Like with like ? !!
So far all we have is a lot of "mine is bigger than yours" in terms of numbers for the fuel burn of the PA-28.
Not one of you has stated the power setting used and with out this all the numbers are useless.
The fuel burn will stay constant with increase in altitude (but the TAS will increase) so flying high will save money and the rough altitude vs distance on this class of aircraft is 5000ft for a trip distance of 100NM with the best alittude to be for range being between 6-8000 FT.
A NON INJECTED O-360 Lycoming should be using about 36LT/HR at 60% power and 40 LT/HR at 75% power. You will have to read the POH for the RPM vs Altitude for your aircraft to get the RPM settings (these nunbers include taxi and climb and should work with a 2 hour leg).
Please remember anyone who tells you that XXXX is cruse RPM without quoting a presure altitude on a fixed pitch prop is talking rubish.
Most of the posts above show how few people have an understanding of the POH for the aircraft that they fly.