By on August 9, 2016

1967 impala 05

TTAC Commentator Land Ark writes:

Hello again, Sajeev!

I’ve been wondering about tire pressure for a while. I changed the wheels and tires on my car which originally came with 14-inch wheels and high profile bias-ply tires. The new tires are 255/40/18.

How do I account for the drastically different sizes when it comes to determining the correct air pressure I should run in the new tires?

landark before 010001

I have the original sticker which says to run 24 psi up front and 28 in back. That seems low so I usually keep it around 33 in the front and 38 in the back. But I have no basis for these numbers other than what it says for my other cars. Is there any sort of guide to figure it out?

Keep up the great work!

 

Sajeev answers:

ZOMG.

That is a fantastic restomod, proving once again that the 1967 Impala is a seriously under-appreciated Bowtie.

No matter, your owner’s manual or body sticker is intended for application to bias ply tires, so it’s now irrelevant. Those shoes behave completely different than radial tires, reflected in their pressures. From my first car’s (’65 Galaxie) owners manual (24 psi all around, up to 36 psi rear on overstressed wagons with 8-ply tires) and some Googling, bias ply tires are rarely inflated over 30psi. But how many radial tires are inflated under 30psi, without pitfalls? But I digress…

Because tires are an integral part of suspension tuning, your bespoke restomod has no correct PSI found via Internet searching.

I know nothing of a ’67 Impala’s bias ply specific tuning (much less a suspension not 100 percent reconditioned?). So play suspension engineer with your new wheels: start at 32 psi all around.  Drive on bumpy, twisty and wet roads.  Then bump it up to 35 psi or down to 30 psi. Feel the difference in ride, in how quickly it breaks into under or oversteer, and how the steering feels at low and high speeds. If the higher or lower number feels better overall, try adding (or removing) 1 psi and see if it improves further. Spend a few days trying this out, perhaps let the tires cool overnight before changing the pressures.

The sweet spot for the vast majority of lower profile radial tires is between 30-35 psi. You can adjust front/rear pressures individually to fine tune under/oversteer, preferably off of public roads and on a road course, please!

Depending on road quality in your neck of the woods, I reckon your restomod is happiest at 32 psi all around. What say you, Best and Brightest?

[Images: LandArk]

Send your queries to [email protected]com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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43 Comments on “Piston Slap: Land Ark’s Radial Tire Pressure Bias?...”


  • avatar
    notapreppie

    Option 1: Optimize for even wear.

    Get some sidewalk chalk or white grease pencil and make a few broad stripes across the width of each tire.

    Drive around a little bit. Check the marks.

    Are they completely gone? Repeat with a shorter driving distance. If they disappear evenly across the tread after successively shorter distances, then you’re in the right range.

    Are they worn off more in the center than the edges? Reduce pressure and repeat.

    Are they worn off more at the edges than the center? Increase pressure and repeat.

    Are they worn more towards the inside or outside shoulders? Get your alignment checked.

    Depending on the tires, suspension, and other things, you may find a range of pressures that work. In that case, set the pressures in that range based on the type of driving you do.

    Slow, comfortable cruising? A little lower for more comfort.
    Faster, more exciting driving? Go higher to reduce sidewall roll-over.

    You can also do this using a pyrometer equipped with a needle probe (fancy expensive thermometer) but chalk or grease pencil is cheaper.

    • 0 avatar
      SCE to AUX

      This right here. Applying a little science is better than just using road feel.

      And that’s a beautiful car.

    • 0 avatar
      heavy handle

      That about covers it. The only additional detail is that lower pressures will give you more grip, which is counter-intuitive to some, and that front-rear balance is more important than ultimate grip.

      In practice, this means that you want to either lower the front pressure or raise the rear pressure (or both) if you have too much understeer. Lower the rear pressure and/or raise the front if you have too much oversteer, all the while staying within the range that gives you even tire wear.

      Tire pressure is a good way to make small handling adjustments. You will want to look at spring rates and sway bars if your car is way off.

    • 0 avatar
      Scoutdude

      While that method will get you in the ball park you must always check a load and inflation chart to ensure that the tire is capable of carrying the given load at that inflation pressure. You also want to be under the tire’s max inflation pressure. See below for the industry standard of matching tire pressure to vehicle weight and tire load index.

      • 0 avatar
        heavy handle

        Scoutdude, you are correct, I forgot about load range.
        You’ll usually be well within the range doing a small size change on a passenger car, but this guy is putting 18s where the factory put 14s, which is a huge change.

      • 0 avatar
        SCE to AUX

        @Scoutdude: Agreed on load capacity, although this 67 Impala weighs the same as a Kia Sportage. I’m sure the tires can handle it.

        • 0 avatar
          Scoutdude

          In this case the tires are more than capable of handling the load, in fact this would be a good application for a LL or light load tire. As it is the min inflation pressure is more than enough to hold up the front of the fully loaded car, since it is relatively light compared to many vehicles today.

  • avatar
    Joe Btfsplk

    You can drive through a puddle on a paved parking lot and check the wet pattern that the tire leaves on the dry. It should be the same all the way across.

  • avatar
    cdotson

    LandArk,

    Sweet ride! Much prefer the black/white VA Antique tags to the nasty yellow/blue ones.

    Sajeev’s right; bias-ply and radial tires behave completely differently and react to changes in air pressure differently as well. I am a mechanical engineer, and in a former job I did suspension design on light off-highway vehicles that used bias ply tires exclusively so I don’t know how much my personal experience is worth.

    In my personal vehicles I run the sidewall pressure from the tire manufacturer, typically 44psi. I prefer the sharper response and improved economy, plus I find if I run lower (i.e., recommended) pressures for a long period of time I suffer excessive shoulder tread wear. The one notable downside to running too high a pressure is an increase in the risk of a blowout when striking a sharp-edged pothole, a risk you already encounter due to your rim size and low-profile sidewall.

    Given your vehicle likely has a front weight bias or at best an even front/rear (unless you have multiple bodies in the trunk, that’s some rear overhang) and vehicle dynamic response is more predictable when it is understeer-oriented I would not run higher air pressure in the rear tires than in the front. Keep them even, or if you start lowering the pressure for improved ride do not drop the front tire pressure lower than the rear.

    • 0 avatar
      notapreppie

      In most cases, the pressure embossed on the tire sidewall the maximum recommended inflation pressure. This is based on the mechanical limits of the tire itself (with a significant margin for legal reasons) but does not take into considering anything about the car, wheel, driving habits or any other aspects that must be considered when setting pressures.

      Using that pressure in normal conditions will usually lead to decreased ride quality, decreased maximum grip and accelerated wear in the center of the tread.

    • 0 avatar
      bumpy ii

      I like the yellow plates better. I think of the black/white ones as being for really ancient cars like Model T’s and such. They just look super-wrong on a Miata (which I saw about six months ago).

    • 0 avatar
      Scoutdude

      The factory tire pressures were based on a fully loaded vehicle IE mom, dad and 3 or 4 kids and all their junk in the trunk for a trip to Wally World. In that case the vehicle would have a rear bias hence the higher pressure for the rear. Of course at this point in this car’s life it may see the bulk of its use with just one or two people in it an nothing more than the spare in the trunk.

      • 0 avatar
        cdotson

        Scoutdude,

        There is a problem with directly picking the tire pressure based on the pressure required to support a particular load beyond just determining the load and the appropriate curve for any given model tire. Dynamic stability required a modicum of understeer. When I developed suspensions the legal office that reviewed our testing always viewed more understeer as preferable to however much understeer your vehicle already displayed. These were utilitarian vehicles with limited total grip that carried loads near their total weight and often had rear weight bias. The legal office also required that front and rear tire pressures be the same as owners could not be relied upon to maintain differential tire pressures.

        We would have loved to specify a higher tire pressure in the front to achieve stable understeer. A lower front tire pressure with a rear heavy car is a recipe for dangerous oversteer. This is why I state, regardless of what load charts say, always run front tire pressure the same or higher than rear. Given that there’s no mention of stagger and an Impala such as this should easily swallow 255 wide tires all around the tires should all be the same and my pressure recommendation holds.

        The way we were able to get away with running equal pressure in a rear-heavy vehicle (without elevating front tire pressure) was running narrower front tires and installing a massive front anti-sway bar. At a later time when I had to increase tire size and make the front/rear width match again we had to run stiffer springs in the front to bring the understeer back.

        • 0 avatar
          Scoutdude

          My main point was that you should always make sure that your inflation pressure meets both the minimum acceptable pressure for that tire and has the weight capability needed for the load at that inflation pressure.

          Yes you can use tire pressures as one of the tools to adjust understeer ve oversteer. As you pointed out though other things have an effect too. Keep in mind that this Chevy was set up to still under steer with the factory staggered tire pressures and a light or full load. So the numbers generated by the load and inflation chart are a perfectly good, safe place to start. You can bet that Chevy used the chart that was current at the time when they selected the original tire pressures.

          Also note that many trucks are set up with staggerd tire pressure because of the potential for a heavy rear weight bias. For example my E-series placard says 60 psi front and 80 psi rear, and you’ll find similar recommendations for most 250/2500 and higher series trucks. Note since I never put the full 3000lb load in my van I run 60 psi front and rear and if I do find that I’m going to stick a pallet of block or shingles in it I’ll pump them up to 80 psi before I load up and then drop it back down once I’m done.

  • avatar
    JimZ

    good on ya for getting rid of that bias-ply junk. the advice already given is sound, just don’t exceed the number embossed on the tire. that’s a maximum pressure the tire manufacturer deems safe.

  • avatar
    87 Morgan

    Fantastic! I love a clean old ride. I keep my PSI at 32 all around on my 57′. I have 19’s outback and 18’s up front.

    Any chance you can submit a readers ride or something? I would love to know more about the car. I love the back story on the work, what was done so on and so forth. Their is just something about cruising that era land barges. They are not fast, though you can make reasonably quick in a straight line, they sound good too. Not every story has to end with…I got a Miata.

  • avatar
    Kenmore

    Sorry, but those 18s are a travesty on that beauty. As if the Liz Taylor of 1967 had neck tats.

    • 0 avatar
      energetik9

      I don’t really know anything about this car. I wasn’t even alive when it was made, but it looks massive. I like the wheels though. It just seems to give the car a little more energy and the style is right for this car. Just my opinion.

      At least they aren’t 22″ wire rims.

      • 0 avatar
        Kenmore

        Guess it’s just an age thing. I’m old enough to have grown up firmly believing that our great nation could erase ghettos, not the other way around.

        • 0 avatar
          87 Morgan

          **Mental Head Slap Occurs**

          What possibly could his whee and tire choice have to do with a ghetto?

          But, you are correct it is an age thing. I grew up firmly knowing that old racist white guys are literally a dying breed.

    • 0 avatar
      JimZ

      you couldn’t even comprehend how much better that car handles with those “travesties” than the bias-ply junk it was originally shod with.

      You and your ilk would do well to understand that a lot of the past is best left in the past.

      • 0 avatar
        Kenmore

        Can’t get 14″ or 15″ radials anymore?

        It’s the garish, glaring bling of the wheels size, the mongo-like-shiny degradation that offends against the original clean & elegant design language.

  • avatar
    -Nate

    Sharp looking short there ! .
    .
    Agreed , chalking the tires as in post # 1 is the very best way to go .
    .
    -Nate

  • avatar
    Paragon

    Here’s my Rule of Thumb: no less than 32 psi for all tires. I always run them at 34-36psi, or even 38-40 with no problems. Unless you are really diligent about regularly checking your air pressure, keep them ABOVE 32psi.

  • avatar
    dukeisduke

    Nice Impala. Do I see the optional running lights on the front? I always thought those were cool.

  • avatar
    rpol35

    As the owner of a ’68 Impala that has gone through some changes I can tell you that Sajeev’s advice is right on. The 24/28 PSI sticker references the Imp’s original 825-14 bias ply tires. The correct new pressure (cold) should be between 30 & 35; just sample until you get the right all around feel, i.e. ride, handling, on-track feel, etc.

  • avatar
    Scoutdude

    Did Sanjeev write this article? Because it is not up to the usual standards of Sajeev.

    There is science behind what the proper pressure to run in a given tire based on the load it is carrying. It is an industry agreed upon standard and there is a thing known as a load and inflation table to look it up. Now most tire mfgs have their own version but since it is a standard the meat of each one is the same.

    This one has nice and simple instructions, if you are replacing tires that you know the load index of. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwiR4dH2xLTOAhUL4mMKHRzPDMwQFggrMAI&url=http%3A%2F%2Fstatic1.1.sqspcdn.com%2Fstatic%2Ff%2F589830%2F23447320%2F1378330097907%2FDiscount%2BTire%2Binflation.pdf&usg=AFQjCNHE-QkXUFUiynSb0f6KGxq_7IwdbQ though you can also just find the axle weight rating and check the load index of the tires you have installed and work from there.

    Here is another option that allows you to look things up based on size. https://toyotires2-1524598101.netdna-ssl.com/assets/lib:toyo%20content/Application_of_Load_Inflation_Tables_20151020.pdf

    On that chart you’ll notice that there are 2 different load indexes for a P255/40-18 Standard Load (SL) and Light Load (LL) and some sizes are avalable in extra load (XL). There are also different numbers of hard metric tires, ie 255/40-18 (no P).

    Preferably you would want to take the car to the local truck scale that can weigh by the axle but in the mean time we will do a little estimating. What I find shows a V8 auto sport coupe tipped the scale at about 3800lbs, add in the typical 6 x 150lbs for passengers and a 300lb luggage allowance and we are looking at a potential 5000lb load. Because those factory pressures were staggared with a higher pressure in the rear I’m going to say the full load distribution is 40% F 60% rear. So we need each front tire to hold up 1000lbs and each rear to hold up 1250lbs. Assuming you have a P255/40-18 SL with a load index of 95 we follow the cart accross and find at 26psi, the min recommended for a P-metric tire it can hold 1168lbs, so our front pressure should be 26psi. Continuing on across the chart we come to 1268lbs which is good for the rear and we find that is in the 29psi column.

    So front 26 psi, rear 29 psi for full load conditions. Of course most people don’t run around with a full load all of the time. So if the car usually sees just a driver or driver and single passenger with no more than a bag of groceries out back I’d run 26 psi all around with the caveat to bump those rears up to 29 with a full load.

  • avatar
    Scoutdude

    Ok for some reason that didn’t seem to post right and it won’t let me edit.

    I did want to add that some times the Light Load tire is the right choice. Those have came about because of the trend to put such large tires on vehicles today. That sometimes causes the car to be over tired such that the 26psi min inflation is significantly more than the load on the tire. Using a LL tire can thus alleviate the problem of the lowest inflation pressure resulting in tire wear consistent with an over inflation condition.

  • avatar
    JohnTaurus_3.0_AX4N

    Land Ark,

    Very nice! Did you restore it yourself? You should write an article on it, the site can call it “B&B Rides: Land Ark’s Impala”.

  • avatar
    olddavid

    I am up a size in width and diameter on most of our cars, yet I stick to 34 front and 32 back (empty) with 34 loaded. I get even wear, except when showing off my power-brake technique.

  • avatar
    raph

    Personally I’d get a baseline pressure for the tires that allowed them to comfortable carry whatever the operating weight of the vehicle. Get the car scaled then add weight for a full tank of gas and whatever you normally carry on board plus a passenger ( as an example ) then its,just a,matter of looking up the pressure required to support that in the manufacturer’s load and inflation tables ( check a tire dealer ). You can then tune the ride from there in in whatever manner you wish by whatever means.

  • avatar

    I say that is one freaking AWESOME ’67 Impala! Underappreciated indeed.

  • avatar
    JuniperBug

    The sticker on my car says 26 PSI, and even on the track lots of people keep them below 30 PSI, including myself. More pressure than that doesn’t make it more “responsive,” it just makes it ride and grip like crap.

    Not all cars are Ford Explorers.

  • avatar
    mik101

    Sajeev didn’t cover tire wear in the article. Longer term if the vehicle is aligned correctly you will see how the pressure affects how your tread wears. If it is eating the edges, you’ll probably need to bump it up some more. If you find it wearing mostly in the middle then the pressure is set too high (which is a trick used by hyper-milers to increase fuel economy by lowering rolling resistance at the detriment to ride and handling). I’m really surprised that was left out.
    Manufactures find a compromise between ride, wear and handling following the tire manufacturer load and inflation ratings as mentioned in the comments above.

    For example the ratings on my Fiesta vary greatly between different markets depending on roads, expected carrying load etc. The European model even carries different ratings for how many people are in the car. In North America they only give one rating in the manual that falls in between both euro specs.

    That is definitely a nice restoration. Cheers.

  • avatar
    Land Ark

    Sorry for not replying yesterday, I’m in Las Vegas for a wedding and not staring at my work computer as usual.
    I will give the first suggestion a try. My main concern is for tire wear since I don’t drive it in any real spirited manner. Unfortunately at this point I rarely drive it in general.

    Thanks for all the comments. I realize it isn’t to everyone’s tastes but few cars are.

    the cliffs notes version is it’s been in the family sine the early 70s and I got it in 93 as seen above. I wanted to restore it instead of buying a new car as my first car. I drove to through out high school and it then sat until I bought my house in 09. Since then I’ve sloooooowly been doing more to it including the suspension, brakes, and trim parts.

    I might write about it in the forums but no one will ever see it.


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