Max. Range difference B767.300ER vs. B767.300F
 

When looking at the Boeing.com supplied range charts and technical figures for both B767.300ER and B767.300F, I notice a severly reduced maximum range for the freighter version compared with its passenger counterpart, although they can carry the same amount of fuel ( around 90.000kg.).

Can anyone explain this to me?

Haven't looked at the charts, but could it be that the freighter charts assume maximum payload whereas the chartsfor the pax version assume a typical two or three class load (210 pax) IIRC? Pax are a fairly light payload vs all cargo.
I have yet to fly a 767 that takes 90 tons of fuel, just under 75 is the max I have seen.

Freighter versions of aircraft typically operate at closer to their max ZFW than their passenger equivalent. A classic B747F could carry almost 110 tonnes whereas the weight of a load of pax + bags would be more in the 50-55 tonnes range. Given equal max TOW then there is less scope for fuel - hence less range!