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Increasing the tire diameter by more than 3% is dangerous because it overloads the brakes and can cause brake failure. Just because everyone does it, doesn't make it right. As you can see by the comparison chart, you at over the safe limit as far as tire size.

From: 1010tires.com

Stock Tire - 245/70R17..................... .....Tire 1 - 305/50R20
Section Width:9.64 in245 mm...........Section Width:12.00 in305 mmRim
Diameter:17 in431.8 mmRim.............Diameter:20 in508 mmRim
Width Range:6.5 - 8 inRim.................Width Range:8.5 - 11 in
Overall Diameter:30.50 in774.7 mm....Overall Diameter:32.00 in812.8 mm
Sidewall Height:6.75 in171.45 mm......Sidewall Height:6.00 in152.4 mm
Radius:15.25 in387.35 mm..................Radius:16.00 in406.4 mm
Circumference:95.81 in2433.5 mm...........Circumference:100.5 in2552.7 mm
Revs per Mile:681.9..............................Revs per Mile:650
Actual Speed:60 mph........................100 km/h
Speedometer1:57.1 mph....................95.3 km/h
Speedometer Difference: -.....................Speedometer Difference: 4.907% too slow
Diameter Difference: -Diameter Difference: 4.69%
 

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I don't think I can accurately diagnose your problem all the way from the Mojave Desert. I am just trying to help you find the solution to your particular problem. Do you have the original wheels and tires that you can put back on to test with?
 

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Is your 06 Crew Cab a 1500, or a 2500? I run the same size on my 06 2500HD with no probs at all. I have 305/50/R20 Toyo Open Country A/T's on Ultra wheels.
I have a much easier time braking with my truck that my Wife does with the same tires only in 18" on her 1500 Suburban because of the brake size difference.
The brakes on the 2500HD are awesome. I have never had better brakes on any truck. I have 88k+ miles on my 06 with the original pads. Had it serviced last week and the pads still are at 75%.:party:
 
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