I would think the smaller tire side walls with the 20 inch tires cannot support the weight vs say a 17 inch tire with a larger side wall.

 

The diameter of the 20" wheel + tire is the same as the 17" wheel + tire.

I believe it is due to the extra unsprung or rotational weight of the 20" wheels over the 17" wheels. It has been said that 1 lbs of unsprung mass is equivalent to somewhere in between 6 - 10 lbs of regular weight. I don't know if that adds up since I don't know the weight difference between the wheel/tire setups, but that is a good reason for a large portion of the difference at least.

 

The min towing capacity for the 2--0 1500 is 5900 lbs. The max towing capacity if 9100 lbs.

I'm not sure what they mean by minimum towing capacity though.

Also - the guide says that you lose between 1100 and 1150 lbs of capacity if you have the 20" wheels. I couldn't find out why though. I might speculate that it has something to do with how close to the ground the 17" wheels make the truck sit - but that question is for someone with a strong background in engineering and physics.

 

If you read the rest of the thread your questions will be answered.

1) 5900 - 9100 lbs. There was a chart posted - towing capacity is based on your cab configuration, gearing, rim size, engine size and 2wd vs 4wd.

2) 20" wheels - normally it's because
a) 20" wheels are not load rated hight enough
b) the low profile tires used on the 20" rims are not load rated high enough. The 17" rim/tire combo and the 20" rim tire combo are basically the same height.

Also, anything over 5000lbs with the class3 hitch and 6000lbs with the class 4 hitch is going to require a weight distributing hitch - otherwise you are exceeding the weight limits of the hitch.

 

I being a paraplegic, went to pick up my ram and well it was higher than I remembered, guess I was just excited that day, extra strong, So for fun we got the keys to one with 17's its the same height I can guarantee this I was there just ask me. More tire on the 17 in fact I would like a set for winter.

 

I am 6'1" and have a hard time getting into my Ram, but I wouldn't change anything

 

the 17" tires are 265/70 r17. This means they are about 31.5" diameter.
the 20 " tires are 265/60 r20. This means they are 33" diameter.

If we consider the 17"s to be normal, a 3.55 gear ratio would be a 3.55 effective gear ratio.

With the added diameter of the 33" tires, that would make a 3.55 ratio effectively 3.39.

On 17"s, if a 3.92 = 3.92, with the 20"s, a 3.92 = 3.74

The effective gear ratios are almost a step lower with the 20"s. That's the main reason why the ratings are lower. They are also harder for the brakes to stop.

The 20" tires are larger diameter, that's a fact.

It only equates to 3/4 of an inch in ride height which may not be noticeable to you, but the gear ratio sure notices it. I was parked next to another 09 that had 20"s today, and it definitely is higher. The tires on the 17's have bigger sidewalls, but the 20s are larger overall.

 

265/70 17 is 31.6"
265/60 20 is 32.5"

So in terms of truck height, the truck with the 20's would be .45" taller than the truck with the 17's

 

Some, but not a whole lot of difference. At 2000 RPM there is about a 1.5mph difference.

52.96 MPH (31.6" diameter, 3.55 gears in 1:1 trans gear)
54.47 MPH (32.5" diameter, 3.55 gears in 1:1 trans gear)

 

According to Tire Rack, they have these numbers:
265/70/17 Wrangler HP is 31.7
275/70/17 Wrangler AT/S (TRX4) is 32.4 http://www.tirerack.com/tires/Spec.j...irePageLocQty=
275/60/20 Wrangler HP is 33.0 This is listed as the OEM tire size for the 20", not the 265. http://www.tirerack.com/tires/Spec.j...3D76SR0WRHPOWL