B4000 gas mileage...

i'm gonna be buyin a 96 B4000 4x4 soon from a friend's dad... he says it gets about 17-18mpg, and that he's gotten 21mpg before on a long trip with just highway driving... now here's my question... i kno when my friend put I/H/E on his little civic he got like 2-3 more mpg... will this also ring tru for the B4000? and if so, what shold i go with? any specific type? idrc about the gas mileage, that's not why i'm getting it... i'm just saying better gas mileage is better gas mileage, kno what i'm saying?

Steven Folchi