BSU
Try the
www.boeing.com website
AFAIK, it would appear that Pax aircraft are designed for range. Pax+bags +limited cargo results in a lower ZFW, so with a given Max TOW, the fuel and range is greater
.
The converted aircraft (pax->cargo) have the plmbing already in place for the extra fuel tanks.
Cargo aircraft, however, are all about maximising payload at the expecnse of range, hence they don't need the range that the extra tanks give them - they much prefer payload.
Some numbers from the Boeing site, suggest the originally designed freighters have a range of 4-5000nm vs pax variants of 7-8000nm. Specifically, the 747-400ER and 747-400ERF give ranges of 7670 vs 4970nm for the same max TOW. The -ERF can load about 6500kgs less fuel. Ths translates into much more payload, probably about 30 tonnes or so.
HTH