I had a Dodge motorhome that did that - had to replace the steering box.

 

My tires rotate every time I drive!

 

One of the forums I'm joined had a thread about this. Most obvious reasons were examined before coming to any rash conclusions too but the thread starter found that the 2nd gen Dodge Rams have an issue with improper caster settings. Most will find that their caster is set equally between the left and right front tire and these trucks need more positive caster on the right side. In saying that, the 2nd gen front suspension responds way better to all bad steering habits with as lots of positive caster angle. It may not completely fix the pull to the right but it will help. I set my caster as far positive as I could (with the right having more) and my steering has never been so kind.

 

YEP, rotating the tires took care of the pull issue. Who would have thought.
So we went to Discount tire and replaced the (4 year old 40,000 miles) dry rotting tread at base kuhmo 265 65 16 tires with Falkin 265 65 16 tires. I noticed right away that per my gps, when I set the cruise at 65 mph on the old tires, speedo indicated about 66 on the dial. The new tire when set at same speedo speed, indicates 62 mph on the gps. New tires must be shorter in height then the ones removed. Tack rpm is also higher with the new tires. So I guess that just because a size is stated, it doesn't mean they are the same size. I guess that is called "cost reduction" from the manufacture and passed on to the customer.