Titanjose

I found this article titled "5 Driving Myths That Cost You Money" at yahoo finance. Take a look at #4. It confirms others' views (e.g. Magellan55, JT29) in this thread about overinflating tires.

http://financiallyfit.yahoo.com/fina...waad=ad0035&nc



4. Over-inflated tires -- $200 wasted (minimum)

The Theory: Rounder tires roll easier, creating less work for the engine and therefore, better MPG.

The Facts: Again, this tip is true ... to a point. Over inflated tires will have less friction with the road, which lessens the effort the engine exerts to keep the car rolling, providing slight gas savings. However, overinflated tires will wear out quickly and irregularly, causing you to need early replacements at a cost of about $50 to $100 per tire. What's worse is that the decreased contact with the road increases stopping distances and limits handling capabilities. This all adds up to a large risk in costly accidents and injuries. Even if you are lucky and avoid a collision, it would take a lifetime (which could very well be short if you're riding on bald and bulbous tires) for your fuel savings to negate the cost of four new tires. According to Edmunds.com's testing, the fuel consumption difference between driving with over-inflated tires and tires at the recommended pressure is negligible. Sometimes, despite what GM's recent track record suggests, carmakers do know what they're doing and the recommended settings and levels do provide the best results.

Pat8635

The key here is to define "overinflated" ... part of what goes into the psi from the manufacturer is ride comfort. I have been filling tires to close to max on sidewall for almost 30 years. I always check treadware and adjust, and usually get much longer life than what was promised by manufacturer. My 40k z rated tires on my last car lasted over 85k miles. On my hs I have settled on 44psi and tires look new and it gives me about 3 extra MPG the way I drive and still a nice ride. I have not tried 51psi which is what is on the tire, so really can't comment, but articles are written to apply to everyone and therefore are not really scientific and our personal results are more telling. So as long as we all post responsibility. Just my 5 cents worth.

noku

A car manufacturer's recommended tire pressure in my experience is on the low side as Pat8635 has suggested. To me this is the minimum pressure that would provide a safe driving experience so as not to deduct from the vehicles ride comfort too much. However, increased tire pressure will provide better mpg.

The tire manufacturer's sidewall pressure is the max fill amount which will provide a safe tire. I would never go above this level and one must allow for heat expansion. So depending on your driving habits (top speed, curves in the road) monitor as necessary.

The perception of a vehicles acceptable ride comfort is subjective so there's no perfect tire pressure on a given vehicle. This would change depending on vehicle load also.

Tire thread life is increased when having equal weight distribution across the entire thread width so as to ensure an even wearing tire. This would also increase braking performance.

Vehicle traction throws everything out the window as lower pressure is best. A low tire profile would increase your handling but decrease ride comfort because the sidewall height is reduced.

As I see it, there is no applicable general rule of thumb which can be used for every vehicle/driver/tire. Each component affects the other. I wish there was a computer program that we can plug our own/tire info in to determine a tire's optimum operating pressure and not some writer making generalizations which may not apply to you or your tire.

Pat8635

very well said Noku. Again, experiment! Change presure by 3 to 5 pounds for a few weeks and check your results...

Titanjose

I consider myself a bit ignorant about this issue and just trying to sort out the wealth of info i'm getting from this thread and 3rd party articles. I just want to increase my mpg by increasing tire pressure but don't want premature tire wear. I only have 2k miles on my HS and don't want to regret setting my psi so high that i would have to replace the tires at 10k miles.

For the sake of this thread, let's just define "overinflated" as the tire pressure above recommended settings that leads to premature & irregular tire wear. (I know that i'm neglecting braking distance, handling, comfort, noise, etc. and just focusing on premature tire wear and mpg)

Correct me if i'm wrong but here are the facts i've established:
1.) MPG increases as tire pressure increases.
2.) "Overinflated" tires cause premature tire wear.
3.) "Overinflated" tires cause irregular tire wear.

So in our case of HS stock tires on 18-inch rims, what tire pressure would start to be considered "overinflated"? With a minimum of 32psi and maximum of 51psi (32-51 range), would overinflated be anything above the median of 41.5 psi perhaps? What's your tire pressure of choice?

DDS62

The HS owner's manual recommends 33 psi front, 32 psi rear. IMO that is to high, but I would never lower the pressure below those values because of potential wheel damage. That is always a problem with the low profile (18") tires - when the tire pressure drops below design values, driving over a pot hole can damage the rim. Increasing the pressure will help prevent such damage and may increase mileage, but the ride is way to hard even at design pressures.

Magellan55

Pulled this from another thread I answered a while back, concerning how to judge the "right" inflation just considering tire wear and grip:

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Just be careful inflating your tires at/near max inflation. Tires are designed to provide maximum traction at a certain shape, dictated by load/pressure combinations. That shape is achieved at maximum pressure only at maximum load. Since the HS250 puts less than maximum load on each tire, even with a full load of passengers, filling it to 51 psi will give you less traction for braking and cornering. Plus there's the additional danger of overinflating the tires if your gauge is wrong.

For some quick math, each 18" tire's max load is 1356. Let's assume 50/50 weight distribution, since I don't know what it is on the HS, so that's a gross load capability of 5424 lbs @51 psi. But, the HS only weighs ~3700, plus 825 max load, making the gross weight 4525 lbs. That's about 83% of the tires' max capacity. A first order approximation of relation of pressure to load is linear, so the "right" pressure for the gross weight of the HS is 51psi * 83% = 42psi. If you typically drive alone, then it's more like 37 psi.

Not sure why Lexus recommends only 32 psi, other than they are trying to give a softer "luxurious" ride (especially on these low profile 18's). The tires on mine read 36-37 psi on the dash, and they came from the dealer that way (nitrogen fill, which I assume was a local dealer thing rather than from the factory). Personally, I've found that adding 5-6 psi over the recommended rating works best in general on other cars, from improved handling, more even wear, and even temperature across the tread (indicates the whole width is contacting evenly). So I plan to run mine in the upper 30's to maybe 40, but I haven't run a temp check yet so we'll see after she breaks in a bit more.

But IMO putting more than maybe 45 psi in these tires isn't worth the tiny improvement in gas savings compared to the safety risks as well as reduction in tread life.
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So, I run my tires about 38 psi now, and I'm getting towards the end of life on these tires. So far, with the exception of some wear on the outside of one tire that looks to be an alignment issue, the wear is pretty even across the tread. So maybe start there, and then experiment up and down a few psi to see what works for you.

noku

I don't know if any of you are old enough to remember tires of yore, 40+ years. During that time proper tire pressure was a lot more sensitive than most if not all of the current designs and tread composition. Just a few pounds over or under this desired pressure would gravely affect it's performance and tread wear. For me it wasn't at all unusual to get less than 10k on a tire which had even tread wear. Actually about 4k when running VW Beetle tires on the front of my '70 Chevelle although alignment was way out of spec. LOL......back then that funny car look was popular.

Most tires had nylon belts, then polyglass belts began to take hold. Later on we began seeing radial and steel belted tires. Now, only God knows what different manufacturer's use. Tire profiles has also changed a lot along with larger rim sizes. From 78's, to 70's, 65's, 60's to the present what looks to me must be no more larger than 35's. Common rim sizes of 12,13,14 inches were the rule of the day and nohow anyone could even imagine 22" rims like some cars have now. Of course tires were cheaper and you could get a recap for like $12. Now $300 and up tires are available as oem's. Along with all of this new technology, tires are able to properly operate at higher psi's.

In my ending days as a vehicle mechanic in the air guard about 15 years ago, every tire was filled to pressure that was embossed on the tire. So I know tires as a whole can easily handle such high psi's. I hate to recommend this as manufacturer's make boo boos and a defective tire is not all that uncommon. But oddly enough tire performance or thread life never seemed to be compromised and we had golf carts, aircraft aux. generators, sedans, wagons, small and large pickups, vans, stake trucks, buses, fuel tankers, almost anything and everything under the sun.

With this being said, the tires on my HS (18" wheels) at the moment are at 40 psi. Not that I think it can't hold more pressure and still perform well but to me the ride becomes just too rough and don't really like the extra shock my suspension is being submitted too. Here the condition of the roads you travel has a part in all of this. As always YMMV.

"Overinflated"? I get "irritated" far before that can happen.

Titanjose

Again, thanks for all of the info! It sounds like 38psi is a good number to start with.

Magellan55: You mentioned that you're towards the end of life on your tires. So what's the mileage (if you don't mind me asking)?

KGenaidy

I've owned my HS 19 months now and have about 17k miles on it. I am not trying to justify a position here ... just providing some "real world" results below.

I tend to drive w/a "hyper-miling" slant and I have kept my tire pressure at max sidewall since I bought the HS. The tires have worn even over this time period. Tread wear has been very good ... about 3/32" used so far.

DDS62

How about this device for extending mileage. http://www.gfchips.com/lexushs250h.aspx
Anybody know anything about it? What does it do? I suspect it simply negates the EPA emission requirements.

Magellan55

About 17k so far, and I'd say I have about 5K left, though I don't recall how much tread they had new. Probably not bad for W rated tires. Rotating them every 5K.

Magellan55

You have to ask yourself, with all the pressure to increase mpg and performance, why didn't the smart engineers at Lexus design this into the car in the first place?

Answer - you do give something up when you "chip" a car. Usually the first thing you give up is regular gas - all chips I've seen advance timing and require premium. And what's the point of a few mpg gained when you have to pay an extra $.30 per gallon? Others may cause emissions issues, or damage your catalytic converter over time. Some require other mods like free-flow exhaust or intake systems to function correctly.

The only chip I've thought was worth it was on a low-pressure turbo like the VW/Audi 1.8/2.0T. I did that on my old Passat, and the difference was huge. But even then, the cost was reduced life expectancy of the turbo. And in my case, the installation had a faulty part and I ended up stranded on a highway at night - the engine just shut down while I was doing 80. That cost me a new ECU ($1200), an 80 mile tow, and my family a night in a roadside motel No more aftermarket for me.

acheparev

This was posted in car chat section of the forum and everyone said it's a ripoff. You can read about it there.

DDS62

Everyone member claimed it is a rip-off except one person - the one guy that bought one and installed it - ?