pressure low for similar effects? While a lot of consumers are concerned about fuel economy, a bigger factor is price. It's cheaper for companies to keep pressures in the mid range to promote a better ride than it is to develop better suspensions and possibly have to increase the price of the vehicle.

Using the Camry as an example, Toyota will sell over 400,000 of them in the US this year. On a mass-produced vehicle like that, how much would it really cost on a per-unit basis to make the suspension a little softer so that an extra 10 PSI goes unnoticed by the new car buyer? Manufacturers tweak suspension / ride setups all the time.

Again, assuming higher PSI = better FE, manufacturers choose to ship their cars from the factory with tires set at 32-36 PSI. A lot of people on this site have noticed a 10% improvement in FE, simply by adding more air to the tires. I'd suggest that manufacturers could easily do the same thing, but don't. I still think safety & liability are the reasons - but in the absence of data, we can only speculate.

[RedDevil]

One interesting example of the way manufacturers determine the numbers that go on the door post.
The Chevrolet Aveo 1.3D LT is one of the cheapest diesels you can buy over here, because it is so economical that it is excempt from luxury tax. By the barest of margins; but they made it, thats what counts.

Can you feel it coming? You should

Chevrolet recommends (for this model&over here) a tire pressure of 3.0 bar or 42 PSI...!
They call it the ECO mode; and that pressure was what the car was tested with.
They also advice a Comfort setting of 2.4 bar or 35 PSI.
A tire fitting company's website advises 2.1 bar (30 PSI). And please visit us soon for the next set
Even Americorean Iron grips the air pump when it suits them for whatever reason.

The test with the AMG Mercedes on skinny tires is fun to watch, but remember it could only be done in the wet.
The car has so much power and the total tire thread surface is so small that it would immediately overheat the tires when drifting in the dry.
It has very little to do with pressure or contact patch size. Needless to say, the heat gets spread as the whole thread will contact the road at each rotation.

Whether or not the tire can handle the heat depends on the size of the thread surface and the amount of power transferred to heat by friction during the power slides.
Space savers are no match for the hundreds of kilowatts the AMG can waste on them.

On our daily drivers we usually avoid the prolonged power slides. Our tires won't heat up like that. Nor would raising the pressure cause more heat because of the smaller contact patch; it does not change the total thread size (and if it does then it is just preventing the deeper flexing and friction from that at lower pressure). Nothing special.

Got a triatlon bike with 8-10 bar tires (110-140 PSI). Both the inner tube and tire itself are almost paper thin. The only reason that they can stand the pressure is that they are so narrow; the tube radius is very small, about a centimeter. As the stress on the tube increases linearly with radius, these tires actually endure less stress than car tires.

[mcrews]

Originally Posted by cbaber

Which is easier, designing a suspension to work better, possibly requiring more expensive parts, or simply keeping the tire pressure low for similar effects? While a lot of consumers are concerned about fuel economy, a bigger factor is price. It's cheaper for companies to keep pressures in the mid range to promote a better ride than it is to develop better suspensions and possibly have to increase the price of the vehicle.

e.

you beat me again!!!

Each part in the suspension can only serve a specific function. Tweaking one requires MAJOr tweaking of another. The tire function is (my quess) "softness'.

I had an infinitiq45 w/ electronic suspension....what a difference. never knew I was at 45psi.
do you know why they don't put it in every car?
too expensive for the mpg gain.
easier to have 32 psi in the tire.

People MAY claim to want better mpg......but not at the cost of a hash ride.
In fact, THAT mpg consumer is already giving u soooo much, at least they want a softer ride.

Not to be insulting, but you view is not at all consumer oriented. You don't 'get' who buys or why. remembers buyers buy on emotion at the end of the day.

[RedDevil]

Soft tires not only 'help' the suspension, they also hide what's wrong with it.
If your shocks are out you will lose grip if you raise the pressure in your tires. Because those shocks are unable to dampen them enough.
If your tires are soft you might never know, until you really needed them...

[gone7]

Originally Posted by RedDevil

There's no way in the world that adding 6 PSI is going to make the difference between easily spinning wheels and not. The rubber is the same (it does not turn into plastic), the power and weight of the car is the same; just the contact patch size is slightly smaller.
Friction has a direct relationship with the force (weight) applied, not with the size of the contact patch, unless that is covered by some gelly fluid or so; and even then, higher pressure will probably help grip, not hurt it.

Road surface otoh can make a big difference of course, if he let it roll through some grease no doubt the wheels will spin.

I found I got better grip with higher pressure, but the troll either did not read that or chose to ignore it. Ah well. Can't have a site like this and expect none of such come around.

You must be right. That's why dragsters use such high pressure in their tires.

[niky]

Originally Posted by qx4dude

Also,

3) Assuming higher PSI = better FE, why do most auto makers choose a door sticker PSI number approximately 30% less of max sidewall pressure.

-snip-

And don't say "it's a conspiracy".

Door stickers have been rising for years.

Unfortunately, as others have said, and has been shown, advantages level off after a certain inflation pressure.

Manufacturers aren't going to spend extra money on better shock damping to make up for extra-hard tires just to claim another half-mpg on the EPA.

Originally Posted by qx4dude

Nothing to do with inflation pressures (space savers are supposed to be inflated to between 60-90 psi).

And it's only partly to do with contact patch size and mostly to do with tire construction and compound. Space savers don't have the same gummy compound and stiff sidewalls as your typical low-profile sports sedan tire.

Hell... motorcycle tires would have been better.

Originally Posted by qx4dude

Perhaps MPG isn't the #1 consideration, but it's easily in the top 3.

Truck sales are booming. Honda Fit sales spiked briefly during the 08 spike, then crashed again.

Most people who are silly enough to buy new typically aren't concerned enough about gas prices to sacrifice (perceived) comfort for another few mpg.

We are. But then, we're not "most people".

Originally Posted by qx4dude

On a mass-produced vehicle like that, how much would it really cost on a per-unit basis to make the suspension a little softer so that an extra 10 PSI goes unnoticed by the new car buyer? Manufacturers tweak suspension / ride setups all the time.

That's actually the wrong way around. cbaber had it right.

A suspension designed around stiffer tires is more expensive. A softer suspension would just destroy wheel control and make the tires bounce, To run an extra 10 psi "comfortably", you will need more expensive, beefier shocks with stronger damping. Perhaps even two-way valving to better control wheel motion if you don't want to go with stiffer springs. (yes, stiffer, not softer)

Then you need firmer bushings, rubber isolated suspension subframes, etcetera. That builds up cost quickly. All simply to ensure people don't complain about the ride.

Just ask BMW about it... as they've had to do all of that to deal with their stupid runflats.

[gone7]

No one will ever win a liability case when they over-inflate their tires. Ever. It will simply not happen.

But of course some people still promote the use of higher than recommended pressures. Those people must believe that the whole professions of engineering and of automobile design is a false science. But then so would be all higher education.....

Oh, wait...... I get it now.

Never mind.

[cbaber]

Originally Posted by ACEV

No one will ever win a liability case when they over-inflate their tires. Ever. It will simply not happen.

But of course some people still promote the use of higher than recommended pressures. Those people must believe that the whole professions of engineering and of automobile design is a false science. But then so would be all higher education.....

Oh, wait...... I get it now.

Never mind.

I'm not arguing against science when I say higher pressures are fine. There hasn't been one piece of credible evidence posted that suggests over-inflating a tire can lead to the horrible things mentioned in this thread. Quoting tire information from a tire manufacturer is not science. I want to see the raw data.

You may choose to blindly accept factory recommendations, but I don't. Not when I have seen so many veteran members have success with over-inflation. As I have said many times already, I'll report my success and my failures.

[niky]

Originally Posted by ACEV

No one will ever win a liability case when they over-inflate their tires. Ever. It will simply not happen.

But of course some people still promote the use of higher than recommended pressures. Those people must believe that the whole professions of engineering and of automobile design is a false science. But then so would be all higher education.....

Oh, wait...... I get it now.

Never mind.

And how many multi-million dollar class action suits have been filed over over-inflation as opposed to under-inflation?

All safety studies have pointed to underinflation being dangerous.

All high-speed failure studies show increased chances of failure at lower pressures.

All hydroplaning tests show increased chances of hydroplaning at lower pressures.

The only data that suggests over-inflation is dangerous is one done by the NHTSA that shows increased bruising. But that was decades ago.


-

If you've actually been paying any attention to the arguments instead of raising your own strawmen to shoot down, none of us are saying anything like that.

We are simply pointing out that automotive design is a compromise. It's much cheaper to design your suspension around softer tire pressures than around higher tire pressures.

At this point, you are simply debating with an opponent who doesn't exist.

-

I suggest you read CapriRacer's post to understand what's wrong with the sidewall pressure. Unless you can explain why it's exactly the same for many disparate tire models and brands, you still don't comprehend what those numbers really mean.