im gonna put 20's on my 94 chev truck with 285 50 rubber, how much is the mileage gonna go down?  

depends on how much heavier or lighter the wheel and tire package is than the stock rollers. It also depends on how the new rollers change the final drive ratio of the truck. Typically, larger wheels and tires are heavier and if you dont re-gear the rear axle your mileage will suffer.  

Very important is the outside diameter of the new vs. old tires.  If you increase the diameter of rear tires on a rear-wheel-drive, it cuts your rpm and could actually INCREASE your mileage but it will not show on your odometer.  Bigger diameter will mean your odometer/speedometer is now underreading both miles and mileage.  Example:  I went from 31.4" stock Ram SRT-10 22" tires to 34.8" and that cut my rpm by 11%.  I do not know what the effect was on my mileage but what I am saying is that going to bigger does not necessarily mean worse mileage and it is the o.d. of the tires, not the wheel size that matters most.