Well, According to the Laws of Physics....it's true.
Dodge can claim whatever they want. Insurance comapnies are clueless to the whole idea.

The reason you gained with 35's is because you aided those 35's with the 4:56gears. That helped the torque numbers and lowered them on a dyno. So, you could produce the torque needed to tow at a much lower rpm than what you would have with stock gears....but I think this part you are already aware of.

 

Why? Just figure out the error (if any) and allow for it.

Not necessarily. Wheel diameter does not affect the speedo/odo.

 

It's the construction of the wheel itself that diminishes the towing capacity. A 17" rim has less distance at the weak points to travel to the center of the wheel. This makes them stronger. The spokes become this weak point. I'd guess if your wheel was uncut solid steel without spokes your physics may apply. But you'll never find an approved racing wheel in 20" because they are too weak to handle the load.

 

IF you have a LT rated tire your towing capacity will go back up! I have the towing chart and did massive research on this since I tow a boat and a fifth wheel when mine and have the factory 20's on mine. BUT you need to make sure the gear ratio is 3.92 for the 20's to have max towing.

 

Physics?

Since you brought it up, let's just start with rotational mass, whereas there is less energy required to rotate a wheel where the majority of the weight is closer to the center of the object. Last I checked, 17" wheels had their weight closer to the center of the hub than 20" wheels.

Rotating components add effective mass which impacts greatly in moving that object (in this case a truck) forward in a straight line. I believe the proper "law of physics" would be: m_{\mathrm{rot}} = I / r^2

Wheel weight plays every bit as much a part in determining towing capacity than tire diameter does. Where over-all tire diameter is reflected in effective gear ratio, wheel weight is reflected in effective mass. If you put a tire that weighs say 10 lbs. more than the stock tire and is 3" larger in diameter, then the engine will have to produce enough energy to actually push an additional 60 lbs to achieve the same acceleration/speed. Now multiply this times four because there are four wheels...

 

There are several elements that have an effect here and perhaps there are aftermarket ways to get around some of them. But I was told, and Dodge customer service confirmed, that if you buy 2 trucks, identical in every other way, one with 17's and one with 20's, off the lot, the one with 20's has 1000lb lower towing capacity. I've been fortunate that the RV's I've owned have still not been heavy enough to be an issue. Still, I'm over the look of the stock 20" daisies and the prices of the tires. At my next tire swap, I'm going with aftermarket 17" rims.

 

Thanks Everyone. Looks like I'll be sticking with my 17's and just go ahead and put some new rubber on them. I'm thinking Continentals

 

I can't wait to get rid if my 20" and get 17" with 33" tire's. So if any want to buy some 20" send me a pm. I will be replacing them next month.

 

Rotating mass does play a role at high rpm and fast acceleration, but the amount of difference needs to be more significant than the difference between the 2 sets of wheels.
If we discuss rotating mass, the biggest factor is that the farther away from the centerline causes more difference than when your closest. Add bigger tires, this adds rotating mass further away from the centerline which is where the 20's are to begin with. I understad the rim itself, but again, it's minor in the grand scheme of things.

Stock 17" rims with a decent 33" A/T tire weighs the same as Stock 20's do! Which is ~76lbs. Go throw 35's on those 17's and now your up to 86lbs...10lbs over 20's plus a rotating mass thats further away from the centerline as well.

I think we are both right in a sense that they will deplete towing ratings. I just think that diameter plays a larger role when the whole gear reduction comes into play.

Now, the manufacturer doesn't say...If you add an A/T tire that weighs more your tow ratings go down...etc BUT I do think if you have a 17" and a 20" with the same size/brand/width/color/wear..etc that the 17's would slightly outperform, but it would be a much smaller difference as compared to 31.5's vs 33's that would be quite noticeable.
So, 3:55's w/stock 17's and 3:92's w/stock 20's have a very close overall effective gear ratio. This is why when guys with 3:55 gears have 20's, they lose towing capacity! So, 17's with 33" rubber for guys that have 3:55's essentially have the same towing capacity as stock 20's do with 3:55's.

Rotating mass will play a bigger role in an average driver(not a racecar truck driver..lol) when it comes to braking because that rotating mass is stored(stolen) energy and takes more energy to come to a stop. So, it's easier to burn/glaze your pads and wear down your rotors with larger diameter tires. THAT is where the rotating mass comes into more noticeable play than when towing.

So, to sum it up..I agree with you Hammer. I was trying to be simplistic for guys to understand and a rule of thumb to "run" with. Just as the manufacturer does.

 

Don't agree with me! I was looking for a worthy adversary for a good, old fashioned debate!!! LMAO...