Thread: Tire air pressure

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I asked this a while back, and if I recall, most said to start at ~28. I did, have but a bunch of miles on the car and----lumber wagon. Not bad on good roads, but since the most local communities with the exception of the town I live in, have no money, which means roads aren't being repaired. I've wandered (search only) a few other sites and folks go all over the place. I'm gong to try 24/22 (front-rear) but will welcome other suggestions.

Originally Posted by IC2

I asked this a while back, and if I recall, most said to start at ~28. I did, have but a bunch of miles on the car and----lumber wagon. Not bad on good roads, but since the most local communities with the exception of the town I live in, have no money, which roads aren't being repaired. I've wandered (search only) a few other sites and folks go all over the place. I'm gong to try 24/22 (front-rear) but will welcome other suggestions.

Dave,
I have not done in on mine yet, but for the wide rear tires the best way to find the ideal pressure for ride and wear is to find big deserted parking lot or stretch of road where you can put a heavy chalk line across the tread, drive across the lot and see where the chalk is wearing off. Start high, like 28psig, chalk the tires and you'll see that it wears off in a center stripe (overinflated). Re-chalk, drop the pressure to 26 and drive across the lot again. Repeat until you're wearing the chalk off across the tread pattern, but not to the point that you leave a band of chalk in the middle (underinflated). I expect that you'll find 24 psig rear about right, but it may even be a bit lower. Harder to do on my 5.60x15 fronts as they're so narrow, so I'll likely follow your approach and run a couple of pounds higher than the rears. My Jeep runs 25psig on 33x12.50's and they wear right, and ride much better than with the 35 to 40psig that the tire store puts in them.

Ahhhhhh-----tire pressure------bigs and littles, skinny and wides, road race, drag race, handling(cornering)braking(fronts or rears lock up)drag racing, bouncing like a basketball-----where to start and where to stop----

Roger - not a bad way to check/fiddle by using the chalk. Unfortunately I'm genetically lazy and was looking for a cookbook recipe. Also, not sure the rears even need air as they are heavy tires.

Jerry - I hear you. Since I don't race this car am just looking for a comfort factor - and maybe some handling with my new springs (which do appear correct for the application vs. what was on there)

I run 24 in the fronts and 20 in the rears. That seems to work well for me. I will have to try Roger's chalk test to see how close I am.

I tend to run what the tire manufacture says as I want them to last . Better ride = more pading in the seat

Originally Posted by cffisher

I tend to run what the tire manufacture says as I want them to last . Better ride = more pading in the seat

Charlie,
Actually the tire manufacture's recommendation is based on an assumed vehicle weight, and if you're running wider tires you will be severely over-inflated with his numbers, and you will wear out the middle of the tires quickly. The chalk trick gets you to the point where you have full contact with the pavement, which will maximize your tire life, and will feel better to boot. I learned it with Jeeps - big meats on a relatively light vehicle.

Originally Posted by cffisher

I tend to run what the tire manufacture says as I want them to last . Better ride = more pading in the seat

Now, let's see - the rears, 265-60x17 max pressure is 44 psig (Dunlop SP5000)
Then, the 205-60x15, max pressure is 51 psig (Yokohama AVID)
The OEM 4.75x19 for a '31 Ford - 35 psig

Naaaaah. Big, heavy tires and at 22-24 psig and on a 24-2500 pound car, they will last a loooooong time.

I'll either do a guesstimate or use Roger's good idea.

OK my car Factory wt 3200# Tires are MT 29x15.5x15 20#s A/P at 1425# max weight on each side in rear. I tried the chalk thing and at 20 psi they are flat on the concrete. I think they'll last awhile ,Not a long time probably just awhile.

Originally Posted by cffisher

OK my car Factory wt 3200# Tires are MT 29x15.5x15 20#s A/P at 1425# max weight on each side in rear. I tried the chalk thing and at 20 psi they are flat on the concrete. I think they'll last awhile ,Not a long time probably just awhile.

Charlie,
With that size tire and 3200# 20psig is probably about right, and I would think that they will wear right. The wear factor will probably be affected more by a heavy right foot coming off the stop lights.....

I run 18 psi in the rear of the Cobra, and 22 psi in the front.
Wear is even, but...for every three sets I replace on the rear, I've replaced only two sets on the front.
Dunno what the cause of that is...both front and rear travel the same distance.

Originally Posted by johnboy

I run 18 psi in the rear of the Cobra, and 22 psi in the front.
Wear is even, but...for every three sets I replace on the rear, I've replaced only two sets on the front.
Dunno what the cause of that is...both front and rear travel the same distance.

I'm thinking the rears might get there a little sooner.

low pressure can ride better but can also be a handful if you get out of shape at speed.

Originally Posted by shine

low pressure can ride better but can also be a handful if you get out of shape at speed.

I agree.
And 18 psi and 22 psi are low pressures.
But the car is a light car; when it's full of fuel, oil, water etc, it still weighs only 2315 lbs...1050 kgs.
And it handles extremely well, even when it's sideways.
Which is not that often!

It took a lot of trial and error to come up with those pressures; I wish I'd known rspears trick with the chalk then...that's a goodun, so easy and effective.

Originally Posted by shine

low pressure can ride better but can also be a handful if you get out of shape at speed.

Shine, I agree 100% that underinflated tires are not good, but the proper inflation is a function of weight, tire cross section and tire contact pattern. For the best performance, wear and comfort the tire will have a full contact pattern across the cross section. More pressure will crown the tire, putting too much pressure in the center and lifting the sidewalls slightly, while too little will lift the center, and wear the edges. Neither will provide good handling. The perfect pressure for the street will have the installed tire sitting exactly flat with fuel load & driver. I got 40K miles from a set of BFG 32x11.5x15's on a Jeep using the chalk trick to find the right pressures, and then rotating & balancing them every 6000miles.