How to calculate street pressure for larger tires.

[Toy-Roverlander]

This is exactly how I've always done it. I've done 30k miles more or less on a set of second hand 315-75-16 Mickey MTZ's. Tread wear was very even along its width and it wore of about .2" of the tread. Not bad at all. Used the same method with my new Yokohama's.

Here is what I do to get a BASELINE for the chalk test.

You need to know your front and rear axle weights. No one is running the same pressure front and rear, right?

Example:

5000 lb vehicle with 60/40 weight distribution
3000 lb front axle or 1500 lb per tire
2000 lb rear axle or 1000 lb per tire

Each tire is rated at 3050 lb max load @ 80 PSI
3050/80= 38 so each 1 lb of air supports 38 lbs (real world is not linear but this works here)

So each front tire supports 1500 lbs, 1500/38 = 39.5 psi
Since each rear tire supports 1000 lbs, 1000/38 = 26 psi

This gets you a BASELINE for the chalk test. Test and adjust accordingly.

I have never run less then 4 "extra" lbs in the front on any vehicle I have owned.

https://www.google.com/url?sa=t&rct...58MQt4&usg=AFQjCNGRwja02oW0vd6qI4xjh5KZTiMGWw

 

[e60ral]

That method is close, but not correct

Here is the correct way to do it:

If the customer does not have or is not putting the OE tire size on the vehicle, determine air pressure by doing the following:
1. Use vehicle placard or Tire Guide to find load index and air pressure of OE tire.
2. Select the correct inflation table and find the load carrying capacity of the OE tire.
3. On the replacement tire you recommended, find the load index (use the store system or the tire itself).
4. Use the inflation table that corresponds to the replacement tire's load range. On the selected inflation table:

X Find the replacement tire's load index line.
X Move across that row until you find the load that is equal to or just greater than the OE load recommendation.
X The recommended air pressure is at the top of that column.​

https://static1.1.sqspcdn.com/static/f/589830/23447320/1378330097907/Discount+Tire+inflation.pdf

for your load requirement, 4 corner weigh or get axle weights fully loaded for your vehicle if you have modified it

 

[Meili]

E60ral, "the "divide load by psi" method is wrong (its close, but incorrect because the load-inflation intercept is not at 0)"

I wont say "my" way is wrong, served me well for 35 years, but agree with the second part of your statement. I basically said the same thing, see below.

Reread my post,"(real world is not linear but this works here)"

I also put made BASELINE bold and in caps for a reason, it's a place to start.

Is that the old Firestone chart?

 

[e60ral]

Yes, the pound/psi is wrong because while it is fairly linear it does not have zero intercept. Just pound/psi gives you a y=mx equation but a better approximation would be y=mx+b and there is no reason to even bother with that when load charts are standard.

That's not the old Firestone chart. Load ratings are standard, charts will look similar because of thay. If a tire is E rated with a certain load rating it will have the same load rating per psi as any other manufacturer for a tire with the same rating

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