Piston Slap: Land Ark's Radial Tire Pressure Bias?

Sajeev Mehta
by Sajeev Mehta

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?

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 sajeev@thetruthaboutcars.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.

Sajeev Mehta
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  • Land Ark Land Ark on Aug 10, 2016

    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.

    • JohnTaurus JohnTaurus on Aug 12, 2016

      That's a shame (the last part of the last line). I'm sure you're proud of it, and I for one would like to hear more.

  • TheTireWhisperer TheTireWhisperer on Feb 23, 2024

    First of all, why such huge wheels?


    Which ever size tire you decide to run, when you don't know what pressure to inflate the tires to, you need to locate, somehow, the GAWR(gross axle weight) for the car.


    You are driving a car that might not have a sticker with such info on it, and as you said, the tire pressures listed on the car are for the original late 1960s era bias-ply tires that came on it.


    Once you know the front and rear gross axle weights for it, divide those weights in two, to get the gross weight per corner.


    You chose tire size 255-40R18. In the tables on a site such as tirepressure . org, you want to select a pressure that supports a maximum weight about 10 percent higher than the GAWR/2.


    On my 2010 Honda Accord, the front(heavier) gross axle is 2,403 lbs, or, 1,202lb per side. According to the tables I referenced, the OEM tire size 225-50R17, at 32psi, supports a gross - MAX - load that is, of 1,353lbs, or about 11% over. Perfect.


    You need to plug in the axle weights for your Impala, and consult the standardized tire & load tables for that optimal pressure.

  • 1995 SC PA is concerning, but if it spent most of its life elsewhere and was someone's baby up there and isn't rusty it seems fairly priced.
  • CanadaCraig I don't see ANY large 'cheap' cars on the market. And I'm saying there should be.
  • 1995 SC I never cared for the fins and over the top bodies on these, but man give me that interior all day. I love it
  • 1995 SC Modern 4 door sedans stink. The roofline on them is such that it wrecks both the back seat and trunk access in most models. Watch someone try to get their kid into a car seat in the back of a modern sedan. Then watch them try to get the stroller into the mail slot t of a trunk opening. I would happily trade the 2 MPG at highway speed that shape may be giving me for trunk and rear seat accessibility of the sedans before this stupidity took over. I ask you, back in the day when Sedans were king, would any of them with the compromises of modern sedans have sold well? So why do we expect them to sell today? Make them usable for the target audience again and just maybe people will buy them. Keep them just as they are and they'll keep buying crossovers which might be the point.
  • Kwik_Shift_Pro4X As much problems as I had with my '96 Chevy Impala SS.....I would love to try one again. I've seen a Dark Cherry Metallic one today and it looked great.
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