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Old 04-16-2009, 08:20 PM   #1 (permalink)
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Tires question

Hey, I have a question (obviously) I have a Dodge Neon that has 3 different tires on it that are probably all pretty old but still have tread on them, One of them is was a free tire that I got from this guy because of when I got kid napped and the guy wrecked my car and messed up the whole right side of the car, that free tire I got replacing the broken one leaks air and frequently goes to about 20psi and the others have a max of 35psi.
Now here is the problem: I have some BRAND NEW tires that I payed over $400 for, for my old car (88 Fiero) these tires have a max of 41psi and are perfect but they are for 15 inch rims and not the 14s I have on the Neon, So if I traded them out I would have to keep the Fieros rims with the tires which I think would be heavier because the tire is thicker and just overall bigger.
Should I switch to the Fiero tires and keep them at 41psi or should I keep the probably lighter,thinner crap tires and just keep paying to put air in them? Sorry for the long question....

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Old 04-16-2009, 08:31 PM   #2 (permalink)
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1.) go for the bigger tire and wheel. It will increase your final drive ratio. So you car will claim its going slower than it actually is. So at 3000 rpm you will be going slightly faster than you would with the 14 inch tires.

2.) safer

3.) the higher pressure will allow better FE due to less rolling resistance and it will be more constant and not risk blowing a tire because it gets low(safety again) but also if it has constant pressure its more RR and FE frinedly
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Old 04-16-2009, 09:22 PM   #3 (permalink)
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Noob, you don't have to obey that 41 PSI sidewall max like a zombie. Search around on the forum, you will find that BRAND NEW tires are the best time to start to pump 'em up. You can generally safely pump them up to 50 PSI, which is a very good point where both relative safety and FE are balanced perfectly. After 50 PSI you will experience little real benefit and a LOT of weird glances at filling stations.

The NVH (Noise, Vibration, Harshness) from the pumped up tires will be somewhat higher than the regular 'normal' settings, but not significantly. You will also be better off in the handling and tread wear department with the pumped up tires.

In short, there is a lot of benefit tires kept at 50 PSI, and should you start doing it on NEW tires, it will ensure your tread wear will be uniform and significantly low.
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Old 04-16-2009, 09:32 PM   #4 (permalink)
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I would always be hesitant to go over the rated max pressure. Lots of people here do it, but you have to ask one very important question, is it worth it?

Yeah lots of people on here bump 50 psi and get great mileage. But unless you have the exact same set of tires that you stole from their car there are no guarantees they will be the same.

Industrial produced goods have quality control but there are alot of factors that can cause serious problems in the high pressure, temperature range for rubber and valves. The max rating is what they suspect is safe even if something goes wrong. i.e. your tire has a section of rubber that has a different composition and wears fast and can't take pressure. At 41 PSI their tests of intentionally poorly made tires deflated slowly rather than bursting or deflating rapidly, wore out and then burst. Setting to the max still allows for the engineers to incorporate a safety factor. Higher than that and there are no promises, also your insurance company has the ability to not cover you if you are injured in some form of accident relating to the tire malfunctioning and you wrecking.
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Old 04-17-2009, 07:13 AM   #5 (permalink)
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Quote:
Originally Posted by theunchosen View Post
...unless you have the exact same set of tires that you stole from their car there are no guarantees they will be the same.
ANY set of tyres will coast easier at higher pressures than the SAME tyre at lower pressure, upto ~50 PSI.
Quote:
Originally Posted by theunchosen View Post
...The max rating is what they suspect is safe even if something goes wrong. i.e. your tire has a section of rubber that has a different composition and wears fast and can't take pressure.
Kinda vague, isn't it?
Quote:
Originally Posted by theunchosen View Post
At 41 PSI their tests of intentionally poorly made tires deflated slowly rather than bursting or deflating rapidly, wore out and then burst.
What is the study being quoted here?
Quote:
Originally Posted by theunchosen View Post
Setting to the max still allows for the engineers to incorporate a safety factor. Higher than that and there are no promises, also your insurance company has the ability to not cover you if you are injured in some form of accident relating to the tire malfunctioning and you wrecking.
In spite of every care, tires still get recalled. There is no silver bullet here. But as per the feedback on the very same forum, several members have got a good tread life and no safety incidents by pumping up to 50 PSI. As in any case where you exceed manufacturer specification, it is important to avoid stupidity and be safe and scrupulous when driving.
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Old 04-17-2009, 08:55 AM   #6 (permalink)
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I was not referring to LRR or FE at the outset. I was referring to the potential safety. Any process that involves any form of polymers are substantially more variable than ones that involve metals. Metals its relatively easy to keep it moving along at a brisk pace and avoid accidently making the frame too thin anywhere or having disproportionate concentrations. Polymers its not so easy because its a chemical reaction. That said some reactions are pretty easily metered(nylon), but even there sometimes the line snaps at the reaction point because the two reactants either wanted to react too fast too slow or in a different way(the latter is very uncommon). Rubber is kind of like this but not as easily measured(nylon breaks thread when something is amiss).

Not all things involve funded studies. This falls under the category of quality control. Its like most steel companies. They intentionally don't go get a perfect slab to do stress testing on. To avoid being on the receiving end of a law suit they use a slab thats like any other slab and in some cases use steel that is slightly flawed to ensure that the average will always perform above the test conditions(I had a friend who worked for E-Steel and this is how they demanded their specimens were measured). Its an added layer of our safety factor.

Yes I am entirely aware there is alot of feedback about tire pressure. I am also aware that you are the only one to advocate to jack straight to 50 PSI without slowly moving forward and closely monitoring wear on the tire. If anyone else has done it I didn't catch it. Also Ecomodder is not really even big enough for a case study, no offense at all, one day we might be but I am pretty confident that we have less than 2K members.

The last factor is I'm an engineering student and if an engineer tells me not to exceed this number I'm not likely going to do it. True most of the time you can go pretty far above that number and be fine, but some days its not true and then your tire bursts at 60 mph. You should try having your tires go away at 60+ and then you'll probably be back here with me at max rated pressure.
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Old 04-17-2009, 09:21 AM   #7 (permalink)
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Very likely your best option is to sell all your current tires and also the Fiero wheels, and get new tires (rated 41 psi or higher) that are the right size for your Neon. More notes on that further down below.

People, people... With all due respect...

The question is not so much about the relative merits of higher pressure, but which tires + wheels should HypermilingNoob use on his Neon? You've seen me discuss high pressure tire use here plenty, but that's not the biggest issue here.

First item to address:
What is the stock tire size and outer diameter for each of these cars?
HypermilingNoob knows they are 14" and 15", but that's only the rim diameter.

Tire outer diameter is what "drives" the final drive ratio (I use the term loosely) and is also what keeps your speedometer/odometer accurate. A tire of larger outside diameter will of course give you lower rpms HOWEVER they may not fit your wheel wells, especially in curves or on bumps when the springs compress. And, personally, I prefer to stay with stock diameter to keep the speedometer/odometer accurate. True that you can use GPS as a substitute but only if you have one, but HypermilingNoob may not have one.

Anyway, find the tire sizes imprinted on the various tires you have. If the Neon's four 14" tires are NOT all the same size that's a good reason to ditch any that aren't a match. You should have matched pairs front and rear, at least. Most would say that all four should be same make/model, and of course should be same size.

Knowing the sizes of all your tires, use this calculator to see if the outer diameters are the same:
Tire Size Calculator
Likely the Fiero tires aren't the same outer diameter as the Neon but go have a look.

Your Fiero wheels may or may not fit the Neon. Use TireRack.com as a resource to figure out wheel fit specs. If you can't make sense of it, call their 800 number and get them to tell you. Best option for the Fiero wheels/tires is to sell them to a Fiero driver. Craigslist.

Here's another tip: Even if the Fiero 15" tires have the correct outer diameter they may be wider than the Neon spec size. If so they might still fit in the wheel wells but maybe not. Further, if wider they likely will be heavier. Heavier tires hurt FE, all other things being equal (which of course they never are). You can use the "specs" references at TireRack.com to learn about tire weights. Find a specific make/model in your size and find the weight. You can get an idea of how weight changes for different size tires by scanning the chart.
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Last edited by brucepick; 04-17-2009 at 09:26 AM..
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Old 04-17-2009, 09:44 AM   #8 (permalink)
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I think he has the fiero in wheel with mounted tire. at least thats how I took it. In which case slap them on and measure the gap between the tire and the well and make sure the alignment is still good. If those are all clean then use the larger.

My speedo is off but I just do the conversion. . .its really not that difficult once you do it you can remember most of the important ones.

If the tire is the same profile size and the wheel is larger he gets a larger final drive, which I suspect would just about have to be the case because he did not specify either were wide-wall tires or low profiles so they are probably both hovering around the 55-60 range.

That said if the tires are not already mounted Bruce has a point and I defer to him and then to the first paragraph if they fit.
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Old 04-17-2009, 11:33 AM   #9 (permalink)
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If the tire fits, wear it. I've never liked running on old mis-matched tires, just doesn't sit well with me in the safety department. Ditch the mixed 14" tires, but keep the rims. Run the 15" set until you find a better more suitable 14" set of tires. I'm assuming the fiero has wider tires than your neon, so that'll hurt, but the taller tire may offset to put you back where you started.

to address the air pressure statement, the sidewall max is rated at the tire's weight max, something I doubt most people run within 70% of. Do contact patch tests first however, before jacking the pressure above and beyond. All vehicles are different, the blanket statement of "pump those to 50psi" doesn't fit.


Safety. New tires are the reason you should change, regardless if they drop your mileage by 1-2. Safety of you, your passengers, and others on the road. Never sacrifice safety for better mileage.
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Old 04-17-2009, 11:44 AM   #10 (permalink)
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Quote:
Originally Posted by theunchosen View Post
1.) go for the bigger tire and wheel. It will increase your final drive ratio. So you car will claim its going slower than it actually is. So at 3000 rpm you will be going slightly faster than you would with the 14 inch tires.

2.) safer

3.) the higher pressure will allow better FE due to less rolling resistance and it will be more constant and not risk blowing a tire because it gets low(safety again) but also if it has constant pressure its more RR and FE frinedly
Quote:
Originally Posted by almightybmw View Post
If the tire fits, wear it. I've never liked running on old mis-matched tires, just doesn't sit well with me in the safety department. Ditch the mixed 14" tires, but keep the rims. Run the 15" set until you find a better more suitable 14" set of tires. I'm assuming the fiero has wider tires than your neon, so that'll hurt, but the taller tire may offset to put you back where you started.

to address the air pressure statement, the sidewall max is rated at the tire's weight max, something I doubt most people run within 70% of. Do contact patch tests first however, before jacking the pressure above and beyond. All vehicles are different, the blanket statement of "pump those to 50psi" doesn't fit.


Safety. New tires are the reason you should change, regardless if they drop your mileage by 1-2. Safety of you, your passengers, and others on the road. Never sacrifice safety for better mileage
.
My max tire weight is actually pretty close its rated for a total of 4000 lbs(car is 2050 plus me(2190)) and after a dip in the road at 60-70 its going to get very close to being at max loading for an instant but its not static loading either. Impact loading is much more stressful than static. The max load weight will also have a safety factor incorporated but it won't be a very large one because not many 14 inch wheel vehicles weigh over 3Kips.

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