With a smaller tank, you will need to insure you can adequately pick up the last 1/2 gallon of gas. Use 2 fuel pick ups, 2 fuel pumps, with check valves on each. Use something to create a peak under the center floor of your bladder, with the pick ups in the corner.
I seem to always get about 4mpg with any car I ran on the track, B/S (510), B/P, GT1, IT, or E/P.
When exiting the infield at Charlotte I had problems picking up the last 6 gallons because the cornering forces would move the fuel to the far right side of the cell. My only fuel pick up was on the left side. Was nearly on the flat back straight, exiting NASCAR T2, before I was able to recover any fuel pressure.
If you run a smaller fuel cell, move it as far away from the muffler as possible. I eventually had to replace the fuel cell bladder. It was too close to the lava rock muffler. The residual heat would heat perk the bladder after I got back to the trailer. It eventually blistered the bladder and caused it to leak. Never realized what was happening until refueling one day. Ended my weekend. Try to mount the bottom of the cell level with the rear valance or just slightly below it. It may slightly improve your aero drag.
