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Check your rear tires air pressure. If they have to much air in them it'll cause it to spin easier. )</font>
What is the recommend air pressure for the rear tires? I have them at 20psi right now, and the fronts at 30 psi. )</font>
That depends on tire type.... Radials differ from bias ply.
Industrials differ from AG, and from turf...
I run from 10 to 12 PSI in rears on my tractors. (20 sounds high for any type to me) I have a couple tractors with radials and a few with bias ply (All AG tread) The one 4WD has 16 in the fronts. Most any tractor tires I'm familiar with will wear out in the middle if you run too much air....
Also, look on the tire sidewall. There's usually a recomendation from the manufacturer.
When plowing, both moldboard or chisel, I usually let the pressure down to 8 or 9 PSI (on radials) and 9 to 11 PSI (on bias ply). It doesn't hurt to see the sidewall "wrinkle" a bit under that sort of pull. (Radials SHOULD wrinkle under load or pull)