I read alot of stuff from the 70s were former fighters and writers say jim jeffries in his prime was the greatest. Ali would have destroyed him he may really not have even touched Ali.
I don't think there are any reliable films of Jeffries in action. Also, it's really hard for us, today, to gauge the quality of Jeffries's competition. These points hold true for our analysis of any of the pre-Tunney heavyweight champs, including, possibly, Dempsey.
It would be interesting to see how he handled himself now-days. He had the size to be a modern day HW. I agree with Kenmore that there isn't much known about how good he was. The only film out there that I had seen of him was against Johnson and really shouldn't tell us much about him since he hadn't fought in 6 years and weighted 330lbs prior to it and took no warmup matches either. The fact that he lasted 15 rounds is interesting though.
gene tuney is the only one of the real old timers ali respected...he was the first one to bring actual movement and boxed and used defense
Alot of the old time great fighters from the early 1900's were of the consensus that Jeffries was the greatest heavyweight that had ever lived. He was freakishly strong and tough as nails in his prime. He worked as a boilermaker for years, and it put thick slabs of muscle on him. He is largely forgotten today outside of hard core boxing fans, which is a shame. Most people that know of him, only know him for his last fight that he came out of retirement for. He was considered well past his prime at 35, and had to lose over 100 pounds in a short time for the fight.