the reason that 20 inch rims typically hurt tow ratings is due to rotating mass. With more of the weight farther from the center the ability to brake effectively gets cut.

thats why you lose the tow rating.. because if the truck cant break right with a certain payload... then the DOT comes after them. so they lower the rating for safety

also more of the tire is again pushed father from the center.. throw in the fact that 20s again typically weigh more than 17s even standing still and you lose gas mileage for your truck trying to move those heavy fers...

these are the main reasons i have a set of forged S in my garage Right now ready to replace my daily driving 24s... too much weight

 

The reason towing ratings are different is the engine/rear axle ratio/tire size. For example a 5.7l Hemi with a 3.55 rear end with 17's will pull less weight then a 5.7l Hemi with a 3.92 rear end with 17's. As soon as you very one of those three then the rating changes. The other change is the size of the truck, regular cab is lighter then a crew cab so the regular cab can pull more weight. The brakes don't have anything to do with it as anything over a certain weight must have trailer brakes depending on local laws.

 

This is what I have always been told as well. I just wonder why on the 2010 ratings the 17" and 20" towing capacities are rated nearly identical with gears and engine size being the same.

Also, do you think that since nothing engineering wise has changed from 2009 to 2010 that the higher tow ratings for 2010 are just as valid on the 2009 trucks? I look up the towing capacity for my truck for 2010 and its 2,550 lbs higher (7200 lbs to 9750 lbs). This is coming from Dodge themselves saying they have made no changes.