truck runs great with less gas? HELP

This is the wierdest thing, my truck runs the best when the gas in the gas tank is close to being empty, basically when the guage is over E.What causes this? Seems to run faster/ more torque and be more responsive. I have no idea why it don't do it when it has Full, Half or even 1/4 tank left?

fuel weighs quite alot

I know but it shouldn't make the rpms and mph go faster with less gas.

Fuel weighs alot, so it takes more power to move it.  With less weight, the engine doesn't have to work as hard... rpm's increase more rapidly, therefore being more responsive.  You're truck isn't actually running better, it's just moving less weight.